Calling Nick Lappos - Blade Stall [Archive]

dClbydalpha

28th Sep 2016, 10:10

AnFI - You are correct that each point can be considered separately and then integrated. I suggest you try carrying out the integration, taking chord elements along the blade from root to tip, and post the equation that results. Sorry if I am telling you something you already know, but to start it is easier if you assume a constant chord/aerofoil and the ideal twist to give you a uniform downwash. Concentrate on the change in alpha along the blade as the tangential velocity changes with respect to the induced velocity. The equation you post, if correct, will tell you why I say that your overall assertion is only valid at a point and therefore why it cannot be applied to the rotor.

Centripetal v Centrifugal is not nitpicking, they are not colloquialisms, it is engineering terminology. Centripetal is a tendency to cause movement towards a centre. Centrifugal is a tendency to cause movement away from a centre. The definition is in the Latin name, centre-seeking and centre-fleeing. The two must not be confused, a movement of the blade CG toward the centre would add to and not oppose the lift. While physics tells us that centrifugal force is a "false" force, it is actually an inertia effect seen in a Newtonian frame of reference. It accepts that it is perfectly valid to calculate within a non-Newtonian frame of reference, such as one in constant rotational motion. In this case "false" forces become relevant and directly usable. It is much more convenient for certain rotor calculations to work in such a frame as it simplifies a number of equations, and as such the terminology is crucial to establish what is being worked with so a particular calculation doesn't cross frames of reference. Of course some who practice engineering experience this regularly, and some even have to work with a Newtonian, a constant rotation, and a relativistic view of the universe all in the one system.

But back to the original post. I assert that once you have calculated the lift along the blade it does not simplify to a direct relationship between stall and coning that can be applied generally, even for a simple rotor in hover, let alone one that employs varying aerofoils, varying chords, non-linear twist, that is translating as well as rotating.

[email protected]

28th Sep 2016, 11:01

Well that's AnFi put back in his box:ok:

He will be back though, complaining you are being rude to him because you don't agree and then arguing some infinitessimal point to try and win his argument - then extending that to argue he was right all along:E

SASless

28th Sep 2016, 12:44

Anyone got some Duct Tape to seal that Box?:oh:

Canute

28th Sep 2016, 14:15

Take it to the extreme case.

If all the blades are pointing vertically up, are they all stalled?

If so, then he is correct.
If not, then he is not.

Ascend Charlie

28th Sep 2016, 14:39

If the blades are vertically up, then they must be generating lift, so they can't be stalled, ipso facto ignoramus platypus.

AnFI

28th Sep 2016, 18:05

that integration is fairly easy
and i think you'll find it proves my point

( and your explanation of why u use false forces is a laugh
" In this case "false" forces become relevant and directly usable."
thats ok with me )

dClbydalpha

28th Sep 2016, 18:26

:) which is why I look forward to you posting the results of your attempt AnFI. With the starting point I've given you it's only 4 steps to the standard form of the equation that any rotorcraft engineer would recognise.

Don't forget to state clearly any assumptions and particular notation forms you choose to use. I'd hate to think you'd got it wrong because of the use of different terminology, like centripetal being used to describe a force that acts away from the center of a circle.

[email protected]

28th Sep 2016, 18:59

which is why I look forward to you posting the results of your attempt AnFI don't hold your breath waiting for that one..........

dClbydalpha

28th Sep 2016, 19:13

Thanks Crab, I'll try not to let it keep me awake ...

megan

29th Sep 2016, 02:19

( and your explanation of why u use false forces is a laugh
" In this case "false" forces become relevant and directly usable."
thats ok with me )Even I, a non engineer or academic, am familiar with the term "false force" as used by academia and engineers, also referred to as "fictitious force". You might like to cogitate if gravity is a "real" or "fictitious (false)" force.

paco

29th Sep 2016, 03:51

And even whether it is a pushing force instead..... :)

Phil

Bell_ringer

29th Sep 2016, 04:29

Anfi will take issue with centripetal and centrifugal forces being involved in any equation as why use two forces when statistically any equation with just one force is as good :E

SASless

29th Sep 2016, 06:40

Bell Ringer....you are a very naughty boy!:=

[email protected]

29th Sep 2016, 12:22

But very funny:ok:

AnFI

4th Oct 2016, 18:05

Coning angle is the result of the integration of some of the local forces on each element of a blade.

At each element the local forces that are relevant to coning are the moments about the flapping hinge. (Ie those forces in the plane normal to the flapping hinge at their respective distance from the flapping hinge.

They arise from two forces
They are
1 the apparent outward force (generated by the reaction of a mass to Centripetal Accn), call it Centripetal Force (Cf) if you like.
AND
2 the aerodynamic forces in that plane, traditionally thought of as ‘lift’ (although it’s not strictly true, nevertheless the aerodynamic forces are still a product of local v^2 notionally)

The first can be expressed thus
Moment = ∫(from 0 to R) mxω^2 xsin(κ) dx
m is the mass of a small length of blade (dx)
x is the position along the blade up to R the radius
ω is the angular rate in radians per second, ω≈RRPM/10
κ is the coning angle in radians
MCf is the Moment about the flapping hinge causing the blade to (not) cone up
That integral gives
MCf = -1/3 mR^3 ω^2 sin(κ)≈ 1/3mR^3 ω^2 κ (for small κ)

The second moment can be expressed like this
Moment = ∫(from 0 to R) 1/2ρA©x^2 ω^2 x dx
Where
ρ is the density of air
A is the Area of a dx sized piece of blade (ie Chord)
© is the ‘Coefficient’ of Aerodynamic force in the plane normal to the hinge (similar to CL ) but it also assumes an ideal twist (T, or ‘washout’) that reduces CL by an amount that ensures the Lift and Cf remain in proportion to minimize unnecessary longitudinal blade bending. So it embodies an x^-1 term, (so something like CL (R/(R-x))
ML this is the Moment caused by aerodynamics in the plane of the flapping hinge
x is the distance along the blade and ω is again the angular rate in Radians/second
so xω is the local v (speed of airflow, at that x)

That integral gives
ML = 1/6ρA©R^3 ω^2

Since ML + MCf = 0
Then ML= - MCf
1/3mR^3 ω^2 κ = - - 1/6ρA©R^3 ω^2
solve for κ (THE CONING ANGLE)
κ = ρA©/2m
SO regardless of how exact that rough working is you can see clearly that the coning angle is not dependant on RRPM (equivalent to ω)
What you can see is that coning angle depends on the ρ (density) , A (the chord), the shape and Angle of Attack (CL) and the mass of the blade.

Since there is a ©STALL then there is also a κSTALL and it does not depend on RRPM

It is irritating to have to pointlessly integrate your pointless formula, especially since you obviously barely understand it yourself

Since you have chosen to align yourself with the playground oaf I seem to have no choice but to perform the pointless maths you ask for.

I say pointless because the variable cancels out prior to having to perform any integration anyway.
So your (pointless) set of ridiculous assumptions (eg twist for constant Induced Flow, and I used a similar (but better) assumption) are unimportant since.

The whole thing can be stated far more simply (which is generally a good thing) like this

Κ = arctan(L/CF)
Which for small L and Large CF is (to 4 decimal places) the same as
Κ = L/CF
Well it should be obvious that RRPM cancels out of that but here it is
L = a times RRPM^2
CF = b times RRPM^2
So
Κ = L/ CF = a times RRPM^2 / b times RRPM^2
CANCELLING OUT you get Κ = a/b
(where a is the collection of other (unimportant here) factors (density of air that sort of thing. b is another collection of other (unimportant here) factors (mass of blade etc))

Don’t blame me for giving that answer, I was goaded into it by an over self satisfied aeronautical engineering degree holder and a playground oaf.
I’m sure there’ll be all sorts of nit picking over the detail of my maths, but I don’t have time to improve it so please don’t bother. I think it is enough to prove my point though

212man

4th Oct 2016, 21:00

I’m sure there’ll be all sorts of nit picking over the detail of my maths, but I don’t have time to improve it so please don’t bother. I think it is enough to prove my point though

Which was what, exactly? Sorry, I'm easily confused, but I didn't see any reference to how many engines were involved......

Arm out the window

4th Oct 2016, 21:05

If you're saying local speed at any blade point is distance out x angular rate, that ignores forward speed, which was one of the fundamental sticking points way back at the start.

Bell_ringer

4th Oct 2016, 21:06

Which was what, exactly? Sorry, I'm easily confused, but I didn't see any reference to how many engines were involved......

Ironically there wasn't one point.
There were at least two which is, statistically, better than one - ok this is wearing a tad thin.

Over and out..

Lonewolf_50

5th Oct 2016, 12:48

AnFI, I think you forgot the QED at the end: Quod Erat Demonstrandumb

[email protected]

5th Oct 2016, 13:19

Oh..AnFI,,,you are so clever, so intellectually superior... how could any of us ever have doubted you???

I am sure there is a proper adult with more maths knowledge than my poor O level here on this forum who will blow big holes in your 'Proof' that you were right all along (as ever:E) and that we are all idiots who shouldn't even be allowed to breathe the same air as you:ugh:

Not sure what all your pseudo-intellectual posturing is all about but it's certainly not winning friends and influencing people.

Ascend Charlie

6th Oct 2016, 04:06

AnFI, you said:
you can see clearly that the coning angle is not dependant on RRPM

However, I can clearly see that a chopper doing an engine failure at the hover will have its coning angle increasing with decreasing RRPM, even though the lift is slowly decreasing.

But I suppose that, as the revs decay, the CL would be increasing with the pilot pulling in pitch to slow the descent and cushion on, which from your equation would increase the coning angle. Apparently the change in angle between low RRPM (pitch high, CL high) and high RRPM (pitch less, angle less) is in fact due solely to CL and not RRPM.

What a clever little secretary. Learn sumpfink every day. But yer average dum student can easily comprehend it if we (mistakenly) say that coning angle is the result of lift (upward force) and RRPM (outward force), the resultant vector of which lies neatly along the span of the blade.

dClbydalpha

6th Oct 2016, 13:10

AnFI I haven't aligned myself with anyone. But I did make one mistake, I assumed that you were someone who was here, like a great number of users of this forum, to expand your own knowledge by interaction with others. I therefore chose not to directly post a refutation of your analysis, but instead gave you an opportunity to look at the conditions and statements that I and others had posted and see if it revealed to you where the inconsistency lay through your own exploration of the physics. From the tone, attitude and content of your post it is obvious to me now that that isn’t the case, so I won’t waste too much time here.
We'll use your equation.

κ = ρA©/2m
© used here by you to denote coefficient of lift, is a normalised value of lift, for any given aerofoil. An aerofoil has a curve associated with it, often obtained empirically, that links © to α. Assume we are in the linear part of the lift curve and we'll call dClbydalpha "a" and substitute.

κ = ρ.A.α.a /2m.

Unlike a fixed wing, a rotary wing is rotary. Therefore the "v" that any aerofoil element experiences relies on rrpm and the element's distance from the hub let's say Ωr . However the downwash the element is seeing is independent let's call it u.
Therefore
α = arctan(u/ Ωr)
Assuming a small angle
α = (u/ Ωr)
And substitute

κ = ρ.A. (u/ Ωr).a /2m.

As you can now see, there is a term for rotor speed in your equation. Q.E.D.

AnFI

7th Oct 2016, 09:15

Hi a

(we'll call dClbydalpha "a" and substitute) ;)

I am afraid you are wrong
I am trying to get an important point across.

Please analyse your friend crab responses and see if he is helping get the point across. That is where the frustration comes from.

I think your assumption that downwash remains constant is incorrect. Really the downwash in your scenario would increase with RRPM so there's the RRPM term on top that you 'accidentally' left out.

ie it still cancels.

But that's not the point, the point is there is an interesting way to look at an Ultimate Coning Angle

Yes there is some bathwater (like Airspeed, which would reduce the UCA)
but it is useful to think of UCA being dependant on the terms left in my previous math

You can see it is useful because there is a frequent error made in thinking that coning of 'say' 45deg might be acheived.
In 'layspeak':
This 'can't' happen because if a blade is going slowly enough to make feeble Cf then it's going too slowly to make enough lift (and the attempt to make more lift by increasing AoA will have resulted in stall prior to that point (actually at (or close) to the UCA)

dCl/dA I hope that you can find the valuable point here, and help me explain it. (rather than showing that you learnt some maths once, it's nice that you did, why not use it for the forces of good, truth and the American way? Instead of encouraging that moronic oaf.)

AscendC quite right

[email protected]

7th Oct 2016, 11:30

As ever AnFI, it is never clear exactly what point you are trying to get across - and even less clear as to how it will improve a student's knowledge and make him fly a helicopter better.

Also, as usual, when someone who does know the subject (maths in this case) shows you the true path, you immediately refute it because it doesn't fit your argument (whatever that is).

Enjoy your posturing but, as the oaf in the playground, I do know who is rapidly becoming the big-headed bluffer in the classroom;)

dClbydalpha

7th Oct 2016, 16:00

AnFI, the discussion of such concepts is important, but fundamentally the premise that for a given rotor system there is a "coning angle" that is the equivalent to a "stall angle" for a fixed wing does not stand up against the physics of the situation.

I am glad you acknowledge the nature of downwash. Downwash is the important factor that appears to be missing from your conceptual model, and where my musings began. The reason I suggest a uniform downwash, and a linear twist is that it makes the first calculations easier. If you can envisage the form of the integral that incorporates a linear twist and a uniform downwash then you can see how it can be extended to more exotic implementations.

So there are a number of forms of the equation below but it is pretty well recognisable

dL=1/2 ρ c dr a [ θ - u/Ωr - τ(r/R ) ]

c is the chord at that section
dr is the element of radial blade
a is the coefficient of lift of the aerofoil with α
θ is the original pitch of the blade
u/Ωr is the difference of α generated by the downwash with respect to the tangential velocity of the blade
τ(r/R) is the difference of α generated by the twist of the blade

If you choose not to have a uniform downwash, u, then you replace u with a function that represents the value of u at blade element r. Similarly for τ. But even in its simplest form it shows the importance of the induced downwash term when calculating lift.

Your suggestion of providing a blade design that matches lift to centrifugal force is intriguing, but can only really be considered as self-defeating to your hypothesis. I'll explain the steps why in words rather than equations.

Change CL such that it is proportional to the distance along the rotor.
Lift is now changed with proportion to the distance along the rotor, except for the fact that the change in lift has induced a change in the relationship between local downwash and tangential velocity i.e. it has changed the α and therefore the lift, so the lift is slightly off of the expected value.
You compensate this delta by tweaking the rate of change of CL along the blade and so now the lift is as you required.
What you find by doing this is that the "tweak" is only valid for a given rotor speed. You change the rotor speed and you need a different relationship between CL and r. Without adaptive aerodymanics this is impossible.

So what have we discovered?
The concept of relating coning angle to a "stall" condition can only be done for a known rotor system, with a specific relationship between CL and r, and at a particular rotor speed. Something that I hoped I had pointed to in my original posted reply. This is a long way from the concept of a generic, rotor speed independent relationship hypothesised.

This of course is totally academic as it has so far only touched on the rotor in hover. As soon as any cyclic commands are input, then the disc tilts and the axis of the cone along with it. Having done that we will start to translate, creating an asymmetry of lift. The whole concept of "stall" then involves flapping, advancing blade and retreating blade dynamics.

There is a relationship between aerodynamic and inertia forcings, called the Lock number, which is important to rotor design. But that is another topic.

So I'm genuinely sorry AnFI I don't see what this important topic is. I must be missing the scenario you are thinking of as there doesn't appear to me to be any reason to monitor coning angle as a critical flight parameter. To the pilots out there, have any of you flown with a coning angle indicator marked with a critical value?

Ascend Charlie

7th Oct 2016, 18:59

Yes, the indicator is called the Collective, and the critical mark is when it reaches your armpit.

The other indicators are "RPM LOW" and "beeeep beeeeep beeeep".

AnFI

7th Oct 2016, 22:27

dCl/Da
I did cover all those points didn't you bother reading?

and it is important because there are many accidents from attempting to pull more g resulting in less lift

elegantly the rotor actually physically performs the integration (not that it's neccessary), and as AC simply puts it coning is the ratio of 'lift' to Cf. you just can't get that ratio beyond a (fairly low) value even if you want to. (because if the Cf were low (although that's not the point, the normal RRPM case is) then the 'Lift' that can be made before stall isn't that great either)

yes, nobody flys with a 'critical cone indicator' (perhaps they should!) but it would help pilots understand why you can change pitch of the aircraft at a greater rate but should not neccessarily expect a greater TRT hence why they annoyingly hit the lake/snow/ground so often.

I alluded to higher speeds, the points dCl/Da makes are not relevant (especially Lock number)
really what happens when you pull more g in a helicopter at speed is parts of the disk stall but the effect is distributed, there's a load of vibration and the disk as a whole starts to make less TRT rather than more.
It's a little like the real stall in an aeroplane in as much as the whole wing (generally) does not stall at the same parameters, so it's a 'soft stall'. For a helicopter it is very like the phenomenon described in the manual extract by (i think it was) SASless, about 'mushing'.

valuable ? possibly would have been to the pilots with egg on their face and might help other pilots to avoid egg on thiers

Crab nothing valuable to add as ever, you really are an oaf, and nowt wrong with my maths either thank you (dCl/da neatly avoided the fact that there's an RPM term on top missing in his simplification !) since you obviously don't understand this stuff why don't you just shut up and contemplate a little longer before opening your unhelpful and loutish mouth?

anyhow i've said enough, take it or leave it, it might be useful to some pilots

megan

8th Oct 2016, 02:11

and contemplate a little longer before opening your unhelpful and loutish mouth?You should take up some of your own advice. You work on the principle that bull**** baffles brains, an educator lays out the subject matter in a manner that is appropriate to the audience, in this case, one that doesn't involve maths.

Ascend Charlie has given the simplified detail of what concerns the PIC.

[email protected]

8th Oct 2016, 08:36

AnFi - now you have slid sideways in your arguments and its suddenly all about G stall not coning angle.

Have you ever pulled more than 1.4G in a helicopter? Ever backflipped a Lynx hover to hover (a 3G manoeuvre)? Ever taught advanced high speed, high G manoeuvers ie air to air combat in a helicopter? Ever taught and utilised rapid descents and recoveries (high G and close to the gorund) to get through a threat band?

I have done all of those, yet I am the oaf in the corner who doesn't understand.........Yet you can pontificate about how many pilotsannoyingly hit the lake/snow/ground so often something you clearly know less than the square root of c*ck-all about.

Now get back to your preening and let those of us who do know how to teach students how not to crash get on with it.

Frying Pan

8th Oct 2016, 10:58

Oh dear, I realise Nick Lappos is recovering but I also imagine he's read this thread and thought why? It started with good intentions but has declined into farce and willy waving. And yes I get the irony that this post has nothing positive to say either. Carry on.:ok:

Bell_ringer

8th Oct 2016, 11:31

It started with good intentions but has declined into farce and willy waving
I would be careful of waving too vigorously as the angle of the dangle could result in issues with lift and performance resulting in a severe stall with the ladies :mad:

AnFI

9th Oct 2016, 01:52

Crab

You tell me you've done these things, you must be very good, but you add nothing.

If you have you'll understand what I have been saying from the beginning. (wrt the Greek Apache, remember?)

If you don't then you are part of the problem.

What do YOU think happens as you begin to pull ultimately in a helicopter? No answer I guess !
(just re-read my posts it's all there)

NickLappos

9th Oct 2016, 03:48

I am following this thread, and am a bit perplexed! Most helicopter accidents where the maneuver falls short of the need (a pull out that can't stop the descent before ground contact) are the result of too little engine power, since most occur at speeds where there is no power other than the engines to fuel the blade lift demanded.

I would argue that even in the video we watch where the Apache plunks in. I know that a light Apache can pull about 3 g's before the rotor stalls, and yet the engines can only produce about 2 or a bit more. I wonder what the G level was of that machine as impact occurred.

I know these issues quite well (I authored the papers that helped define Maneuverability and Agility back once upon a time), work that helped produce the LHX maneuver requirements.

I seriously doubt that blade stall produces accidents, mostly it is lack of available maneuverability as limited by engine power. Let me be sure and state that if sufficient speed is had (well above Vy) then the speed can be sacrificed to produce extra thrust. The video proves that the Apache in question was very very slow (25 to 30 knots?) and that maneuver was a pure power maneuver.
In a nutshell, stall had nothing to do with it, and the simple fact is that the pilot misjudged his altitude, and had insufficient climb performance to stop the descent prior to impact.

The best video is at https://www.youtube.com/watch?v=xOW44sydqtw

xOW44sydqtw

megan

9th Oct 2016, 05:07

Thank you Nick, very much obliged, language we can all understand, and exactly what we all thought, save one. :ok:

AnFI

9th Oct 2016, 08:30

Hmm thank for your interesting view which carries much weight.

I note you say
"a light Apache can pull about 3 g's before the rotor stalls, and yet the engines can only produce about 2 or a bit more. I wonder what the G level was of that machine as impact occurred."
Me too!!!!
What coning angle would an Apache make whilst pulling 3G?

We could measure the coning angle from the video and then we'd know !!
Looks like quite high cone to me, about 3 times (ie approx 3g)?

and
"speed can be sacrificed to produce extra thrust. "
well he didn't run out of speed
and as for "running out of climb performance" is concerned ??
he's not even nearly converting Fuel and Kinetic Energy to Height Energy

and what is you opinion of this one
https://www.youtube.com/watch?v=FqRgkur-RpQ

"I seriously doubt that blade stall produces accidents, mostly it is lack of available maneuverability as limited by engine power."

anyhow thanks for your view, they'll be delighted !!
get well soon

ShyTorque

9th Oct 2016, 09:37

Nick, thanks for injecting some authority into the subject. I think most of us have been trying to say the same sort of thing.

Here is an illustration of the problem we're faced with:

Dw-vflrjkQI

Ascend Charlie

9th Oct 2016, 09:50

AnFI, you want g performance, talk to the BK 117/ BO 105 people, +3.5 to -1 is the envelope. They can probably inform you of the coning angle, and it sure ain't stalled at the limits.

[email protected]

9th Oct 2016, 10:17

As I mentioned AnFI, your knowledge about the cause of helicopter crashes wrt failing to pull out of manoeuvres is pitiful.

Almost all the type of videos you link to may well end up in an 'annoying' crash into the scenery but the end result was predictable - usually at the entry stage.

You won't have done aerobatics but it is all about entry gates - strict heights and speeds to achieve before you start the manoeuvre - the Apache one and so many more were the result of insufficient height on entry to safely complete the pull out. (and in the Greek one a pretty poor wingover technique)

The classic wingover crash is where the pilot doesn't get halfway round the azimuth before he gets to the apex - the temptation is to roll on more bank and pull harder to try and get round (this accelerates you towards the ground) - then it is only in the latter stages you realise you don't have enough height and pull hard with both hands - at that point, as Nick says, it is lack of engine power due to the high pitch angles and massive drag that prevents a 9 G pull out, coning angle is just a symptom of the overpitching.

But, since you won't even listen to an absolute expert like Nick, I am sure my views won't cut any ice with such a self-proclaimed know-it-all like you.

As for my abilities - I am simply the product of a very good training system which, in turn, I have used to pass on the same lessons learned - normally with simple explanations and demonstrations on the ground and in the air - all without trying to score intellectual points over my students.

I wonder what you bring to the instructional party..............

AnFI

9th Oct 2016, 16:18

"You won't have done aerobatics but it is all about entry gates"
If I had your experience in Aerobatics in helicopters and your training background I'd probably think the same as you.

I am sorry I wasn't able to explain. That is either because I am wrong or insufficiently clear I suspect, but possibly because you find it difficult to understand new ideas.

"As for my abilities - I am simply the product of a very good training system which, in turn, I have used to pass on the same lessons learned - normally with simple explanations...."

Please demonstrate by explaining to me (us) what you think happens when pulling out of a steep dive where the airspeed is say 120kts (ie no lack of available Kenetic Energy to tap and suppliment what limited Energy the engine can contribute form fuel burnt (/time, ie power)).
I am keen to have your understanding of what the Angles of Attack would be doing and where the conng angle would be.
Are you saying that I could pull 9G if I had sufficient energy(/time) available? Would the coning angle be about 9 times the 1g condition coning angle?
What would be the ultimate physical limiting factor to pulling more G? (would it be Angle of Attack?)

I would be more than happy to receive an explanation that would alter my view and 'educate me'

I think NL was saying in the specific case of the Greek Apache that his estimation of the speed (energy available) was too low to be able to suppliment the engines ability.
(although a coning angle of 10deg would imply in the order of 3 G, which he says is limiting for a lightly loaded Apache)

Looking forward to a clear explanation ( I won't hold my breath either ;) )

JohnDixson

9th Oct 2016, 17:01

Just to add some technical " color commentary " to Nick's post:

1. Most modern helos design their boosted controls for 100% travel per second or more.
2. Thus, if the pilot of that Apache ( or a UH-60 for that matter ) applied that sort of collective increase the first result would be some slight NR droop because he would be increasing the power required faster than the accel schedule designed into the engine.
3. The next thing that happens is that the engine reaches its power limiter and from that point on the Nr droop becomes larger and the ability of the rotor to modify the vertical path is compromised.
4. I'm pretty sure the Apache rotor inertia is similar to the UH-60, and the engines have always been in step with one another. So, historically, the best Nz we ever achieved at hover thru the low speed range on the UH-60/SH-60 machines was just over 2 G due to the above engine factors combined with relatively low inertia. I was told by a US Army Flight Standards engineer who was monitoring one of our UH-60 structural demonstration test programs that the Bell 214 guys with a higher inertia main rotor achieved their highest Nz test point in the jump takeoff manuever and could get 3 G very transiently. Makes sense.

None of these attempts to achieve max G at hover thru slow speed ( the military target is to achieve the 3.5 G spec requirement ) ever came close to blade stall*. Of course the other element involved is the G achievable by flying the helicopter symmetrically so that the G results from the basic equation of pure pitch rate times airspeed, but the reality is that at slow speed, one cannot get enough pitch rate to have enough effect on the vertical path, and you get back to what Nick described.
* One other comment deserving mention in passing is that in all of the new model development testing I'm familiar with, which includes the flight envelope definition, and maximum manuever tests, coning angle is not a parameter of importance, is not a limiting parameter, is not a parameter used by the telemetry team to predict blade stall onset or anything else for that matter. Flapping is, at least for us articulated types.

NickLappos

9th Oct 2016, 17:16

The coning is actually a measure of many factors, as you have carefully stated, AnFI, but in a case like the Greek Apache, it probably indicates that the power demand that the pilot made was excessive (and yet not enough) so that the engine power (torque) limiters allowed the rpm to droop, thus making coning angle higher because the centrifugal stiffening was much lower as the rpm was pulled down.
All this is great speculation, the data recorders on the aircraft should have captured it all and made it available.
You raise interesting points, but a central one is mostly overblown and should be discussed.
Rotor stall does not affect the large percentage of CFIT accidents, most rotors stall at load factors far above the load factor that the engine power can produce. Unless the airspeed is very much higher than Vy, the speed decay that can fuel the "extra" load factor is just not available.
Here is a chart that shows the energy available for "fueling" maneuvers from the typical sources. Note that below 80 knots, kinetic energy is vastly out weighed by engine power and even by stored rotor energy.
https://scontent-lga3-1.xx.fbcdn.net/t31.0-8/14633218_10207512003981008_9128110073794482769_o.jpg

Here is actual maneuver data from a Utility helicopter during air combat trials, plotted against Ct/Sigma which is actually a stall factor. It represents the absolute maximums that can be squeezed from the machine by an experienced test pilot. The rotor stalls at about .18 to .21 Ct/Sigma, so the typical low speed maneuver maximum is far below stall. Note how the shape of the typical maneuver is the same as the engine power shape in the previous chart? No coincidence! For comparison, .15Ct/Sigma is about 2 G's in this case. BTW, advance ratio is the tip speed ratio, so that .2 is about 85 knots in this case. Note that when speed is higher than Vy extra G is available as transient, powered by a deceleration.
https://scontent-lga3-1.xx.fbcdn.net/v/t1.0-9/14573005_10207512005021034_6218301555795789286_n.jpg?oh=fb20 9af77730440d57674fb9e4abb6a4&oe=5861D2A9

[email protected]

9th Oct 2016, 18:03

So there you have some explanations from test pilots AnFi - doubtless you will refute elements that don't fit your present and ever-changing argument.

For a simple pilot like me - in a steep dive when I need to pull out to avoid an 'annoying' visit to the scenery I can use aft cyclic either with or without the addition of extra collective. At this point the coning angle isn't high on my list of priorities.

Now the aft cyclic will change the direction of induced flow (flare effect) and increase rotor thrust but also Nr - because the blades will tend to cone up - I still don't care about the coning angle because the ground is coming up quickly.

Now I add a big handful of collective to further increase my rotor thrust - because I don't want to be 'annoying' to AnFi and hit the ground - does the coning angle increase? Probably but I don't care and can't measure it anyway.

Now I am at max engine limits and pulling hard with both hands - the amount of thrust I need to avoid being 'annoying' can't be achieved because the Nr starts to decay from the high levels of collective pitch and the blades cone up further - I still can't measure the coning and it is still the least important thing on my 'to-do' list - apart from cancelling my booked late lunch.

I hit the ground with both engines at max chat, full collective and the cyclic in my groin - does it matter what the coning angle is or whether the blades reach the stall???

Maybe if I had listened to AnfI and calculated my desired and critical coning angle before I went flying - now where is that graph in the RFM? - I might have been saved.

Perhaps I should have just given myself enough height to pull out and listened to crab:E

NickLappos

9th Oct 2016, 23:13

As I read this thread in more detail I find more to comment on:

1) the coning angle is only loosely tied to the load factor - rpm variance is a big influence, so I would not take any coning angles measured as proportional to the load factor.

2) the character of blade stall is very different than wing stall in an airplane - there is no loss of lift and "falling through" in any case I have experienced. The rotor stall usually occurs where the local lift coefficient is maintained, at least within 10% or so, and the coefficient of moment falls rapidly as the blade center of lift shifts. In other words, the rotor stall characteristic is a big change in blade pitching moment (and the ensuing control system loadings) and not a reduction in rotor thrust or load factor.The basic premise of this thread seems an attempt to prove other wise.

3) Generally speaking, any maneuver that produces significant load factor must have a pronounced pitch rate (disk angular rate, continuous node up rotation) that increases rotor flow into the disk. Generally, the load factor produced is proportional to pitch rate times airspeed. Below about Vy, there is little such load factor available, that is why autorotations that slow down below Vy generally produce poor flares and very little rate of descent reduction - the auto cyclic flare is simply a load factor maneuver used to reduce rate of descent by swapping forward speed for vertical G.

4) Note that the Greek Apache has almost NO pitch rate as it approaches the water. The pilot is attempting a purely collective pitch recovery, because that is all he has left - first because his airspeed is very slow and no cyclic rate will produce mush lift, and secondly because he knows he will put his tail rotor in the water if he rotates much.

krypton_john

10th Oct 2016, 01:35

Ascend Charlie - no coning hinges on the BK117/Bo105 rotor head... was that your point?

212man

10th Oct 2016, 08:53

I am reminded of that expression about taking a knife to a gun fight.....:E

Bell_ringer

10th Oct 2016, 09:49

I am reminded of that expression about taking a knife to a gun fight.....:E
In this instance a butter knife.

AnFI

13th Oct 2016, 18:14

2 Second loop of G Apache

http://ytcropper.com/cropped/xO57ff80c3359db (http://www.pprune.org/"http://ytcropper.com/cropped/xO57ff80c3359db")

and some reasonable calculations

AnFI

13th Oct 2016, 18:25

Nick what you say is very high quality and is mostly correct.

Firstly I’d like to take your second post The 2 graphs are really excellent, and to see energy measured in feet, just goes to show how many things you can actually measure in feet! (it normalizes for aircraft weight and gives the pilot what they want to know ft.lbs/lbs)

In the second graph I presume Ct/Sigma is derived in test flight by actual measurement of g and then factored appropriately? If so the limit load is not approach very often, and if the objective was to get there by ‘pulling hard’ then it might go some way to helping make my point that some of those data points may have been as as result of pitch rates that were higher than optimal and resulted in lower g measured.

In fact it would be quite surprising if once one ‘pulled’ harder than the rate that gave best Ct/Sigma that the Ct/Sigma remained the same and did not ‘fall off’. You would expect pulling harder than optimally to result in a lower load factor, that is fairly intuitive, don’t you think? You don’t seem to agree with that point, surprisingly? (your point 2 in post 3, seems to say there is no dropoff in load factor, so I guess it would be the sort of surprise that might cause ‘misjudgment’)

Evidence for that is that there is not a ‘grouping of data points clustering up at the max Ct/Sigma line, although it could be by chance that not many aggressive attempts were made or that load factors were carefully and incrementally progressed, and no concerted effort in this test was made to exceed pitch rates beyond optimal Ct/Sigma. (so it is not absolutely conclusive, but merely probable) What we would need is a different data set showing load factor against pitch rate, anyone got a graph of that?

The whole thrust of what you say about energy available from engines and Height and speed energy is of course quite correct.

I don’t really want to take the Greek example as a good example but if we do analyse the footage we find that the

Pitch Rate is +21Degrees/second (quite rapid) The Speed is 90kts (not your estimated 20-30kts) The coning angle is 9.5 degrees, indicating that the RRPM was drooped, or the helicopter was ‘heavy’(average over the 2s sample, so probably with higher peek figures)

Your point 1
Is central to what I am saying

Far from being barely relevant coning is a direct measure of the lift the disk is making compared to the Centrifugal force the blades are making (both are proportional to RRPM^2). If the RRPM is drooped then the ultimate load factor will be lower If the RRPM is not drooped then the load factor would be higher BUT THE CONING ANGLE WOULD BE (approximately!) THE SAME
Ie the Ultimate cone angle – regardless of RRPM

Your Point 2

Yes of course at stall we’ll have a moving Center of Pressure and in unboosted controls (as in the H500) one can feel that when pulling hard, it’s a nice piece of feedback (as opposed to the AS350 where the Jack forces are overcome) But that’s not the point, the point is, yes it’s different from an aeroplane, but it is implausible that there is not a dropoff in the g one can pull with increased pitch rates (from say high speed and high power dive). As more of the disk becomes less effective, I do accept this point though, since the rest of the disk still has capacity to do more, so the characteristic is docile rather than sharp, what stops me being able to pull 10g? (just that I run out of energy to keep trying I suppose)
At the very best one could state what you say as ‘when we pull at a higher rate than the ultimate loading then the loading remains approximately constant despite the expectation of more g with higher pitch rates’, would that be fair? In which case one can understand why the pilot might be surprised that an increased pitch rate gives no greater g. I guess that’s the (secondary) point, Pull a greater pitch rate and achieve no increase in g, is a surprise to a pilot expecting more.

If the loading does not increase with increased pitch rate then the coning angle will be the highest you can achieve, if the RRPM is not drooped, and if the RRPM is drooped then the coning angle will still be (approximately!) the same, because both the Max Lift possible and the Centrifugal force will have been reduced together (both dependant on RRPM^2), so my point about Coning is still intact, even if there is only a perceived dropoff in Loading with pitch rates in excess of that which achieves Ultimate Load

Point 3

Yes, true. and in the Greek case, (although it isn’t a great example) the speed is 90kts (against your guess of 30kts), where is Vy in an Apache (guess 80kts?) so it is at a speed where most power is available to make g, and it has a feed of Height Energy (as well as ultimate power).Is 21dgrees/s the max Pitch Rate for an Apache, or could the chap have pulled harder?

Point 4

In the 2 seconds prior to impact the nose is raised by 42degrees, I don’t know if it is fair to say “almost NO pitch rate”? Are you saying that in the last 0.1 seconds he stopped the pitch rate? And in any case the Inflow from his path of motion compared to his disk attitude is the thing. It has little (nothing) to do with absolute attitude (which is flat)

I’m guessing 1.7g is all you can pull in that helicopter at that (probably drooped) RPM and speed. You can see the coning angle is as high as you’ll get it, can’t do more.

So I hope the subtlety about coning angle isn’t lost in there, it’s in bold above, as you say:

“You raise interesting points, but a central one is mostly overblown and should be discussed.”

I have changed (with reservation) one view, that is the Pull More get Less, I think it may be not less, or as you say perhaps 10% less. But for a pilot who is experiencing increased loading for increased pitch rates to hit a pitch rate beyond which perhaps a slight decrease occurs, he would perceive this as a decrease. It marks the end of the Pull Harder Get More Party

Or as Crab brilliantly points out, if he were higher he would not have hit the ground

Lonewolf_50

13th Oct 2016, 20:08

Yes, the indicator is called the Collective, and the critical mark is when it reaches your armpit.
Is that measured in degrees from the floor board, or inches of penetration into the armpit as indicated by bruising? :8 :}
The other indicators are "RPM LOW" and "beeeep beeeeep beeeep". When we integrate those two dependent variables, the typical result is a decrease in the angle between collective and floorboard. :E :ok:

@John D
I have been told that the cutting edge bearingless rotors (Bell Viper/Venom) required that the design and test teams had to keep an eye on flapping during test and development, but do not have personal experience in that program.

[email protected]

13th Oct 2016, 20:13

Nick took 4 small paragraphs to neatly explain a few points in a language pilots can understand.

AnFi, you took almost 20 to waffle, obfuscate and generally confuse any logic there was to the arguments here - as usual.:ugh:

Perhaps, rather than perpetually theorising AnFi, you could take an aircraft and fly specific manoeuvres, taking data points, measuring cyclic position, pitch rates (and maybe even coning angle) so that you have some empirical evidence collected in a scientific manner to support whatever it is you are trying to prove this week.

Start with a level, 45 degree AoB turn at constant speed which will give you a feel for 1.4 G, then take it 60 deg AoB which will give you 2 G - perhaps get someone to video the disc so you have some idea of coning angle.

But don't be surprised at 2 G that your aircraft doesn't have enough power to maintain level flight and constant speed but you will be able to experience how pulling hard will help increase the rotor thrust until the speed washes off below about 60 kts.

At least if I thought you had actually done steep turns in a helicopter I might think you knew what you were talking about.

[email protected]

13th Oct 2016, 20:33

Just one point - In the 2 seconds prior to impact the nose is raised by 42degrees, I don’t know if it is fair to say “almost NO pitch rate”? Are you saying that in the last 0.1 seconds he stopped the pitch rate? And in any case the Inflow from his path of motion compared to his disk attitude is the thing. It has little (nothing) to do with absolute attitude (which is flat) it is his inherent training that if he is going to hit the ground/water hard he needs to do it in a level attitude - that and the fact that continuing to pitch nose up would put the TR into the water first. he realised he was going to crash and tried to make it survivable.

I’m guessing 1.7g is all you can pull in that helicopter at that (probably drooped) RPM and speed. You can see the coning angle is as high as you’ll get it, can’t do more. you have said that you can use coning angle as an indicator of G load and I know an apache can pull more than 1.7 G so why is the coning angle 'as high as you can get'??

dClbydalpha

13th Oct 2016, 20:55

@John D
I have been told that the cutting edge bearingless rotors (Bell Viper/Venom) required that the design and test teams had to keep an eye on flapping during test and development, but do not have personal experience in that program.

It is not unusual for an instrumented head to provide "flap" angle when developing a rotor system. I've not often seen this fed as a direct instrument to the pilot and I've never seen "coning" angle as a specific monitored value. I was interested to see if anyone on here had.

Crab, shame that Apache didn't have any flotation gear fitted.

Lonewolf_50

13th Oct 2016, 21:06

Why feed flapping to the pilot? I must not have been clear.

I am referring to instrumentation to measure flapping to see if it was doing as they wanted/needed to get the rotor to perform to their spec/design goals. I think I presented that badly.

Why feed coning angle to the pilots?
By the time you see a problem with coning angle while you are flying, you have either already screwed the pooch, or you are too busy trying to get the bottom of the auto right that it isn't your primary nor your secondary scan. :cool:

I'll go back to reading this thread, there are a few very good posts here from folks who explain things very well, while others are all noise, fuzzall insight.

JohnDixson

13th Oct 2016, 22:39

No reason for cockpit indication that I can think of, Lonewolf.

Main rotor shaft bending is a function of flapping and that is important during test operations in various parts of the flight envelope.

For articulated rotor heads with elastomeric bearings, it is important to balance the tail incidence and CG range such that cruise conditions will result in steady state flapping numbers below that of the elastomeric bearing endurance limit. ( Endurance limit defined as that number below which the fatigue loaded part has unlimited life ). Also, because of the flapping vs main rotor shaft bending relationship, this factor is relevant to keeping the steady state ( cruise condition ) main rotor shaft bending below its endurance limit.

Of course the performance engineers care only about ( well, maybe that's being a bit rough on them, but not by much ) keeping the fuselage level at cruise, and devil take the shaft bending, shaft life and bearing life, right? So, for the UH-60 with its FBW stabilator, we spent quite a few flights adjudicating this three cornered discussion. Finally made everyone happy.

megan

14th Oct 2016, 02:11

Nick what you say is very high quality and is mostly correct.Heaven forbid, Nick got something wrong. Obviously failed to attend AnFI's school of aeronautics. Tell us where he is wrong ace, or not correct.

dClbydalpha

14th Oct 2016, 07:31

Thanks JohnD, thanks Lone wolf. I was certainly only asking for use in development flight test.

Flap angle is a very interesting parameter to monitor in instrumentation for a few reasons during experimental flight. I was interested to see whether any of the experienced test pilots here had ever had it fed directly to them real time. I've never seen coning angle directly used even in instrumentation, although it obviously can be derived from the other parameters.

[email protected]

14th Oct 2016, 08:29

dCl I've never seen coning angle directly used even in instrumentation, although it obviously can be derived from the other parameters. according to our guru, AnFi, it is directly proportional to G load so all you need is a G meter with an additional scale on it:E

You're right about the Apache but no-one wants to ditch in one because they are a nightmare to get out of and typically roll inverted due to the high C of G.

AnFI

14th Oct 2016, 11:54

Crab
"according to .... AnFi, it is directly proportional to G load"

NO Crab keep up, how can you argue when you don't even understand?

A coning guage would indicate how close you are to the Ultimate Load independantly of RRPM
whereas a G meter would not. It's subtle and not very important, cept if you're designing, where you need to know what Ultimate Coning angle it's worth trying to accommodate. I doubt pilot's really want to be looking at that, although it would be more useful than a G guage.

(1st order, that ultimate coning angle would reduce for DA for example, but would be valid for different weight, g etc)

[email protected]

14th Oct 2016, 17:37

how can you argue when you don't even understand?you seem to manage it most of the time :E

[email protected]

14th Oct 2016, 17:38

So you are a helicopter designer now.............

[email protected]

14th Oct 2016, 17:40

How many engines will it have............

Canute

14th Oct 2016, 18:53

Crab.

You are not really bringing anything useful.

Right or wrong (well above my brain), at least Anfi is trying.

p.s.

I don't agree that the cause of the Apache crash was entering the wingover too low. It is perfectly possible to exit a wingover higher than you enter, particularly in something like an Apache.
I reckon he lowered the lever slightly on the initial entry...

AnFI

14th Oct 2016, 19:56

Thank Canute
Seriously, Crab you say "I know an apache can pull more than 1.7 G so why is the coning angle 'as high as you can get'??"

One last sincere try
Because the RRPM may be drooped (as NL postulates) and/or it is heavy. It's the coning angle that's the 'give away'.
ie regardless of whether the RRPM is drooped or not that coning angle would be the maximum reachable in those circumstances. ie you could not pull harder and get more cone.
So
If light weight at normal RRPM, max G would be (say) 2.5g and the coning angle would be (say) 9.5deg
If heavy weight and low RRPM the G would be LESS (say) 1.7g and the coning angle would still be 9.5deg

That's why cone angle is an interesting (useful) indicator (especially in video)

A helicopter coned to 2deg at 1g would be coned by 6deg at 3g
If 3g were it's max capacity, then 6deg would mark it's limit at other weights and particularly RRPMs and the g would be correspondingly different, but the max coning angle would be the same.

Anyone know what the normal coning angle of an Apache is ? (at that weight!?)

(the greek apache averages at 21deg/sec and 90kts during the pull out phase.
That's just the average, it may have been less (1.2g) in the 1st second and correspondingly more (2.2g) in the 2nd second)

It is assumed that one might acheive an unstalled 20deg of cone for example. That would be wrong, since if the blades are going slowly enough to cone that much then the blades would be going too slowly to make the lift required to cone them that much. So you can't cone a 2deg coned helicopter to 20deg unless it is capable of 10g.

Does THAT make sense? yet?

(Examples of this accodent are, Apache at altitude in Afgan, Dennis in da USofA, Italian Lake disintegration, H500 Belarus. I am sure if any of these folk could have pulled harder they would have)

[email protected]

14th Oct 2016, 21:00

It is assumed that one might acheive an unstalled 20deg of cone for example. That would be wrong, since if the blades are going slowly enough to cone that much then the blades would be going too slowly to make the lift required to cone them that much. So you can't cone a 2deg coned helicopter to 20deg unless it is capable of 10g.

Does THAT make sense? yet? No, not at all to anyone except you

Canute - I don't agree that the cause of the Apache crash was entering the wingover too low. It is perfectly possible to exit a wingover higher than you enter, particularly in something like an Apache what don't you understand about a wingover??? Exiting higher than you enter is a climbing turn. not a wingover!

AnFi - I note with interest that you have stopped engaging with Messrs Lappos, Dixen et al because you know your arguments are weak and not technically accurate - strangely because you are not a test pilot or even vaguely close. You claim to be a flying instructor but refuse to declare your qualifications or experience in the field - I am a flying instructor and regularly declare mine. If you want the credibility your ego clearly demands then why not come clean and let people judge you by your achievements rather than your rhetoric?

Arm out the window

15th Oct 2016, 04:04

Exiting higher than you enter is a climbing turn. not a wingover!

Not always - "What are the four reasons for a wingover .... Bloggs?"
"Speed to height, height to speed, positioning for further manoeuvres, and lookout ... SIR!"

Canute

15th Oct 2016, 05:19

crab

You are on sticky ground mentioning ego after your willy waving about lynx.

Are you suggesting that a wing over entered at 100ft and exited at 101ft is not a wingover?
I still think that the cause of the crash was not entry height as you stated, rather mishandling the wing over.

[email protected]

15th Oct 2016, 08:49

at least Anfi is trying yes he is........very trying

I take your point re the wingover, I misunderstood what you were trying to say - it is very probable that he flew the wingover badly - many people do because they haven't been taught properly - he didn't gain enough height for the recovery and his base height was stupidly low - that was my earlier point about entry gates, you have to ensure you reach a height at the apex to ensure you can complete the wingover safely. He clearly didn't.

As for ego - I merely stated my level of experience and knowledge regarding advanced manoeuvres - something that AnFi has repeatedly refused to do (type of licence, types flown, instructor quals, hours etc etc etc) on this and many other threads.

I am sure he is cleverer than me (especially at maths) but he has no skill in putting forward coherent, logical arguments in terms or language normal people can understand.

The fact he sees fit to question those like Nick shows whose ego is out of control.

[email protected]

15th Oct 2016, 09:00

AnFi - I do understand what you are trying to say - I just think you are wrong.

I don't think there is an 'Ultimate Load' based on coning angle and certainly not anything useful to be gained from trying to measure the coning angle from a video clip.

All you can say with some certainty is that overpitching is taking place because the disc has coned up - to infer that anything else is measurable ie CT/sigma, G load factor etc from that is guesswork and unproven theory.

NickLappos

15th Oct 2016, 15:53

212man

15th Oct 2016, 16:02

In a former life he probably sold snake oil........

AnFI

15th Oct 2016, 21:01

oh well at least I now know that coning angle is not the ratio of Lift to Cf apparently

and that neither lift nor Cf are proportional to RRPM^2 which is why apparently they don't cancel out (!)

furthermore courtesy of PPrune I have learnt that Vortex Ring a a condition that can be inescapable for 4000ft ! apparently

and I have learnt from Crab that if you crash then you are too low, seems obvious now, silly me.

PoF courtesy of PPrune

Phone Wind

15th Oct 2016, 23:41

Normally after reading AnFI's posts I almost lose the will to live :rolleyes:

Finally, however, amazingly, I agree with something he said:

silly me

:}

AnFI

15th Oct 2016, 23:56

haha (65789)

NickLappos

16th Oct 2016, 01:13

AnFI, this drags on. Cf falls with RPM, but lift must be a constant, because the pilot raises the collective and increases the blade angle to compensate, therefor as RPM falls, the coning angle increases immensely. Please, consider that this is just a bit more complex than your simple analysis, and that some of us have a bit of knowledge about this.

AnFI

16th Oct 2016, 01:30

it does drag but there is a serious point in there Ascend approximately understood it in post 33

But I suppose that, as the revs decay, the CL would be increasing with the pilot pulling in pitch to slow the descent and cushion on, which from your equation would increase the coning angle. Apparently the change in angle between low RRPM (pitch high, CL high) and high RRPM (pitch less, angle less) is in fact due solely to CL and not RRPM.

The Cl will hit it's max at the same coning angle regardless of the RRPM.
eg increase RRPM but add weight to keep Cl at max and the coning angle will be the same (as good as damn it)
get it yet?

(last go)

[email protected]

16th Oct 2016, 09:17

(last go) if only it was..........

But you follow a predictable path AnFi - you come up with an off-piste notion, get challenged on it, move the nub of the argument from place to place, confuse everyone, refute the opinions of those who have an esteemed track record in the discipline and finally claim no-one else understands you and flounce off.

And then in a few weeks you will be back to do the same again.

I was accused by Canute of not bringing anything useful on this thread - he hasn't dealt with you before so that is understandable.

However, in the final analysis, your confusing, unproven ideas are actually damaging to those seeking to further their knowledge of helicopters - fortunately we have our resident experts to guide them back on the the true path.

dClbydalpha

16th Oct 2016, 09:59

Sincerely, AnFI your fundamental premise relies on Lift being proportional to RRPM squared and only to RRPM squared so that you can "cancel" it out. I've posted for you the simplest equation, based on BET, for lift on an individual blade. In calculating α you need RRPM, if you choose to dismiss α as a second order effect when calculating CL then that neatly sums up my concern with your approach.

Rotor design is fascinating because each section along the blade radius is experiencing significantly different conditions due to rotor speed, then each blade experiences significantly different conditions each cyclical rotation. The lift of each blade not only contributes collectively to the total lift, but individually has to cyclically influence the orientation of the disc. Rotor design, and therefore the selection of aerodynamic qualities, has to take this and many other things in to account. The concept of a "stalled" disc is interesting, but is so much more complicated than the concept of a stalled wing - it certainly doesn't distill straight to the CLmax of each blade independent of RRPM which your initial premise implies.

212man

16th Oct 2016, 11:55

dClbydalpha

16th Oct 2016, 13:15

You flatter, 212man, sadly no icing on my account. But my hat's off to you, as one of those greats was my tutor and mentor, he will be horrified :}

[email protected]

16th Oct 2016, 13:35

Was it Simon Newman? I attended a short university course at Southampton courtesy of the RAF a good few years ago and he was one of the lecturers - absolutely excellent.:ok:

dClbydalpha

16th Oct 2016, 13:49

It is indeed Crab :)
He has a rare enthusiastic talent for taking a complex concept and making you feel it is simple to understand, before you had to deal with the challenging maths.

212man

16th Oct 2016, 14:11

I have his 'Foundations of Helicopter Flight' book....

dClbydalpha

16th Oct 2016, 14:51

It's a good book, mine sits on my shelf at work with many post-it notes.

It reads like he lectured. Each chapter is an individual topic and starts with an explanation. For many uses the explanation is sufficient without having to dive into the maths. I always recommend it to young graduates who join the team.

ShyTorque

16th Oct 2016, 17:56

Asking two highly experienced, world renowned, chief test pilots of a major helicopter manufacturer if they "get it yet".....priceless.

megan

17th Oct 2016, 03:44

Nick what you say is very high quality and is mostly correct.ShyTorque, the quote I loved most is the above telling Nick he "is mostly correct". Left me speechless, must admit I've not seen the likes of it on Pprune previously, the sheer arrogance and unbridled condescension.

[email protected]

17th Oct 2016, 17:23

the sheer arrogance and unbridled condescension. the hallmarks and calling card of AnFI across many threads...

ralphmalph

18th Oct 2016, 20:23

I would sincerely love to know who AnFI is, and watch him/her work.....

Lonewolf_50

18th Oct 2016, 22:05

I would sincerely love to know who AnFI is, and watch him/her work..... I'd rather rub lemon meringue pie on my head right before going to church.
1105

AnFI

18th Oct 2016, 22:43

CORRECTION

In the calculation for g of the Greek machine I used a=wv (where a is accelerationn (m/s/s), w is angular rate (rad/s) and v is airspeed (m/s)) brilliant eh?

xept I FORGOT the 1g we start with on this planet. !!!!!!

So the calculation for the 'g' for the Greek machine works out at 2.7 'g' not the 1.7g I wrongly declared.

It's pitch rate is 21deg/sec and avg speed 90kts.
So that means 'g' = 1+wv/10 (a works out at 17ms^-2) or 1.7'g' PLUS 1g = 2.7g

Looks like it's fair to say that was limiting thrust after all.

Lonewolf_50

19th Oct 2016, 13:03

AnFI, the Apache was traveling nothing like 90 knots when that unsuccessful pull occurred, as evidenced by the video. As to how much 'g' it was trying to pull at the end ... ]

I don't recall that Apache's have a 'g' meter installed, so (I mistakenly thought that ) the Greek accident board won't be able to confirm or correct your estimate when their report on this bit of flat hatting goes final.

Edit: LRP has corrected my lack of knowledge on that point. Many thanks, sir.

LRP

19th Oct 2016, 14:05

The 64 does have a g-meter. It is also measured by the FCC for weapon inhibits.

AnFI

19th Oct 2016, 17:51

Lonwolf you may well be right but my working was this:
Speed 5.25 lengths in 2 secs is 58x5.2x3600/6080 = 90kts (I measured 10 times and divided by 10 for more accuracy)

Can you see this video ytCropper | Dramatic: Apache helicopter slams nose-down into sea during exercise in Greece (http://ytcropper.com/cropped/xO57ff80c3359db) ?

i agree it doesn't look very fast but if you measure it in aircraft lengths and time then the result is a surprising 90kts according to my calc, try it yourself see what AVERAGE speed YOU get

I can understand someone thinking it doesn't look as fast as it is. Like a Formula1 car on telly doesn't look very fast either

Angle change rate and speed is all you need to work out the 'G' strange but true (a=wv)

just sayin'

dClbydalpha

19th Oct 2016, 17:52

Without getting into details LRP, for obvious reasons, was that a normal g readout, I'm aware of lateral g being used.

Lonewolf, I suspect the inertials will be measuring normal g, but not familiar enough with the type to know whether it would be recorded for retrieval.

AnFI

19th Oct 2016, 18:17

wot do they use lateral g for ? sideslip?

dCl/da are you actually a helicopter designer?

dClbydalpha

19th Oct 2016, 19:11

Most modern inertial systems will tend to have a triaxial accelerometer orthogonally orientated aligned to the aircraft axes. Given our previous discussion on inertial forces when a body is rotating i'm sure someone as on the ball as you can figure out how to use such an output to give a whole raft of useful information. One obvious one would be how well a turn is coordinated, perhaps?

Lonewolf_50

19th Oct 2016, 19:23

AnFI: The Blackhawk, for example, uses accelerometers to measure lateral acceleration. The signal gets fed into the AFCS, and account for control coupling, to include an input mixed with the stabilator when lateral acceleration is sensed (gust alleviation). (From old lecture notes, Fort Rucker origin, UH-60A)

(4) Provide sideslip to pitch coupling to reduce susceptibility to gusts. When the helicopter is out of trim in a slip or skid, pitch excursions are also induced as a result of the canted tail rotor and downwash on the stabilator. Lateral accelerometers sense this out of trim condition and signal the stabilator amplifiers to compensate for the pitch attitude change (called lateral to sideslip to pitch coupling). Nose left (right slip) results in the trailing edge programming down. Nose right produces the opposite stabilator reaction.

As to your speed estimate, OK, I see how you arrived at that. The last time I did a rotor over type maneuver was a few decades ago, so memory may not serve on how much speed is reduced going over the top and then accelerating ... maybe 90's closer than my guess.

Is your 90 kts the resultant vector in direction of travel, or is it already converted to X axis 90 kts and Y axis (some vertical speed) ? I ask because he picked up a bit of vertical velocity (acceleration even) after the turn over the top.

dClbydalpha

19th Oct 2016, 19:44

AnFI, what did you use as points of reference to measure the distance travelled?

LRP

19th Oct 2016, 19:53

AnFI

19th Oct 2016, 20:06

dCl/da

"i'm sure someone as on the ball as you can figure out how to use such an output to give a whole raft of useful information."
cut the sarcasm, it doesn't suit a helicopter designer? more like Crab's style.

"One obvious one would be how well a turn is coordinated, perhaps? " yea that why I said "sideslip?" (obviously !)

Lonewolf thank you for your informative answer, I wonder if that is to help the fact that the swashplate is not orientated it the same plane as the zero-pitch plane?
(dCl/da any view on that? Maybe try googling it?)

[ and glad you see where the 90kts comes from, some others on here found it hard to understand (surprisingly !) ]

dClbydalpha

19th Oct 2016, 20:13

AnFI, my suggestion was turn coordination not sideslip.
You have not answered what you are using as points of reference for distance travelled.

AnFI

19th Oct 2016, 20:19

dCl/da "my suggestion was turn coordination" then you were wrong apparently (smartarse)
(and sideslip was right, "called lateral to sideslip to pitch coupling")

regarding refrences tail nose tail nose tail nose etc you get the idea , or as you would sarcastically say "I would have thought that would be obvious"

and if you're touchy about 'not answering questions' then you didn't answer this "dCl/da are you actually a helicopter designer?"
are you? (i'd be surprised if you are)

dClbydalpha

19th Oct 2016, 20:20

Tail nose with regards to what fixed points of reference?

[email protected]

19th Oct 2016, 20:24

AnFI - what you are too up yourself to see is that you are being asked how you measured the aircraft movement accurately.

If the camera was in a fixed position, you might be able to assess the movement relative to the scenery BUT the camera is panning to keep the aircraft in the middle of the frame.

So, exactly how did you do it?

AnFI

19th Oct 2016, 20:29

Crab I refer you to the answer dcda gave "i'm sure someone as on the ball as you can figure out...." it's not that hard, I don't think you've said anything useful, you're just a troll (xept perhaps "if you hit the ground then you're too low" but then most people knew that already !!)

[email protected]

19th Oct 2016, 20:36

Excellent work AnFi - you are alienating everyone else on the thread but still not providing any information, there is only one troll on this thread..........

Keep up the insults, they are just more insight into your insecurity.

dClbydalpha

19th Oct 2016, 20:42

AnFI nice googling.
How would an accelerometer detect a constant velocity sideslip?
Still no hint at the reference points you used to estimate the distance.

AnFI

19th Oct 2016, 21:00

dcda googling ? no! Lonewolf's post 108 doh
"How would an accelerometer detect a constant velocity sideslip?" there is a limit to how much basic understanding you are asking me to explain to you, the most basic accelerometer in an aircraft measures sideslip (the ball), I don't know what method they actually use in this aircraft system but it doesn't sound very hard with a "triaxial accelerometer orthogonally orientated aligned to the aircraft axes"
No hint at the reference points? I answered you. Try it. Are you serious that you think that is hard? nose tail etc (yes it does)
.... and you still didn't answer MY question !

Crab almost the only replies i ever get from you are insults, you have a damn cheeck accusing me of it, you barely ever say anything that is not an insult. I'm sorry to stoop to your level but you really are a total.... oh I can't be bothered.

dClbydalpha

19th Oct 2016, 21:20

So in measuring "sideslip" what physical quantity is the most basic accelerometer measuring?

Again, you have asserted that you can measure velocity from the video clip, it is central to your calculation, all I am asking is what points of reference did you use to carry out the measurement, you say you did it 10 times so it should be easy to share with us.

AnFI

19th Oct 2016, 22:48

dc ! this thread is about coning angle

it's not about how a ball works !! you could look it up on google, or if it saves you the time it measures the direction element of the acceleration vector in the plane defined by the arc of the little glass tube, thereby indicating what proportion of the g experienced is lateral

re the velocity of the Apache this thead isn't about that either !! and i have answered it 4 times

and given that you don't understand what a balance ball does, can't work out how to measure distance in units of aircraft leangth, can't see that the measurment requiring 10 repetitions to be averaged is the timing (since 2seconds is a very sensitive number in the denominator), have generally been disingenous in not recognising the central point about the ratio of 'lift' to Cf. Your arguement against it is to do with lift not being proportional to Nr^2, whereas to a first degree approximation it is universally considered to be sufficiently relevent (in all published and respected works). It is not surprising that you don't understand, infact I think you maybe do (or should) understand but you just perversly enjoy leading the 'easily led' and rather simple Crab down the garden path. You dismiss my assumptions yet make your own (uniform downwash assumption). How about this (agree if you have any honour) if you take the premis that Lift is proportional to Nr^2 then you agree with what I have said about the limiting coning angle observation ? and better than that the Ultimate CA wrt a/c speed would resemble the graph of Ct/sigma that NL posted it's a novel way to look at coning angle, it brings something constructive to the party. and all you can do is try unsuccessfully to pick little holes in irrelevant aspects in a way that calls into question your undersatnding of those things. play straight and acknowledge the insight into the crude concept of Ultimate Coning Angle.

Furthermore YOU have refused to give YOUR idea of what the dependancy of Lift on Nr is. you didn't answer THAT. and you want to press me on all sorts of stupid (and fairly obvious points which i am mostly patiently responding to). YOU don't answer my points and i have answered most of yours.
CAN YOU? I don't think so. Nr^2 is a pretty reasonable approximation for these purposes.
How about you give the coefficients of the polynomial expansion on Nr? K(a0Nr + a1Nr^2 +a2Nr^3 +a3Nr^4 ...) (or minus power terms if you wish...) etc

I don't beleive that you can't work out how to measure the distance flown from the video LOOP. Please confirm that you can see that loop?

You can't be a helicopter designer, and if you are then I hope the "people on your team" can help you with some of these ideas. Are you a helicopter designer?

[email protected]

20th Oct 2016, 04:46

Ah, now we are in the full RANTING phase of AnFIs argument strategy - this is where no-one is clever enough to understand his ground-breaking concepts, each person's arguments and logic are systematically rubbished and refuted - without any actual facts or proof.

The only thing he has left is Ad Hominem attacks on the people who dare to contradict or question him and a bit more 'mathematical' obfuscation.

Ever heard the phrase 'When you are in a hole and want to get out - stop digging'?

You don't like those with more relevant experience on a topic than you, you don't like those with a proven track record on your favourite subjects who don't agree with you and you definitely don't like people who are cleverer than you....

At least the AnFI fan club gatherings must be easy to organise................

dClbydalpha

20th Oct 2016, 07:33

AnFI - I'm sure a little thread drift would be tolerated. The discussion included a g meter for the Apache, I was interested to find that this was fed to a cockpit instrument. If you recall you were the one who introduced sideslip "wot do they use lateral g for ? sideslip? dCL/da are you actually a helicopter designer?" empahsis was yours.

I have posted the BET equation for lift, that is sufficient for most to grasp the concept of inflow/induced flow and its impact on lift. I don't intend to get in to a deeper discussion on tip or wake models.

No, I can't measure distance from the video as there is nothing clear to refer the distances to.

It can't be done by analysing the frame, as there is no clear indication of the speed at which the frame is moving. If the frame was stationary then the aircraft itself could act as its own reference scale. With the aircraft and the frame moving it can't be derived as there are two unknowns. It can't be done by comparison to the background as the background is clearly a substantial distance away. This introduces, in essence, parallax. Without knowning the difference in distances this cannot be taken out. Further information is required, or gross assumptions made. The only thing that can be seen in plane with the aircraft is its downwash effect on the water. When this is evident there is much too short a clip to analyse without introducing large deviations in the measurement. This is why I asked you what you used as your points of reference.

AnFI

20th Oct 2016, 07:56

my post full of logical arguement facts and reasoning, answering the questions to me, refuting the validity of incorrect points made against me, explanation etc

your post pure insult off topic no resoned content no helping to get to the bottom of the facts deferential to 'respected members'

"you definitely don't like people who are cleverer than you...." that is not a problem I have in your case
I loved NL inputs, clear cool, insightful, enrgy in feet (ft.lbs/ft)
he's just sadly missin'
the point that he's dissin'

dc/da i respect that he has some degree of technical understanding, it is partially your fault in reducing the quality of the debate that he's gone off down a side track about measuring speed, or how the ball works in an aircraft. He is not and has not answered any of my technical questions to him.
megan i respect that he has some technical curiosity and generally a genuine interest in getting to the bottom of the issue, iam am sure he'd like to focus on the point

your only material response on this matter has been a long post about how you should start a pull out high enough to make it.

if you don't have anything to say why don't you try not saying it ?

Do you think it's a fair assumption that the Ultimate Thrust that a rotor disk is capable of is proportional to the Square of the Nr?
(if you do, then you are in good company and you agree with me)

you consistently degrade the quality of other people's discussions, are you literally pointless ?

[email protected]

20th Oct 2016, 09:25

AnFI - answering the questions to me two of us have asked you how you 'measured' the apache speed but you haven't answered - is this refusal an indicator of the weakness of all your arguments ie they are fabrications and bluster?

The 'ball' was introduced by you when dCl mentioned using an accelerometer for turn co-ordination - and you haven't answered his question except to rant about googling balls. Where is the accelerometer in the balance ball tube? When you were corrected you simply called him a (smartarse)
- very logical and concise......

You keep emphasising that Nick doesn't get the point that you are making - what does that tell you about the point - is it you in fact who are pointless because you are making stuff up and won't acknowledge a professional's analysis?

Do please try and find someone who agrees with your assertion that the Ultimate Thrust that a rotor disk is capable of is proportional to the Square of the Nr?

It is such a simplistic statement that it is very unlikely to have any scientific basis because so many other factors are involved - some of which dCL has tried to point out to you.

You have claimed to be a flying instructor on other threads but now you are straying into the realms of being an aircraft designer - a touch of the Walter Mitty's perhaps....

dClbydalpha

20th Oct 2016, 11:34

"Side track" ... loving the pun. Also interesting the subliminal way you drop italic emphasis in.
The point of the diversion was lateral g. Rightly or wrongly people use the term sideslip both in terms of turn coordination and in terms of a constant sideways velocity. The first is indicated by an accelerometer the other is much harder to derive.

The distance measurement is still a question of interest as it is pertinent to your "g" calculation.

[email protected]

20th Oct 2016, 11:55

AnFi is just googling parallax........:E

[email protected]

20th Oct 2016, 15:34

Still waiting AnFi - it can't be difficult to explain since you said it was easy...........

AnFI

20th Oct 2016, 15:50

Crab the answer was given wrt the 'simple accelerometer that is the 'balance ball' "you could look it up on google, or if it saves you the time it measures the direction element of the acceleration vector in the plane defined by the arc of the little glass tube, thereby indicating what proportion of the g experienced is lateral"

The distance can be measured by noting the position of the reference points (which I have answered many times now) wrt the back ground. So note the position of the nose wrt the background, then when the tail reaches that point wrt the background note the new position of the nose and repeat etc . There is no parallax involved because the position of the camera does not change, there is no issue with the pan, the straight lines from the camera to the backgraound are valid (because the camera is in the same position.

Do please try and find someone who agrees with your assertion that theQuote:Ultimate Thrust that a rotor disk is capable of is proportional to the Square of the Nr?I think you'll find Nick agrees, as do most academics. Anyway it is true that it is not exactly correct, but it is correct enough, and in a practical sense would be even more practically correct when the degree of induced flow is less important at speed (although that does bring other minor complications) it is still a valid first order phenomenon that should be interesting to folk interested in PoF.

The extent to which it is not exactly true should be quantified by dc/da so that we can see how relevant it is , I have asked him to do that several times.

It's like claiming that v=d/t is not true because of relativity, but it is true enough for our purposes
(hence the quip about throwing the baby out with the bath water, it is a shame that Nick has not addressed the point, probably because I did not convey it with sufficient clarity ('my bad')

Crab "- very logical and concise......" sarcasm again

[email protected]

20th Oct 2016, 17:43

So if you fly cross-controlled - ie put in left pedal and right cyclic, you are travelling in a straight line but the ball is out to one side - that is steady state not acceleration.

An accelerometer that feeds into an AFCS to produce coordinated turns senses the lateral acceleration that occurs when you roll into a turn without adding pedal (skid or slip) it then feeds a signal into the yaw channel to reduce that skid (acceleration) thus giving a balanced turn. A balance ball won't do that for you which is why they wire in accelerometers and not balance balls.

With your reference points you have two rates of movement to contend with - one of the camera panning with respect to the scenery and one of the aircraft moving with respect to the scenery. Therefore two rates of angular change which are different with respect to the observer (the camera).

What magic maths did you use to eliminate the parallax error?????

Lonewolf_50

20th Oct 2016, 17:55

If I could ask AnFI and Crab to go back a few posts, and edit out (each of you) the bits where you both play the man and not the ball for the last few pages, there would be less FOD in the thread and the points you are each making would be clearer. For example, Crab's last post is mostly content ... but in the past page this back and forth attracted more turbulent flow over the discussion's airfoil than laminar flow.

AnFI: my question on speed calculation, if your use of measurement is valid, will be modified. If we presume the camera to not be moving (which I don't, since I saw the whole video and the camera moved with the aircraft) you are measuring ground speed with the nose to tail technique.

From the earlier video of the Apache accident, my estimate of the breeze from what I saw is that the air (based on the motion of the water) was not moving much, but you might want to account for maybe 5-7 knots. (I'd not guess any higher). That would make for a delta between entry and exit airspeed for the helicopter between 10 and 14 knots if he started into the wind and finished down wind or vice versa. If, on the other hand, there was a gentle cross wind more or less perpendicular to the flight path, that should wash out.

To restate my earlier question regarding your 90 kts estimate:

Is that based on 90kts (in a X axis) with an unknown vertical speed (down) in the Y axis, or, based on your measurement method, is the 90 knots estimate more or less "along the path of flight" and thus the hypotenuse of a triangle with X and Y being horizontal and vertical components?

The triangle we could be constructed by superimposing a grid on the frame and tracing the flight path/hypotenuse and the constructing a right triangle legs from that. I realize that this bit of video does not give us gnat's arse precision, but I am interested as much in method as anything else. The last five seconds of video are not "level flight" but a nose down descent.

The other problem that arises to resolve is the dynamic nature of the event.

The Apache attempts to decelerate (and let's say he started somewhere around 90 knots as a baseline). The pitch isn't oriented in a purely Y or X axis direction: it's a bit of both.

His "nose up" at the end takes him from nose down to a bit above level, if not level, but he was surely in transition, not steady state, in those last two seconds.

Edited upon a second review of the video.

dClbydalpha

20th Oct 2016, 18:29

Imagine an object at distance x from an observer subtends an arc of 10 mrad. That object moves 5 times its length, which creates an arc of 50 mrad. That can be said with certainty.
If an identical object were at distance 10x then it would subtend 1 mrad and the movement 5 mrad. Again the ratio holds true. However, in relation to the observer one appears to have travelled ten times further.
Imagine the observer turns through 1 mrad, that movement encompasses a tenth of the object at x but all of the object at 10x. The observer's frame of reference appears to have travelled the length of the object at distance, but only a tenth of it close to.
Without knowing distances/speeds there are too many unknowns. You don't know how much of the movement is due to the Apache and how much to the panning as a "length" at the background does not equal a "length" at the Apache.

As for the lift on a rotor blade being directly proportional to RRPM squared. I've previously pointed out that if you deliberately choose to ignore the change of CL along the blade due to alpha changes then that sums it up.

[email protected]

20th Oct 2016, 18:32

To throw another factor in - at the apex of the wingover his speed will be at a minimum and as the nose drops on the initial part of the recovery, he will accelerate. At some point as he starts to flare, the speed will start to reduce (or at least stop increasing) once the nose is level with the horizon, the speed will reduce (but not at such a rapid rate as when he hits the water).

Lots of variables in the 'easy' assessment of impact speed......

What you can say for sure is that the speed isn't a constant 90 kts from apex to impact.

AnFI

21st Oct 2016, 00:54

lone
FOD - agreed
"the camera moved with the aircraft" no the camera did not move it panned ie just changed angle when it pans the line from the camera to a place in the woods in the background stays the same in space although a different place in the frame so when the tail has the same background that the nose used to occupy it means the aircraft has moved one aircraft length. I really don't think it's that hard to understand that, is it?
Wind yup there is some room for difference there but again it's not a result that's obvious to most pilots (i really dont want to open a new contention, but it is a frequent error, the effect of wind sheer means that pulling out downwind is energy advantagous, that wont be agreed with and is counerintuitive and true, let's do that in another thread if neccessary)
"is the 90 knots estimate more or less "along the path of flight" and thus the hypotenuse of a triangle with X and Y" yes it's along the path of flight so NOT the hypotenuse but the length of the arc
"but he was surely in transition, not steady state," correct so the AVERAGE speed along the arc in the last 2 seconds was 90kts and the AVERAGE pitch change was 21deg/s (which is actually quite fast in practice, try it, i did and it is quite 'meaty' which is one reason why NL's contention that he had no apreciable pitch rate is incorrect (regardless of how much of a super hero we all think he is, he is just WRONG about that)

dcl/da
I am sorry but your analysis of the speed measuring methodology is seriously flawed. calling into question your ability to understand anything. I can try and help but short of drawing diagrams etc the best i can do is to say imagine a stationary helicopter at some distance between the camera and the background and then the camera pans the tail will still have the same background and so will the nose if on the other hand one found that the tail was occupying the backgraounfd that the nose used to occupy one could infer that the helicopter had moved one helicopter length. the rest is obvious if that is difficult to understand then I am not convinced that you have given serious consideration to the point i have been patiently trying to make, it is a serious point and I urge you to reconsider its merits

As for your contention that lift is NOT proportional to Nr squared i have on many occasions now accepted that it is not exactly true (but it is good enough) I have asked you to quantify HOW wrong it is. To what extent is lift NOT proportional to Nr squared you have evaded answering that at least 4 times now. ANSWER IT if you think you have a point IF YOU CAN ( if you can't I could do it for you are you going to MAKE me do that too, or can YOU do it? ooops not just cut and paste eh !?)

Crab it is a pleasure to have you address the points rather than insulting me, thank you
yes you are quite right the speed is not constant but an average so we can say that at some stage it is MORE than that over the period
i am not sure that the speed reduces (although it might) it is hard to say (it could be done with some difficulty) but the speed may be fed with height, (on reflection you are probably right that the speed at the bottom is not a maximum, ie therefore reduced nonetheless too subtle for me and the crude approximations are 'good enough' too much 'fine point' is dc/da's affliction , he doesn't even realise that there is a baby in this bathwater. please try and think about it again freshly (please try), I think you'll see what I am saying

can ANYONE see the video I posted of the last 2 seconds ?????????

and anyway this thread is NOT ABOUT the GREEK Apache !!!

good night

NL time to 'come out' and support me while you can still take the high ground of independance and rational fairness i dont care who you are, just the logic of what you say has to make sense, i am totally irreverant (thats IRREVERANT, (i'll set em up you kick them in)) only your logic counts with me and so far i cant see that you even understand the point that you might be disagreeing with,.

XV666

21st Oct 2016, 06:29

There will come a time when you good ppruners will realize that AnFI is able to counter any argument with excellently phrased pap, and make it sound scientific.
That Greek Apache was flown into the water with virtually NO pitch rate, and therefore no maneuver-induced load factor. Blade stall is certainly not a factor. A large collective pull reduced the descent and the RPM and it almost worked.

AnFI has "analyzed" it to prove his crackpot theory of how rotors work, and the result is a sausage casing of misapplied theory and mismeasured "facts".

There comes a time when you are wrestling in the mud with a pig and you realize the pig loves it.

And a dozen posts have followed this

(last go)

For one who has steadfastly ignored requests for providing some background of his (her?) experience, the sheer gall of demanding and criticising another Rotorhead for choosing to remain anonymous is another facet of AnFIs attitude whereby only his POV can be correct and any rebuttal will be met with more and more pigswill.

hoss183

21st Oct 2016, 09:19

A wise man once said "Battle ye not with monsters, lest ye become a monster"
I don't know why you guys do. Even without any/much understanding of helicopter physics, i can see this guy contradicts himself, blusters and switches topic. He's just a troll, ignore him.
Whilst discussion is good, and indeed has brought up all kinds of interesting matters, misinformation written in an authoritative way is dangerous, someone might believe it.
My 2c

[email protected]

21st Oct 2016, 12:17

Lonewolf50 - I know I have been guilty of playing the man but only one man whereas AnFi sees fit to play everyone who doesn't agree with him.

My transgressions are due to the frustration of having dealt with him many times before and the fact that he follows pretty much the same MO each time - not adding any value and just confusing what could be quite interesting discussion topics because he thinks he is right about everything.

You can see he hasn't repsonded about balance balll vs accelerometer and he hasn't responded to dCL regarding the angular change wrt the observer - that is because he has made his point (usually with insults) and won't see that he might possibly be wrong - (it's an ego thing).

Similarly, he keeps refuting dCL's accuracy of maths - saying it is irrelevant in the same breath as claiming his own maths are beyond reproach, with any terms he hasn't included being 'good enough' to 'prove' his point.

I know I should just leave him to fester and walk away but there are some very good commentators on this forum whose opinions are highly valued - to let AnFI think he is among that number would be doing them a disservice.

[email protected]

21st Oct 2016, 15:34

AnFI - place yourself in a position where you can see an object in the near or middle distance clearly with an absolutely perfectly aligned reference point in the far distance.

Close one eye and ensure your objects are lined up perfectly. Now rotate your head - simulating the effect of panning a camera - and you will see a parallax error appear.

The camera, it's lens and its focal plane will move at a distance from the pivot (ie the human holding it) and not rotate simply around the focal plane. There is your parallax error (change in position of the observer) and there is the weakness in your speed assessment.

Your video has no such perfect position references and you don't know the focal length of the camera lens (another variable causing distortion) or whether the camera was held at arms length or close to the camera operators eye - therefore you cannot calculate the amount of parallax and your speed assessment is pure guesswork.

Simple enough really;)

Btw, the oaf in the playground worked that out sitting in a café with a large mocha:)

JohnDixson

21st Oct 2016, 16:05

I was reading that all AH-64D/E's have a Maintenance Data Recorder, so, assuming someone was smart enough to take advantage of that, plus the fact that the machine has digital data for those parameters typically needed for recreating a flight profile, the hard data that answers the conjecture in this thread are probably well known by the investigators.

AnFI

21st Oct 2016, 18:35

There is an IQ level below which it is difficult to engage seriously.

People ask me for clarification on a point, when I give it i'm going off on a tagent, when i don't i am criticised for that too.

The point is the point and it isn't parallax its about coning angles.

ONLY dc/da has said it is not correct and I've asked him how much incorrect, a pertinent and important question , on topic, that he has evaded
e)
if he says I am wrong because the induced flow increases with higher induced flow (which is making a set of assumptions that are not neccessarily the case)
then I think it is reasonable for me to ask him 'how wrong?' ( i have said many times the inaccuracy that he points out is true but its throwing the baby out with the bath water, ie it's tru but barely relevant) if he thinks it is relevant he should say so

ie HOW far does the (first order) relationship deviate when you take into account some secondary effects based on a (flawed) model with (flawed) assumptions
HOW WRONG

It like the red herring that crab is pressing, yes he is right there is a parallax error that i ignored, the reason i ignored it is that the measurements i made were not accurate to the extent that the paralax error would be significant. here I am wasting my time and yours coming back on a point of noise from the oaf with the mocha, if we assume that the guy has 2ft long arms and holds the camera at full extention, furthermore lets say the ratio of distance from the camera to the helicopter compared to the helicopter to the background is 1:25, furthermore lets say the camera moves through 30degrees then the distance the camera moves is about 1ft, the error created DOES exaggerate the speed because that helicopter would have travelled a further 1ft times (24/25) during that time. You can see why its a wast of time considering it. You can see why I HAVE to answer it You can see where the waste of time comes from.

SO hoss183 "and switches topic. He's just a troll, ignore him." I am not switching topic they are the topic is the balance of TRT to Cf if people coulsd stay on topic that would be great,
but 183 do you see why I have to answer these dim and off topic points? I am accussed of not answering the question. My post 183 is a waste of everybodies time in having to answer PATIENTLY everyones side issues.

Heli "...steadfastly ignored requests for providing some background of his (her?) experience, the sheer gall of demanding and criticising another Rotorhead for choosing to remain anonymous "
can you see the hyopcritical, contradictory and illogical nature of that absurb statement? I'll spell it out you appear to agree that the desire for a member to stay anonymous is reasonable, yet it becomes a criticism againt me that I chose that. furthermore I am told that I am flying in the face of established expertise of a test pilot and two aircraft designers, well NL is a exTP and megan easily has enough talent to work for Airbus I guess, so I am faced with the implication made to me that dc/da is a designer, I find that astonishing, since his aero BET maths is straight out of an undergraduate degree module on helicopters, and all the designers I have met have allways had a pretty direct grip on the fundamentals, etc etc etc

so you can't win eh
no one will engage on the point they all refute red herrings they won't answer where the disagreement with the point is i you resond to a criticism (eg about parallax, or to what extent a ball is a type of accelerometer) then its a waste of time side track , if I dont respond to a sidetrack I'm avoiding it and repeatedly pressed. can win in a dedate with an idiot. Wasting my time.

The short summary of the relevant points are

1 my assertion
2 dc/da's refutation (which is technically correct)
3 my request to quatify that, to see if anyone agrees that it materially alters the assertion

all the time that is happening we are having to contend with the low IQ knee jerk reaction of mad folk screaming and wailling

and the occasional intevention from the very charming and civilised legend who is John Dixon ;)
a few irrelevant but beautiful technical inputs from NL, combined with some erroneous noise (20-30kts for the Greek helicopter, NOT TRUE)

this does not appear to be a place where a fellow can have a substantive conversation, without the mocha drinking oaf baying and goading etc

THE ONLY QUESTION I WOULD LIKE TO BE ANSWERED BY DC/DA (WHO IS THE ONLY PERSON WHO HAS SAID I AM WRONG)
is
HOW WRONG? Quatify it !!! (there's wrong and wrong)

dClbydalpha

21st Oct 2016, 19:50

Finally a breakthrough AnFI.
I used a concept from lecture 2 rotorcraft.
Then again you quoted an equation from lecture 1 aerodynamics, so it seemed appropriate.

But we make progress because you now acknowledge that BET is a fundamental concept in rotor blade design. BET gives an equation that has both RRPM squared and RRPM terms. So when you say "how wrong are you?" the answer is "completely." When you chose to "cancel rotor speed squared" you appear to have left a variable based on rotor speed. So your theory is not "independent of rotor speed" as you so clearly stated.

When it is that simple why go further? We could of course introduce forward flight and resolve the different demands on advancing and retreating side. We could introduce tip effects, perhaps play with some vortex wake models. I could task some BEM work ... but then why bother? Something academically accepted as fundamental, admitted by yourself, contradicts your assertion.

Remember that you brought in your ability to extract the Apache's speed from the video in sufficient accuracy to support a calculation of g to one decimal place in order to support your theory.

Your explanation as to how you can use the background as a reference when it is moving at an unknown speed through the frame and is at an unknown distance will not pass the red face with anyone who has tried to optically track targets.

LRP

21st Oct 2016, 20:36

dClbydalpha

21st Oct 2016, 20:51

Or is "damaged" beyond recovering data ...

[email protected]

21st Oct 2016, 21:15

So AnFi - you are 3 nil down now - ball vs accelerometer - failed, speed calculation not including parallax error - failed, maths in rotor calculations - failed.

Anything else to add? Bearing in mind you have said your 'final' piece several times now.........

Oh, I forgot - the ball and parallax were just diverting sidelines that you were forced to argue - except that you introduced them to try and show how clever you were.......

[email protected]

21st Oct 2016, 21:19

As for MDR - that might not include cockpit information, it depends on whether it is part of CVFDR set up or a stand alone maintenance data recorder.

dClbydalpha

21st Oct 2016, 22:05

Crab, my understanding is that the MDR is an ED55 / 56 device, if so then it should have 4 audio channels ... of course depends if they were wired up.

Phone Wind

21st Oct 2016, 23:28

This whole thread is starting to resemble something from a Monty Python sketch

Monty Python: The Argument Sketch (http://www.montypython.net/scripts/argument.php)

I'm astounded that AnFI hasn't used the 'S' word yet :ugh:

Still, his red-faced and screaming Herr Hilter impression :\ is jolly amusing to a mere know-nothing Ukrainian tractor driver - but at least I can spell - even if I don't understand all this advanced mathematics about conning angles :confused:

AnFI

22nd Oct 2016, 00:07

Crab "So AnFi - you are 3 nil down now - ball vs accelerometer - failed, speed calculation not including parallax error - failed, maths in rotor calculations - failed."

NO !

ball vs accelerometer ... all i said was that the ball is a crude accelerometer in as far as it shows the direction component of the the g experienced in that plane - TRUE

Speed calculation parallax NO the paralax error is negligable being 50% ish of the length of the arm if at full extension through an assumed 30deg ie about 1ft in 250ft NOT RELEVANT (even if taken to the extreme)

I have allways been saying that there is indeeed a very small (baby out with the bathwater) difference , I have repeatedly asked dc/da to quantify that term which HE HAS REFUSED TO DO. Most experts agree with me that the Nr^2 model is universally accepted to a reasonable degree of accuracy.
How WRONG IS IT ? Answer NOT SIGNIFICANTLY. it is further LESS significant if dealt with at greater speeds and induced flow representing a pullout scenario....
although it is true that that does bring in the additional considerations (that I have already alluded to) of different speeds across the cycle.... and I have suggested that that is well mapped by the Ct/Sigma curve introduced by NL

SO NO !!! it's nil 3 crab

Fohn spelling not my strong point ask Crab (even though I also ironically had to teach him some basic grammer wrt An FI, he was TOTALLY WRONG about that too)

the 'S' word !!! ??? I HAVE used it repeatedly SSSSSSSquared !!!!

HOW WRONG IS IT ? Not significantly! if you want to refute that then QUANTIFY IT !!!!! you have not, probably cannot. You are throwing the baby out with the bathwater and you know it, or PROVE OTHERWISE (you can't) what is the coefficient of the Nr^-1 term normalised to TRT = 1 ?????

You can't or won't because it shows that your clever little nit picking point is essentially irrelevant, and you know it.

NL and most texts in a crude model accept that Nr^2 is valid, it is only in as far as those assumptions are used that I claim that my statement is valid too.nyou can't dispute that and if you do you have no honour

this is not a place where you can have a serious debate with sincere people attempting to explore the validity of a hypothesis, NL has not uderstood the point and has checked out i think I should follow his lead

Ascend Charlie

22nd Oct 2016, 00:39

...and the judge's decision is......reserved.

It is just pure coincidence that the coning angle goes up as the rotor RPM go down. There is no relationship, nothing to see here...everybody please move on and take your angry little arguments with you.

Thank you.

JohnDixson

22nd Oct 2016, 01:03

Here is a link to the MDR. Answers some posted questions:

http://ktferrera.com/images/STSI/FTP/MDR.pdf

Looks like it should have survived the crash.

[email protected]

22nd Oct 2016, 07:46

You can't or won't because it shows that your clever little nit picking point is essentially irrelevant, and you know it. Not really - you constantly claim the moral and intellectual high ground yet when holes in your arguments are exposed, you dismiss them as irrelevant as an excuse for not realising they were there in the first place.

The complete irrelevance is this ultimate coning angle - do the blades cone up as Nr decreases? Yes we all know that.

Is there an ultimate angle beyond which they won't go because of some special mathematical relationship that only you understand? I don't think so.

You are forgetting the practical when dealing with the theoretical - different rotor designs will have different physical limits to their coning - the blades themselves in a teetering head, the physical limits of the hinges in an articulated head, and the limits of deformation in the elastomeric hinges in a semi-rigid head (titanium star bending on a Lynx).

So why postulate that there is an ultimate coning angle? Do designers use it? No. Could or would pilots use it? No. Do engineers consider it? No.

So therefore, on the subject of irrelevance - where does your rather muddled argument and proposed theory stand????

[email protected]

22nd Oct 2016, 08:21

And, just as a reminder - these were the statements from the beginning of the thread Interesting youtube is littered with these 'high speed stall' accidents

The point about coning angle is really interesting. There is effectively a conning angle at which a disk is in effect stalled, regardless of RRPM.

RRPM 'cancels out of the maths' when you look for stall.
and
Quote:
cone is proportional to the ratio of Lift (L) to Centripetal Force (Cf)
both L and Cf are proportional to RRPM^2

take a helicopter in the hover at low load and low RRPM (such that it is very close to stall AoA)
measure the cone
if load is increased the RRPM has to be increased to increase Lift
the AoA remains just short of stall and the coning angle is unchanged.
that coning angle is the coning angle just short of stall

Accurate?????Relevant????

dClbydalpha

22nd Oct 2016, 11:17

AnFI - I don't need to "measure how much". I've posted something that readers of this thread can go check for themselves, think about and come to their own conclusions if they want to.

Ascend Charlie - I agree it is evident to anyone with experience. However to the casual observer, or someone who is ab initio, the simplicity of AnFI's original assertion, accompanied by shiny maths and glittery technical words, may seem quite compelling. This is my concern.

The concept is, from my interpretation of the original posts, as follows

Lift is proportional to rho and v squared
Inertial forcings is proprtional to mass, radius and v squared

given a constant density, mass and radius, and substituting in rotor speed for v then through the mathematics of the equation for coning angle, it is possible to "approximately" cancel out rotor speed leaving a constant relationship between lift and coning angle.

The concept is explored within the context of limiting thrust from the disc, i.e. As rotor speed slows, it is necessary to apply more pitch to the blade to derive the same lift, so you hit CLmax "earlier", beyond that the blade stalls and can no longer sustain the lift required, but the inertial forces are lower due to the reduction in rotor speed. Both the aerodynamic and the inertial forces are proportional to rotor speed squared, and so, according to AnFI's proposition there is approximately an "ultimate coning angle" fixed irrespective of rotor speed.

This concept is used in the original original post to try and explain why by looking at the coning angle it can be determined when a pilot ran out of lift capability during a manouevre and it isn't necessary to concern yourself with rotor speed. But if we turn it on its head things are more worrying

If the CLmax is reached approximately at an "ultimate coning angle" irrespective of rotor speed then the disc is essentially self-limiting. The inertia will balance out the aerodynamics, any attempt to put more pitch on when at that "ultimate coning angle" will result in loss of lift as CLmax is exceeded. What is true for the disc must be true also for the blade, therefore an "ultimate coning angle" must also represent an ultimate self-limiting flap angle ... all independent of rotor speed.

The proposition ultimately leads to a concept that any rotor is self-limiting in flap allowing any amount of blade pitch to be applied at whatever Nr% and the physics just takes care of it as CLmax is always reached at approximately the same angle, close enough that it doesn't matter.

I'm not going to recommend any pilots give it a go on rotor run down that's for sure.

[email protected]

22nd Oct 2016, 15:21

However - the pig is getting tired:E
Speed calculation parallax NO the paralax error is negligable being 50% ish of the length of the arm if at full extension through an assumed 30deg ie about 1ft in 250ft NOT RELEVANT (even if taken to the extreme) but you don't know any of the distances involved - the helicopter is at least many hundreds of feet from the camera and your super accurate reference points (somewhere in the woods) are thousands of feet away.

Given how 'exact' you claimed your speed measurements were and the fact that you ignored (because you hadn't even realised it existed) the parallax - how on earth is anyone expected to trust any of your other 'approximate' assertions and convenient assumptions.

Bell_ringer

22nd Oct 2016, 15:37

I admire the many reasonable attempts to arrive at a sensible conclusion through logic, science and debate. However if logic and common sense always prevailed we wouldn't need nearly as many warning labels as seem to exist, one of which springs to mind - don't feed the animals..

Lonewolf_50

22nd Oct 2016, 18:39

Here is a link to the MDR. Answers some posted questions:

http://ktferrera.com/images/STSI/FTP/MDR.pdf

Looks like it should have survived the crash.
Ah, but did it survive saltwater immersion? Hopefully, yes. Thanks for that tidbit, not sure how "public" the Greek armed forces have to be in their accident reports.

AnFI

22nd Oct 2016, 19:07

Yes dc/da !!!

that is very close to exactly what I am saying, the points of difference are not worth getting into at this stage, you are the first person (other than perhaps AC) who seems to be getting it. If we can keep this constructive then I think we could make a difference.

I still think to have the difference would be interesting
If you have a think about it it would be a curve (i suspect fairly subtle, practically irrelevant) of how UCA changes with Nr, to go with the function that is dUCA/rho (straight line?)
I think it would be fair for YOU to elucidate the difference to UCA that the departure from Nr^2 would give. can you quantify it ?

After all I accepted your challenge to derive some maths and did so within the assumptions that I made, (including addressing points you LATER made wrt twist etc) Anyway don't bother if you don't feel like it, I might do it if I get the chance.

[Crab ONLY - do we really have to waste our time arguing about an irrelevant point that you are disingeuously abusing to try and demean me?
Especially when you are not even right. I was assuming that the camera was stationary as far as is relevant whilst panning. If we allow for your (ridiculous) supposition that it moves laterally as it pans, and we take the MOST EXTREME example of moving the camera whilst panning, we say the holder of the camera has 3 ft arms, we assume the ratio of distance to the helicopter to distance from helicopter to background is about 1:25 seems reasonable we find that it makes no difference within the tolerance of the measurments 1 part per 250 is not relevant {{{EDIT INSERT, that is 1 in 250 of the [I]measured path length, NOT the distance of the helicopter or background which is completely unimportant}}} and I most definately don't claim such an irrelevant degree of accuracy.
and in any case the measurments I never claimed were particularly accurate. The only accurancy I was going for at that stage were to see if the EXPERT OPINION was anywhere near correct , which it WAS NOT (20-30kts, ridiculous !!! ). please don't come back to me on this parallex w.o.t. please try and understand what dc/da has said, he's really close, and I think you would enjoy the point if you 'got it'. try it you'll like it ]

AnFI

22nd Oct 2016, 19:18

Crab "So why postulate that there is an ultimate coning angle? Do designers use it? No. Could or would pilots use it? No. Do engineers consider it? No."
If they don't they should, and i think most do, if there are any that don't don't even go there of course there aren't ANY designers that don't get that idea and its importance

the danger is that helicopters will be designed according to criteria that regulators understand/mandate and they definately don't understand THAT.
Nick can you speak with SKSY designers, what Coning angle to they aim to accommodate?

[email protected]

22nd Oct 2016, 21:33

AnFi - even I, who greatly enjoys pricking your pomposity whenever it rears its ugly head, have tired of your pointless, circular arguments.

You are not a test pilot, nor an aerodynamicist, nor an aircraft designer nor a recognised academic in aviation-related fields yet you seek to lecture, browbeat and harangue on what is a professional pilot's forum.

I do not understand why, nor do I really care, what motivates you to believe you have such an intellectual grasp of factors that even those who design, make, test and produce the things we professional pilots fly do not (according to you) but perhaps you would be better off writing your scientific treatises to Airbus, Boeing, Sikorsky et al and see how far you get.

Perhaps you should look at the impeccable conduct of someone who clearly is a professional in his field, dCL, and compare the quality and humility of his posts to your insulting and fractious ones.

If you want to be taken seriously - and it would appear your ego demands it - then try logical, concise, scientific and relevant arguments. You never know, someone might actually think you have a point.

JohnDixson

22nd Oct 2016, 22:22

Lonewolf, the document for the MDR says that it meets ED 55 and ED 56A. Those are the specs governing airline FDR/CVR equipment crash conditions, so the answer looks like yes,indeed.

LRP

22nd Oct 2016, 23:03

As I said before, barring any unusual events, all of the information needed to analyze, recreate, and animate the accident will be available when the MDR is downloaded.

megan

23rd Oct 2016, 05:45

So why postulate that there is an ultimate coning angle? Do designers use it? No. Could or would pilots use it? No. Do engineers consider it? No.

So therefore, on the subject of irrelevance - where does your rather muddled argument and proposed theory stand???? Exactly. The aerofoil on a Tiger Moth has an MCRIT, but it's of absolutely no concern to anybody - designers, regulator, engineers, pilots.

AnFI

23rd Oct 2016, 08:28

megan
"The aerofoil on a Tiger Moth has an MCRIT, but it's of absolutely no concern to anybody"
Great news! I guess that means you do understand it now? and just can't see its relevance?
You don't have to find it interesting. but I propose step 1 understand the point step 2 work out if there are interesting implications

dcl/da's post understanding it and going on to point out the intersting implications is useful reading in terms of determining whether it is interesting.
i would not have chosen to open so many other fronts in the implications, but then I have to fight hard to get points past my stalker.
but yes
"The proposition ultimately leads to a concept that any rotor is self-limiting in flap allowing any amount of blade pitch to be applied at whatever Nr% and the physics just takes care of it as CLmax is always reached at approximately the same angle, close enough that it doesn't matter. "

that's slightly extreme but it is the nature of the physics, and then the POWER AVAILABLE would in practice most often become the limiting factor (as NLappos rightly says) which is why we rarely look for this as the limiting factor,

normally if you pull harder than UCA the energy is just washed out by the additional drag and so helicopters generally have very docile characteristics in the extremes (with lot's of fatiguing vibration)
but to go back to the begining:

you do see quite a few of these accidents where UCA has been reached and so increased pitch rate does not help the pilot (xept to wash off energy)

megan I guess it would be interesting to some of the pilots who not knowing this have crashed ?

{{{{{ Crab only Crab such a shame that you are allowed to degrade debate here
"even I, who greatly enjoys pricking your pomposity whenever it rears its ugly head, have tired of your pointless, circular arguments."

Crab self appointed judge of pomposity! Circular arguements!!! 90% of which are to address your pointless and stupid goading how much time did you waste on your pointless parallax? no body else has to contend with this level of moronic and insincere deliberate sabotage. You either understand the point or not, whether you think it is interesting or not is opinion you are welcome to. "I, who greatly enjoys pricking your pomposity" the reason that you are upset is that over the last 10yrs you have repeated demonstrated that you have an IQ problem stalking me on any topic just try re-reading your (Crabs) post in this thread very little content mostly insult }}}}}

Crab pops up everywhere I do, just trying to pop away, continuously demonstrating his moronic MO total waste of time

Self loading bear

23rd Oct 2016, 10:00

Crab pops up everywhere I do, just trying to pop away, continuously demonstrating his moronic MO total waste of time

Come on guys,
Take a step back and look what you two have produced on this thread.
I guess 50% of the other rotorheads has stopped long ago following your "discussion". 80% of the ones still following this thread, do this to see how far and how big the pieces of dung fly either way.

It even came to my mind how big the surprise would be if ANFI would turn out to be the alter ego of Crab? (Or the other way around, which is of course the same)

I will duck for incoming dung.

Cheers SLB

[email protected]

23rd Oct 2016, 10:11

the aerofoil on a Tiger Moth has an MCRIT, but it's of absolutely no concern to anybody except that the designer will have selected his chosen aerofoil section based on many criteria - one of which will be Mcrit - so it is of concern to him. The manufacturer will present the finished aircraft for certification which will, of course include the production of figures such as VNe ( definitely related to Mcrit) so both of those parties are interested. In the same breath, the pilot will be presented with limits not to exceed and he will be able to ensure he doesn't reach Mcrit by keeping within the flight envelope.

Contrast that with coning angle - the designer will select his rotor design, size, shape, aerofoil section or sections based on a host of criteria which won't include the ultimate coning angle - the rotor will be tested in a wind tunnel without worrying about coning angle and the manufacturers test pilots will fly a fully instrumented aircraft which won't have a coning angle gauge. The certification process won't include testing or measurement of coning angle and when the pilot flys the aircraft he isn't able to measure coning angle or prevent himself exceeding a mythical coning angle.

megan I guess it would be interesting to some of the pilots who not knowing this have crashed ? how?

BTW have you considered that many modern rotor systems have different aerofoil sections along the blade - with different CLmax for each section which will, of course mean that stall not only occurs at different points on the radius of the blade but also at different AoA depending on the section. How do you factor in high lift element such as a BERP tip?

AnFI

23rd Oct 2016, 12:40

SLB
quite funny, some of the conversation has been good , but engagement with Crab has not been productive, by his own admission he is only trying to goad me, not very helpful.

The productive elements i suggest from this are

1 the idea that coning really doesn't have the capacity to continue smoothly to very large values but rather has an approximate coning angle band (UCA) that does not get exceeded regardless of RRPM

2 Rotor heads need only accommodate a coning range, plus a suitable margin, I am sure designers do consider this, anyone confirm that?

3 Pilots should understand that the load they can pull hit a sort of brick wall beyond which pulling harder will not give greater TRT, it will just wash off energy (agressively possibly, probably with stressfull vibration) and if there is insufficient supply of energy RRPM will reduce but the coning angle won't keep increasing at that point. (despite the wrong intuitive idea that it does). It has undoubtably resulted in many of the accidents where the pilot expected to be able to pull suffieciently, but could not, sometimes due to high DA or heavy weight, or indeed reduced RRPM

4 A photograph of a helicopter showing a 10deg coning angle can tell you it was at limiting load, regardless of whether the RRPM had drooped or not. if 3degrees is normal cone and the helicopter can pull only 3g in that condition (at normal RRPM) then it'll reach it's limit thrust at 9deg cone, regardless of the RRPM (although obviously that will not represent 3g at the lower RRPM, it'll still be essentially the same Cone Angle UCA (this is subject to some small second degree variations for actual circumstances (like the reduction in Ct/sigma with Speed, and to some small extent the subtle variation for different induced flow conditions))

Beyond that it is only interesting as a curiosity of understanding.
I don't hold out that it should be an instrument for the pilot, but it might be interesting.
It might for instance show you a Thrust Reserve (equivalent) for different weights and DAs for instance. FWIW

(Crab I don't think the BERP blade will make significant difference to the essential quality of this observation since the first order variables are still the same. I wish you'd try and understand it since i think you'd like the idea if you could see beyond trying to diss me!)

Lala Steady

23rd Oct 2016, 13:30

Whilst I hesitate to get involved in your love-in with crab, can I just check a few things?

Are you saying that as the blades flap up ie cone, that the stall AoA (CLMax) is reached along the whole blade as a result of that flapping/coning.

If so, are you saying that because the blades stall they will stay at the same coning/flapping angle.

If so, why won't they flap down again as they stall?

I just want to be sure I understand where this argument has been heading - its been rather difficult at times.

puntosaurus

23rd Oct 2016, 14:37

I'm sorry AnFI, but you really have to be called on that lot.

1. No-one had this idea, except possibly you. Most people thought and still think that coning increases with reducing rrpm until the blades stall at which point the disc blows back and cuts off the tail.

2. Designers will presumably confirm this one way or another, but those we've heard from on this thread don't seem to see this as important.

3. Pilots already understand that there is a limit to how much they can pull. Beyond that they call it overpitching, and they are warned they have reached that point by the low rotor warning.

4. At least one person with a great deal of credibility on this thread has said that coning angle without knowing rrpm is unlikely to be a proxy for limiting load factor.

I profoundly disapprove of the bullying you've had on this thread and others but you do rather bring it on yourself.

Lala Steady

23rd Oct 2016, 15:21

The other thing i wanted to clarify was are we talking forward flight or no-wind hover?

If we are talking forward flight, which I presume because of the apache video, then surely the retreating side must stall first which must make for all sorts of complications with flapping/coning.

AnFI

23rd Oct 2016, 15:45

Lala
1 You are effectively correct with a few small clarifications it's not so much that the whole blade anywhere in the disk stalls more that there is a CLmax for the whole disk, it's quite 'soft' because as some parts of the disk reduce the lift they can make other parts still have tha capacity to increase. So it's like the whole disk plateaus

2 and that will happen at more or less a CA that is not dependant on RRPM* so effectively there is a CAmax ie 'UCA'

3 why they don't flap down again there are considerations of whether they do and that could give rise to another vibration mode where that sets up an oscillation but lets leave that complication out of it the 'softness' of the arrival at CLmax refered to above should answer that well

4 sorry it hasn't been easy for me either !

(just saw, crossed post, about speed, that is delt with by the curve that NL kindly posted, Ct/Sigma)

Punt thank you

1 yes coning increases for a helicopter with a constant weight as the RRPM is reduced, but that is not what we are saying here (though some seem to think that's what it is). What we are saying was illustrated by the example of taking a light helicopter and reducing it's RRPM until it is at CLmax (measure THAT coning angle) Increase RRPM and add weight to the helicopter such that the heli remains at CLmax , measure the CA at those increased weights and we see that they are about the same. ie Coning angle is independant of RRPM, but not independant of CL which is why there is increased coning in your example. Your example has a changing CL (this was the point Ascend Charlie perceptively identified, but was lost in the noise) (it's only an example for illustration of the point, there are other qualifiers, not important for this purpose)

2 I think designers are aware of this and maybe we'll hear from one. They do at least need to know what coning they should accommodate (pitch links etc) some people presume that there are upper flapping stops, (maybe some types actually have them?)

3 sure BUT careful here it's not always the RRPM droop at this overpitching, if there is sufficient energy available we don't get an RRPM drop but we still get a CLmax (at about the UCA) this is the surprise to a pilot that thinks maxing out is allways indicated by RRPM droop. You don't have to be drooping RRPM to have reached the limit of how much g you can pull. (try 30degrees pitch up per second at 120kts in an Apache for instance). Therein is an important point.

4 I know he did, but I think that if he had a moment of clarity he might agree that it in fact does, hopefully
( I thinking he is thinking about Coning in the context of reduced RRPM, like you were, whereas really it 'doesn't matter' how you get to that Coning Angle, whether by low RRPM to reach CLmax, or with the normal RRPM and CLmax for any other reason, like pulling g).

we are really talking about circumstances where you do not run out of energy to fund the manoeuver, but the idea still holds

hope that helps? as Crab kindly said "the pig is getting tired"

Lala Steady

23rd Oct 2016, 16:40

AnFi - this soft clmax is where I start to get confused, if the reality is that the retreating blades stall, causing pitch up and roll then how can you have a coning angle that is significant in any way.

Whether the coning is caused by too much collective - ie overpitching - or too much load factor - from flaring with the cyclic it will surely always be the retreating side that stalls first, followed by a flap down, maybe some dynamic stalling and unstalling.

Just how is the coning angle in this case relevant or a limiting factor?

Confused.com.

I believe this Ct/sigma also has to do with rotor solidity (which I think means the number of blades) does that factor into your equations somewhere?

many thanks

puntosaurus

23rd Oct 2016, 16:50

1. You have postulated a thought experiment to illustrate your aerodynamic point. AC found it interesting, maybe even insightful. I thought it was an attempt to describe a real world situation, but I think that has to wait 'till point 3.

2. Nothing to add on that one.

3. Isn't that what G limits are about ? The designers here are more than capable of designing an 'UCA' meter that made all the appropriate allowances so that your approximation wouldn't be required. But they chose to install G limits instead.

4. Nothing to add on that one.

[email protected]

23rd Oct 2016, 17:37

Punto - I think you are starting to see the nonsense being peddled here.

AnFI

23rd Oct 2016, 18:08

Punt
1 the thought experiment helps to see the real world example, the only reason for it is to illustrate the point, lest it be thought of as just standard low rrpm extreme cone

3 well a light helicopter may be able to pull 3 g whereas a heavy helicopter only 2g but in either case the 'UCA' indicator would be valid. I am not suggesting a UCA meter other than it might be interesting.

LaLa , sorry if I confused you but untill the rotor stall is well developed the characteristic is a 'soft' arrival at a CLmax, it becomes dirty and stressful but probably not catastrophic.

Crab, I am trying to help in a serious way, you are not.

Lala Steady

23rd Oct 2016, 18:51

LaLa , sorry if I confused you but untill the rotor stall is well developed the characteristic is a 'soft' arrival at a CLmax, it becomes dirty and stressful but probably not catastrophic. I'm sorry if I am being obtuse but I just don't understand any of that - what is a soft arrival at CLmax supposed to mean versus a dirty and stressful one???

You say the whole disc stalls but that is quite clearly not the case - every book I have read says the stall will start on the retreating side because the AoA there is higher - are you disputing that?

puntosaurus

23rd Oct 2016, 19:04

3. So you are saying that there may be some flight regime for some helicopters, within the limiting G envelope, where UCA limits TRT ?

I would respectfully suggest that if that were the case there would be an UCA meter in that helicopter.

AnFI

23rd Oct 2016, 22:58

Lala
Sorry "what is a soft arrival at CLmax supposed to mean versus a dirty and stressful one???" what happens when you pull hard is that you hit a Ct that no longer increases, when you pull harder you get 'stuck' at about the same Ct, the whole disk doesn't stall in the sense that it actually stalls , nore like part of it does and locally Cl reduces but other regions have capacity to increase still. So the effect is a 'soft stall' compared to an aeroplane where they also try to make stall 'soft' by arranging higher angle of incidence on the inborb part of the wing.

The whole disk arrives at a Max Ct and that doesn't drop off significantly despite pulling harder (or trying to!) it just gets 'dirtier' and absorbs more energy/s it's one of the pretty things about a helicopter. It makes it 'docile' except for when the pilot thought he might be able to extrapolate his experience and pull out before the ground, and finds that embarrassingly and inconveniently he cannot

Anyway in the 'real world' they dont have a CA meter because it is rarely usefull for normal sensible handling, it's only relevant at 'the edges', where a pilot should take this concept into account anyway.

Punto
are you being serious or pulling my leg? (i might be getting oversensitive sorry) if a helicopter can pull 3g when light and Ctmax is reached (UCA is reached also) then the same helicopter when at heavy weight might reach Ctmax at only 2g but it will be about the same UCA.
So UCA would be more relevant than a g-meter for a helicopter.

If YOU want to suggest a CA meter then that's up to you, I'm not suggesting it, but it is a fair suggestion, I just think it would be interesting, and should be a fairly intuitive instrument in a helicopter.

Punto incidentally I'd like to thank you for engaging seriously and concisely i'm not particularly 'hung up' on this observation and am happy to receive sincere challenge to help me improve
as dc/da has rightly and fairly done
i think there is a serious point in there.

but the thread is 'Calling Nick Lappos etc' !!!!!! I don't blame him for checking out it is tedious and if i were he i would also have let this thread roll on alone but has he now understood the point?

puntosaurus

24th Oct 2016, 00:45

Well there is a serious point if CA is limiting, but I don't see any bites from the designers, aerodynamicists, or test pilots.

AnFI

24th Oct 2016, 05:05

"but I don't see any bites from the designers, aerodynamicists, or test pilots.

you're right no original thought? Do you think its right?

"is a serious point if CA is limiting"
the CA doesn't actively stop anything, its just indicative

restatement test
The more coned the blades the closer to Ct max you are regardless of RRPM.
So coning shows how close to Ctmax you are. When you get to Ctmax then you are also at the UCA,
the CA doesn't actively stop anything, merely indicates how close to Ctmax you are.

puntosaurus

24th Oct 2016, 08:21

Well there are two possibilities at this point:

1. There is a collective failure of imaginative thought and/or wilful concealment of the truth by designers, test pilots, and trainers, allowing pilots to enter the pull more get less zone without warnings or;

2. Helicopter designers, test pilots, and trainers have considered these questions and are satisfied that other limits and warnings (low rotor and G) are sufficient to keep us away from that zone.

You've made your point, and the rest of us (including the designers, test pilots and trainers who generously share their time and expertise here) will no doubt make up our minds.

Lala Steady

24th Oct 2016, 08:37

Are you saying that reaching CT/sigma max due to high speed disc loading where there is no RRPM reduction ie pullup or steep turn as high G manoeuver - is the same as overpitching with collective where there is obvious RRPM reduction (not enough engine power) where the might only be a very small increase in G.

Surely the 2 don't correlate - you have achieved the same aim eg producing as much TRT as possible but by 2 different means and with 2 clearly different coning angles.

Brian Abraham

24th Oct 2016, 10:09

I guess it would be interesting to some of the pilots who not knowing this have crashedAnFI, can you point me to an official accident report that has rotor stall as being causative in an accident. Many thanks.

[email protected]

24th Oct 2016, 10:17

And you know there won't be one.......

Bell_ringer

24th Oct 2016, 11:59

And you know there won't be one.......

I suspect that ANFI would pontificate that the investigators haven't "Had a moment or clarity" that would allow them to discover this oversight.

JohnDixson

24th Oct 2016, 12:01

Apologize in advance for the diversion, but am a bit surprised that with all the " commotion " regarding coning, it hasn't been mentioned that the S-70 rotor head, since initial design, has " pre-coning " built into the head, which of course would go to support the premise advanced in this thread, right?

Wrong: the pre-coning was incorporated for a single non-related reason: to reduce the steady state amount of flapping required of the elastomeric bearings, thereby allowing for the goals set for their component replacement time. Same goes for the pre-lag.

212man

24th Oct 2016, 13:34

B212 has pre-coning too....

[email protected]

24th Oct 2016, 16:08

Hmm, could it be that this whole thread was just about AnFi thinking out loud to see what people thought of his idea?

That probably would have been OK if he hadn't insulted and rejected anyone who tried to give him advice or disagreed with his point of view.

AnFI

24th Oct 2016, 16:40

Punto
thank you
"allowing pilots to enter the pull more get less zone without warnings"
you have been warned and the vibration and the rapid increase in energy consumption are thought to be sufficient

Lala
The first happens when you do have sufficient energy available from somewhere and the second occurs when there is not, just pulling more that the engine has doesnt get you to the limit of the blades untill the RRPM has been reduced whereupon the CA will be approximately the same for both cases

Brian Abraham
No official accident reports mention limit of Rotor Thrust being the cause. You are right, Strange when there are so many accidents like that, you'd have thought 'they'd' notice.
I think they just say if you hit the ground then you were too low (as Crab said) indisputably true and reasonably obvious.

JD
and the many of the Bell family because they don't have independant flapping hinges or coning hinges and the difference needs to be taken up by sheer loads on the blades (or elastomerics where present), so chosing an average coning value minimises the stresses. As you know well. (and some tail rotors that don't have coning hinges have pre-cone)

Crab
Still pricking away, eh? "AnFi thinking out loud to see what people thought of his idea?" yes that is partially true, and I am a little surprised at either how few got it or were willing to support it out aloud who did get it.

thank you for your time, especially those that gave it genuinely

ShyTorque

24th Oct 2016, 17:09

and the many of the Bell family because they don't have independant flapping hinges or coning hinges and the difference needs to be taken up by sheer loads on the blades

I think you meant to write "shear loads" but in any case, shouldn't that be "bending loads"?

Lala Steady

24th Oct 2016, 17:27

Lala
The first happens when you do have sufficient energy available from somewhere and the second occurs when there is not, just pulling more that the engine has doesnt get you to the limit of the blades untill the RRPM has been reduced whereupon the CA will be approximately the same for both cases But you have said that the amount of coning is related to the amount of G loading - eg 10 degrees for 2 G and higher for more G.

In the power limited example there will be a high angle of coning for minimal G.

You have asserted that you can assess G loading from the coning angle but that can't be true.

The amount of coning in the high G manoeuver will be far less since the RRPM hasn't decayed and you haven't reached the engine limits.

[email protected]

24th Oct 2016, 17:32

"AnFi thinking out loud to see what people thought of his idea?" yes that is partially true, and I am a little surprised at either how few got it or were willing to support it out aloud who did get it. yet you have tried to present this idea as absolute truth with mathematical proof which is clearly disingenuous - it is your take on what MIGHT happen based on nothing but speculation.

Trouble is, people are starting to see through you.

Can you not see that people might not get it because there is nothing to get?

ShyTorque

24th Oct 2016, 17:53

Surely, G loading on helicopters is mainly a result of flapping rather than coning, in any case?

Lala Steady

24th Oct 2016, 18:30

And you haven't really addressed the issue of the stall characteristics of the rotor - your soft or dirty stalls don't correlate with the experience of retreating blade stall which will be the limiting factor of what the rotor system can produce.

If you try and pull through RBS, you will just aggravate the condition which will manifest itself as a pitching nose up and a roll towards the retreating side. Niether of the conditions you talk about (pulling hard to avoid the ground or overpitching without enough power) show any indications of RBS so the conclusion is that the rotor hasn't stalled and that coning angle is irrelevant.

In the case of the apache video, it is likely that more TRT was available because he still had speed (which you calculated to be circa 90 kts) to convert to additional load factor. There is no indication of RBS (no pitching or rolling) so no evidence that the rotor had stalled.

dClbydalpha

24th Oct 2016, 19:06

The bit I struggle with logically is if there is an ultimate coning angle then the disc is self limiting. It can't pull any more lift. The pilot can pull as much collective as they want. So all that needs to be monitored is Nr as the most lift is at the ultimate cone and the highest Nr. So why worry about the proximity of the ultimate coning angle?

JohnDixson

24th Oct 2016, 19:40

AnFI, sorry to disappoint, but that isn't what I wrote and you took it to be something it isn't:

The pre-cone was put in so that the 1P flapping number ( that's the max during one revolution ) remained under the endurance limit for the bearings. After the stabilator was added during the development period, we used that as a tool to fine tune the stabilator schedule to walk the fine line between the flapping endurance limit and the everpresent need to pay attention to the level flight performance numbers ( and both of those factors as they applied at the extremes of Center of Gravity ).

So, nix on der average coning nonsense.

[email protected]

24th Oct 2016, 19:57

Perhaps AnFi is confusing aerodynamic damping with his UCA.

212man

24th Oct 2016, 20:46

Perhaps AnFi is confusing aerodynamic damping with his UCA.
He's confusing everything with everything....

AnFI

24th Oct 2016, 23:40

Lala you are Crab, no! hitting the Max Thrust the rotor can make is described well by NL, go and read that. Just because a rotor doesn't actually do what you think it does, doesn't change the point and its all been written now I think, there is no further clarification to make I think. Ctmax gives the max thrust the rotor can make. partially stalled blades just cause vibration and the loss is spread around the disk, the power required goes up. If you get catestrophic then I gues you will see blowback etc

dc/da
"The bit I struggle with logically is if there is an ultimate coning angle then the disc is self limiting." Yes effectively

"It can't pull any more lift." Correct

"The pilot can pull as much collective as they want." er NO how does that follow? 'Pulling' can be by pitch up as in pulling out of a dive. You probably can pull as hard as you want (stressful vibration) in terms of pitch rate but it won't do you any good in terms of extra g because you are already at Ctmax, just more energy washed off. You'll be at UCA

"So all that needs to be monitored is Nr as the most lift is at the ultimate cone and the highest Nr." Nr tells you wether you are using more energy from the rotor that you are supplying to it. Coning angle tells you how much of what the rotor can do it is doing.

"So why worry about the proximity of the ultimate coning angle?" dont worry about it just note that regardless of the RRPM when you hit the Ctmax that will happen at approximately the same CA regardless of RRPM, if your RRPM is drooping you are not supplying the rotor with the energy it needs so you have bigger problems (but you'll get to UCA)

Shytalk
"Surely, G loading on helicopters is mainly a result of flapping rather than coning, in any case? "
Not sure you mean that?
G loading is the ratio of Thrust Force to weight(at 1g). So it is about how much Thrust you make compared to your '1g-weight'. You make more thrust with either the Collective Lever OR the Cyclic Stick (by pitching nose up whilst at speed), or both, this increases Angles of Attack.
If you use only the lever you will probably reach the engine limits NOT the Thrust limit of the disk, so the RRPM will droop, before Ultimate Thrust is reached. ONCE the RRPM has drooped then the Max Thrust the rotor can make will be less, if you continue to that Max Thrust (whether high at high RRPM or low at low RRPM) the UCA will be reached, and it'll be about the same regardless of RRPM.
Coning is just a symptom of Thrust (TRT). It indicates how much thrust you are making as a proportion of your Cf (Nr), because CA=TRT/Cf
Maybe you are thinking about feathering rather than flapping?

let's try this , open question
if a Rotor can only make enough thrust to pull 3g when light, and it's 1g coning angle is 3degress, then what coning angle would it have at 3g ??
Everyone's answer
please

Evil Twin

25th Oct 2016, 00:56

Been away for a couple of weeks, can't believe this is still rumbling along. Did nobody get the memo about not feeding the troll?

Ascend Charlie

25th Oct 2016, 02:08

let's try this , open question
if a Rotor can only make enough thrust to pull 3g when light, and it's 1g coning angle is 3degress, then what coning angle would it have at 3g ??
Everyone's answer
please

Who really gives a stuff?

If it takes a yard and a half of calico to weave an elephant's underpants, how long will it take a double-breasted cockroach to tap-dance through a barrel of treacle??

Frying Pan

25th Oct 2016, 02:32

AC, is that an African or European cockroach?

Lala Steady

25th Oct 2016, 06:37

AnFi - I am starting to have some sympathy with crab as it happens - you don't answer the question except in riddles and you immediately insult the intelligence of anyone who questions your theory.

An aircraft can sustain a 2G turn at Vy at just under max power at 100% RRPM - that same aircraft can generate 3G from a 130kt cruise by pulling aft cyclic, also at 100% RRPM and there is no blade stall in either case - therefroe your continued assertion that Ct/sigma max or TRT is dependent on Nr squared is false.

[email protected]

25th Oct 2016, 08:22

Having just re-read the start of this tedious thread, it is quite clear that dCL blew large holes in AnFI's maths about halfway down page 2 - then Nick produced some graphs showing Ct/sigma which AnFI grabbed like a drowning man and tried to use to prove his point - it doesn't.

Thereafter the insults flew (and yes, guilty as charged for part of that) and a circular argument ensued.

The problem is that AnFi has produced (or developed) a theory that is based on NO PROOF whatsoever, no empirical test data, no academic or scientific papers, no accident reports and not even any anecdotal eveidence - only the flaky maths that don't deal with 'insignificant or irrlevant (according to him) factors.

And the invented justification that knowledge of this mythical UCA might prevent pilots from crashing into the ground is about as useful as expecting rainbow-coloured unicorns to give you more lift when you have made a mees of things.

Btw AnFi this isn't pricking your pomposity - we are way past that as you don't feel any sense of humility, shame or embarrasment.

If I had such a random idea as UCA, I would probably PM some of our learned contributors to see what they thought or try contacting aircraft designers for opinion. I certainly wouldn't parade my stream of consciousness on this forum and try to propose it as fact. The Emperor's New Clothes springs to mind...

Brian Abraham

25th Oct 2016, 09:19

No official accident reports mention limit of Rotor Thrust being the cause. You are right, Strange when there are so many accidents like that, you'd have thought 'they'd' notice.
I think they just say if you hit the ground then you were too lowIn that case, post a link to an official report in which you deem rotor stall to be causative, and explain why you so deem.

AnFI

25th Oct 2016, 09:42

If it's not interesting to you then please don't engage. Stalking and insulting has only prolonged the difficulty in explaining the point.
-------------------------------------------------------------------------------------------------------------------------------------------------------------
Let's try this , open question
if a Rotor can only make enough thrust to pull 3g when light
and it's 1g coning angle is 3degress
then what coning angle would it have at 3g ??

Answer 9degress

supplimentary question
if the helicopter weighs twice as much and the Rotor is capable of the same thrust then it can only pull 1.5g

What would the coning angle be?
Answer 9degrees

ie same coning angle 'UCA'

-------------------------------------------------------------------------------------------------------------------------------------------------------------

there are accidents where the rotor has hit it's max thrust (regardless of energy). 2Apaches, 1 H269(possible energy issues too), 1 NH90

(we're talking about the rotor capability, not the engine's ability to supply that energy/s)

-------------------------------------------------------------------------------------------------------------------------------------------------------------

please don't reply if you just want to say how uninteresting it is to you. Crab your noise has degraded the quality of engagement here.
If 'the expert' says it's not true because the Greek helicopter was 20-30kts and it would have to be Vy or above and it turns out that the speed of the Greek helicopter was in fact about 90kts and the expert was wrong then it is only fair to make that point. eh?
Fact shouldn't be determined by popularity contest.

dClbydalpha

25th Oct 2016, 10:00

My point about collective is that according to the original proposition, max lift is at the "ultimate coning angle" and this value is self-limiting. Therefore the pilot is free to request as much collective as they wish, the lift will max out at this limit. The only thing then determining the available thrust at this point is the rotor speed.

I question your source of "fact" about the speed of the Apache in the clip. So far you've only referenced your opinion. If we ever see it, the data recorder should clarify. My opinion is (with substantial error bars) is about 45 knots. Please quote your source of the data so we can peer review. As you say, facts are facts.

[email protected]

25th Oct 2016, 10:20

I am actually interested AnFi but you seem incapable of answering anyone's questions.

Instead you ask your own
Let's try this , open question
if a Rotor can only make enough thrust to pull 3g when light
and it's 1g coning angle is 3degress
then what coning angle would it have at 3g ??

Answer 9degress

supplimentary question
if the helicopter weighs twice as much and the Rotor is capable of the same thrust then it can only pull 1.5g

What would the coning angle be?
Answer 9degrees

ie same coning angle 'UCA'
to which only you appear to know the 'answer' - again based on your shaky theory and maths and absolutely NOTHING else.

[email protected]

25th Oct 2016, 11:16

the difficulty in explaining the point. the difficulty is understanding what point you are trying to make.

Your exam question above - have you actually measured this increase in coning vs G loading or coning angle vs AUM? Have you any actual data to support your answer?

AnFI

25th Oct 2016, 18:04

Crab
"I am actually interested AnFi"
i wish you were, if you are you have a strange way of going about it

ok then
re coning 'maths exam', it's only arithmetic (and it's not exact but it uses accepted approximations, like arctan(a/b) = a/b for small angles)

The relevant arithmetic is
c=a/b
so if a doubles then c doubles
if b halves then c also doubles
if a and b both double then c STAYS THE SAME

which questions do you think I havn't answered?
(mostly many times in many ways, and then I get criticised for when I try to patiently explain answers, you can see the discussion does some good, (African!), and even if just to get people thinking it through)

why don't you see what the point is and take it from there? most people think its about Ca getting bigger when the RRPM goes down, but surprisingly it gets bigger because the Cl has gone up! Clmax being the useful limit and the point where UCA will be acheived regardless of RRPM.

dc/da
"Please quote your source of the data so we can peer review. As you say, facts are facts. "
as I say I measured 5.25 a/c lengths in the arc that took 2seconds, using aircraft length 58ft
That has error bars on it too, I'd say 10%, so it's definately not 45kts, try it on a calculator or try the measurment yourself.
How big are YOUR error bars? 80% would be unreasonable!! and anyway this isn't about the Greek Helicopter !!
and
you say "max lift is at the "ultimate coning angle" and this value is self-limiting. Therefore the pilot is free to request as much collective as they wish,"
Not really, because you are muddying the water (like the Apache did!) with running into the engine limit to deliver the energy required. When Clmax is hit and so UCA reached the energy consumption rate goes up, if you are able to fund this with Potential Energy and Kinetic Energy then you'll just sit at UCA/CLmax, while you can continue to do it. So in a sense it is self limiting to a degree, discovering this limit is when the pilot realises that his increased pitchup rate does not give more TRT for increased energy input, it just funds an increased Cd . If you have less Cl then you have less coning too, so coning will only ever get to where Clmax takes it. (until you've properly stalled the blades, some way down the road, then they'll probably come off)
(you could go to a higher RRPM, and that would increase the rotor's capacity to make more TRT, if used it would make more g but it would still get stop at UCA, as it would for a lower RRPM at a lower g still at UCA)

Do the 'coning exam' please

dClbydalpha

25th Oct 2016, 18:17

So AnFI, just an opinion, not a fact then.

My estimate, for my opinion, is based on the two electricity pylons seen on the hillside. These can be seen on aerial photography of the region. Using a GIS their position can be obtained and therefore the distance between them. I compared the length of the Apache, obtained from public info, to that apparent distance between the pylons. This allowed me to calculate a scale factor (similar triangles). From that I could determine how far the aircraft had traveled in the last second before the impact. Something like 23m.

[email protected]

25th Oct 2016, 18:25

The relevant arithmetic is
c=a/b
so if a doubles then c doubles
if b halves then c also doubles
if a and b both double then c STAYS THE SAME That isn't arithmetic, it is algebra and it doesn't have any numbers (other than the ones you have selected) in to to show how you arrive at your answers.

You still haven't put any actual data on here or into your equations - therefore they are just algebraic manipulation and not mathematical proof.

why don't you see what the point is and take it from there? most people think its about Ca getting bigger when the RRPM goes down, but surprisingly it gets bigger because the Cl has gone up! Clmax being the useful limit and the point where UCA will be acheived regardless of RRPM. this is still JUST your opinion, not backed up with any evidence whatsoever. Whenever you are asked for evidence you just say we don't 'get it' and repeat the same mantra over and over - that doesn't make it true.

The whole point of presenting a theory for public scrutiny is that you provide data to support your position - this has been lacking from the start and shows no sign of being rectified.

[email protected]

25th Oct 2016, 18:31

So in a sense it is self limiting to a degree, discovering this limit is when the pilot realises that his increased pitchup rate does not give more TRT for increased energy input, it just funds an increased Cd When you reread your Prouty you will see that the pitchup does increase Cd but, due to the inflow angle, that increase in drag actually increases Nr - just like the driving force in autorotation. It's called flare effect.

AnFI

25th Oct 2016, 21:37

dc/da
"From that I could determine how far the aircraft had traveled in the last second before the impact."
How? By measuring the distance of the background adjacent to the tail at the begining of the last second to the position of the background adjacent to the tail against the at the end of the last second, measured as a proportion of the distance between the pylons and then applying your scale factor? If so that doesn't work because the background distance is not all at the same distance as the pylons (by a large factor), and if not how? It is very sensitive to the duration of 1 second, how did you acheive that? I did it by looping the video and timing 10runs and dividing by 10. My method is independant of the scale or distance of the backgroud, the distance unit is the Apache length at the distance of the apache. If you measure a boat travelling by seeing how many 'apparent boat lengths' it moved and multiplied by the length of the boat, you'd have found how far the boat moved, if you time it acurately you've got the speed. Did you acount for different scale horizontally to vertically? Do you know what a pitchup rate of 20deg/s feels like?

crab
its a hypothesis, based on the maths (algebra) above, on the basis of which anyone could go and measure this and discover that it is supported in reality. Just because we don't have the data doesn't mean the hypothesis is wrong.
If I hypothesised that hydraulic pressure of 1500psi would exert a force of 1500lbs on a 1in area ram and twice that for a 2in area ram, it would be a reasonable hypothesis.. Do you think what I have said makes sense as a hypothesis. Do you understand what I am trying to say, how would data help?
Do you accept that Ca=Lift/Cf ?
re Prouty
no comment

dClbydalpha

25th Oct 2016, 22:59

"... So note the position of the nose wrt the background, then when the tail reaches that point wrt the background note the new position of the nose and repeat etc . ..."

" ... measuring the distance of the background adjacent to the tail at the begining of the last second to the position of the background adjacent to the tail against the at the end of the last second, measured as a proportion of the distance between the pylons and then applying your scale factor? If so that doesn't work because the background distance is not all at the same distance ... "

I made sure that I established my reference points and then calculated scale. How did you establish that in your method, as you have stated you used the background AnFI?

Brian Abraham

26th Oct 2016, 02:09

Despite being asked it seems AnFI is unable to provide an example of an actual accident where he considers rotor stall to be causative, and why. Have to conclude therefore that it's all bollocks. What other conclusion could one come to?

Lonewolf_50

26th Oct 2016, 02:42

AnFI: two points to ponder from the video.

1. The last second or so before impact is, due to change of pitch/nose attitude, something like a flare (back cyclic) which tends to increase Nr, see crab's point on that.

2. The aircraft was flying in a nose down descent after the turn/course reversal, but it was powered so the Nr would be expected to have been within normal range until the flare at the end that immediately preceded impact. What this means is that once the maneuver began, through its course reversal, and as it descended and regained speed, airspeed was not stable/constant ... and ... at that flare to level from nose down and the last pull to stop descent (too late!) airspeed was not constant during the period during which you are trying to establish 90 knots as a quantity of relevance to the event. I presume that during the descent some collective may have been increased before that last pull, but without in flight data cannot know for certain.

Just out of curiosity, when is the last time you flew a rotor over like the one the Apache attempted?

If you are going to model a flight phase, or a rotor disc phenomenon, trying to do so in the process of a dynamic maneuver seems an odd place to start.

[email protected]

26th Oct 2016, 05:32

If I hypothesised that hydraulic pressure of 1500psi would exert a force of 1500lbs on a 1in area ram and twice that for a 2in area ram, it would be a reasonable hypothesis.. Do you think what I have said makes sense as a hypothesis. Do you understand what I am trying to say, how would data help? The workings and physics of hydraulic systems are extremely well documented and available in many textbooks. When you know the areas of the pistons, the distances they move and the pressures involved then the ARITHMETIC to prove the hypothesis is easy to display.

By contrast, you have just a hypothesis with no data to prove it - that is the prime weakness in your argument but you don't have the courage to admit it.

I think Brian has the conclusion spot on.

dCL has show how the use of real accuracy - measuring a physical distance in the background - uses actual data to put forward an assessment of the apache speed. You used no such data and claimed you are accurate because you looped the video and measured it 10 times - if what you are measuring isn't right then no matter how many times you do it, you will get the wrong answer.

You have previously dismissed this speed assessment as irrelevant to the argument - which it is - but you made such a big deal about how Nick's estimate was so wrong, in an attempt to discredit his criticism of your pet theory, that it has become the pivotal issue on your claims of accuracy and credibility.

You might have noticed a distinct lack of sympathetic or supportive posts in the last few pages - does that not tell you anything????

Bell_ringer

26th Oct 2016, 06:38

This thread is becoming reminiscent of trying to debate with an avid member of Scientology - When someone is unable or unwilling to distinguish between fact and fiction little will be achieved.
El Ron Hubbard would be proud :E

puntosaurus

26th Oct 2016, 11:00

AnFI. Before you post back, just look carefully at these last few posts, particularly Lonewolf and Brian.

Your original observation was that when the disc is producing the maximum lift that it can, then (all other things being held the same) it will be at its most coned. So far so good. The problem is you've then tried to say that the max coning will happen independent of rrpm. Maybe, maybe not, probably not actually, but actually who cares ? As lonewolf says, in a governed machine, the vast majority of this manoeuvre would have been performed at constant rrpm anyway.

So IF (and that's a huge IF without any actual evidence) this manoeuvre managed to power the disc past the linear part of the lift curve (the end of the pull more get more party), the coning angle might indeed have maxed out briefly. Well done, as Ascend Charlie would say, what a clever little secretary. As a tribute to your persistence we could call that the UCA @ 100% or AnFi's limiting angle if you really need an ego boost.

But that dynamic situation (IF it occurred) would have only lasted until the flare ran out of steam, because we all now know that the engines alone cannot power the disc to stall (Thanks Nick !). At that point other phenomena would have kicked in, or to put it colloquially, the pooch was screwed.

AnFI feel free to take credit (from me and AC anyway) for the original observation, but realise you've stretched it way beyond its ability carry useful weight.

Maybe Crab or Lonewolf can finish of with a coda from personal experience which tells us what it feels like when you get close to or maybe even past the linear part of the lift curve. And then we can all sit down with a cup of tea and be friends again.

AnFI

26th Oct 2016, 12:13

Ooh
so many points to come back on with that tirade of criticism
but I'll try, many of them are repetition

BA you might have missed it but I did list some well know incidents by shorthand "there are accidents where the rotor has hit it's max thrust (regardless of energy). 2Apaches, 1 H269(possible energy issues too), 1 NH90" , re a load of bollox, fair enough, different opinion about that, see Punto below.

Lone
point 1 the Nr would have stayed stable due (as Punto rightly points out) due to governing, until the probable excessive pitch pull at the end, which would not have helped, and through the droop the coning would have stayed at UCA (or slightly below, according to NL)
point 2 already covered, yes the speed I gave was the average speed, indicating that it must have been greater at some stage and less at others, also the pitchup rate is also an average with the same consequence, an average pitchup rate of 21deg/s is quite meaty and produces 3g at 90kts, clearly beyond the capability of the rotor even if energy funded adequately.
(ask yourself why, if the rotor had more capacity to produce thrust, the pilot didn't bother using it? Max Pitch rate? I don't think so)
Ref My experience in these manoeuvers, not going there, as previously flogged to death.
Ref Dynamic manoeuver, odd place to start, well the example that led to this discussion was one, so that's where it started, the averages and the nature of averaging shows that those average vales must have been exceeded or held, so it's not such a detriment.

dc/da
You have not answered any of my questions. Yet keep asking. I take it that by implication you agree with my summary of your method, I think your method is invalid, for the reasons stated. My method does not fall foul of that error because in each instance the actual distance on the background that the helicopter travels is eliminated. You could compare it to the stars, and it wouldn't matter how far away the stars were. An Airliner at an unknow height, covers an arbitrary distance against the stars behind, but if you measure tail to nose how many aircraft lenghts it covers in a time you can derive the speed, the actual distance bewteen stars would be irrelevant and introduce error. There is a 'ruler in the sky' in units of aircraft length, which is conveniently placed at the arbitrary distance of the aircraft. If the aircraft and the ruler are at the same distance you don't have to 'scale' the ruler. It might help if you could answer some of my questions. Do you agree with the 'coning exam' arithmetic? Have you seen the 2sec loop video I posted?

Crab
1 if you think it is bollox then you don't think it is interesting as you have also said. If you think it is bollox the we know where you are with it, opinion noted, no need for you to add more.
2 wrt your opinion that dc/da's method is right I refer you to my answer to him, he is wrong and you agree with him, so you are wrong again too. The thing that is sensitive to being measured 10 times is the timing, since it is such a sensitve number in the denominator. dc/da has not answered THAT either !!!! Have you seen the loop video I posted? Do you agree with the arithmetic of coning, in the coning exam? Yes that coning arithmetic is well covered and well known.
You have not answered any of the questions I have asked you either. "Do you understand what I am trying to say, how would data help? Do you accept that Ca=Lift/Cf ?"
"dismissed this speed assessment as irrelevant to the argument - which it is - but you made such a big deal about how Nick's estimate was so wrong, in an attempt to discredit his criticism of your pet theory," not dissmissed, just said that the Greek helicopter is incidental to the point. No no big deal that NL was wrong at his speed guess (actually quite the opposite, I said it is an easy error to make, understandable, look at an F1 car etc, no interest to discredit him, much of what he said was very good and I said so, but not addressing the point, which he did not address. So I have not disagreed with him. If he was wrong about the speed then it's ok to say so I think. anyone can be wrong. He did say for me to be right the speed would have to be in excess of Vy, now we know that is the case, just) (what is the Vy of an Apache?))

BellR
yes sorry about that. but to address a specific issue here one has to wade through alot of treacle.

Punto
ref Looked back esp BA and Lone. Yes done and addressed above.
Yes everything else you say is as close to correct as it's not worth argueing with (with the possible exception of "Maybe, maybe not, probably not actually, but actually who cares ?"). you seem to be one of the few that understand the point, as to whether it's interesting or not is a matter of personal opinion.
"personal experience which tells us what it feels like when you get close to or maybe even past the linear part of the lift curve." I can tell you that, vibration goes up, TRT is not increased, your path no longer continues to alter at the rate your brain would extrapolate to, pulling the lever up more doesn't help, you are grateful that you had more height margin than you thought was neccessary (credit to Crab for that observation) "(the end of the pull more get more party)"
'Cup of tea' anyone?

puntosaurus

26th Oct 2016, 12:34

Wow ! You really could start a fight in an empty bar ! I give up on the peacemaking front. Have at it boys. I'll do the tea on my own.

[email protected]

26th Oct 2016, 12:41

That is just reiteration of the same stuff AnFi - no new arguments and definitely no NEW DATA.

You haven't linked to any actual accidents, just listed some you think might be relevant.

Do you know how they measure the movement of stars??? They use accurate reference points not guesswork.

You only acknowledged the 'average speed' when it was pointed out, your original assertion was that 90 kts was the impact speed. Very magnanimous of you to say Nick made an 'easy' mistake but you haven't proved your accuracy at all.

an average pitchup rate of 21deg/s is quite meaty and produces 3g at 90kts, clearly beyond the capability of the rotor even if energy funded adequately. how have you 'measured' this? More guesswork presented as facts:ugh:

As I pointed out - your 'coning exam' is simply an algebraic expression that you have inserted convenient numbers into - it is not proof in any way shape or form.

Ref My experience in these manoeuvers, not going there, as previously flogged to death. what you mean to write is NONE.

NO evidence, NO proof (mathematical or otherwise) and NO credibility AnFi.

[email protected]

26th Oct 2016, 13:12

Punto - as you have seen, trying to get clear, unambiguous answers from AnFI, especially without having your intelligence and understanding questioned, is like trying to nail jelly to the wall.

puntosaurus

26th Oct 2016, 14:28

The heart of the issue, despite many diversions, is whether reaching the non-linear part of the lift curve and pilots lack of understanding of that phenomenon is a contributory factor to these incidents.

Nick, the eponymous hero of the thread, has I suspect made up his mind, but is wisely keeping his own counsel. He knows that the protagonists enjoy mud wrestling more than he does. Besides which, if he wants a technical discussion on this issue, he can probably reach in to his rolodex and find about a hundred people better qualified than us to engage with.

Given that Nick has actually participated in engineering design and actual manoeuvrability flight tests that underpin the certification of this actual aircraft, I'm inclined to defer to him. Nevertheless, the data he posts do indicate that it is theoretically possible that the non-linear part of the curve could be reached transiently.

So we must acknowledge that possibility until such time as the Gods choose to descend again and provide us with further runes to read.

AnFI

26th Oct 2016, 16:54

Punto
Sorry about that
I thought I had done what you asked in respect of Lone and BA
then agreed with what you said, agreed that you had got the point, and just suggested that it is interesting
Then answered your question (not to me it's true) about what happens.
Then agreed it's tea time !

as well as patiently answering all kinds of diverse points about how to measure the speed etc etc etc etc
what NL had to say was fantastic and i've only complemented him on that. that he's so wrong about the speed is no adverse reflection on him, it's hard to guess a speed from a video clip

and no one has answered any of my questions once.

did ANYONE actually see the 2 second loop video I posted a link to?

Crab you are just antagonising again. answer any of my questions to you, like I have the decency to do for you.
"how have you 'measured' this? More guesswork presented as factshttp://www.pprune.org/forums/images/smilies2/eusa_wall.gif" NO! and I included my measurement and working from that about 100 posts ago. 21deg/s
I don't mean to say that you are stupid but anyone else can make up their mind by following your question about speed measurement and then your response about measuring the speed of stars. I challenge anyone to do that and not reach the conclusion I have. (Nailing a jelly fish to the wall ! ANSWER MY QUESTIONS TO YOU !)
Challenge !

[email protected]

27th Oct 2016, 00:38

did ANYONE actually see the 2 second loop video I posted a link to? yes, and it took me a while to realise why you were so insistent - you have adjusted the video to make the sample ie just 2 secs as small as possible so it is more difficult to see how you have fudged it.

In order to make the distance travelled over time match your assessment of speed, you have increased the speed of your clip relative to the original - not by a lot, but by just enough to make anything but a detailed analysis look OK.

It is much like the crude way that filmmakers used to speed up actions shots in movies by running the film through faster.

It is only evident when you watch the original alongside your doctored clip.

Brian Abraham

27th Oct 2016, 01:57

BA you might have missed it but I did list some well know incidents by shorthand "there are accidents where the rotor has hit it's max thrust (regardless of energy). 2Apaches, 1 H269(possible energy issues too), 1 NH90"Well known accidents? I have absolutely no idea to which accidents you may be referring. I'm afraid you've not attempted to answer the question at all. Listing a number of aircraft types doesn't constitute an official accident report, nor do you explain why the particular report is incorrect in its summation as to cause. Flip me off if you wish, I couldn't care less, but should you do so it only confirms you're on a hiding to nothing.

Evil Twin

27th Oct 2016, 02:47

AnFI mate, when you're in a hole, stop digging. This entire thread, though entertaining, is increasingly farcical.

jellycopter

27th Oct 2016, 03:30

I don't know conclusively whether Anfi is right or wrong. I guess by the number of nay sayers that are vastly more qualified than I, that the collective knowledge would suggest the latter.

However, I do have to hold a degree of respect for anyone that thinks a little bit outside the box and is prepared to voice it openly. One of the things I have discovered in my 30+ years as a helicopter pilot is that many of the 'facts' I was taught as a junior pilot were utterly wrong. (See Nick Lappos' Urban Myths thread). I also happen to have a great deal of empathy with Anfi's 'single engine' logic that he thrusts upon this forum on a regular basis.

I applaud you Anfi for being prepared to challenge the accepted norms. Whether you are right or wrong is irrelevant to me really, as I don't plan on exploring Ultimate Coning Angle any time soon. However, I personally find the 'meat' of this discussion (not the purile mud slinging) very interesting.

JJ

Lala Steady

27th Oct 2016, 06:07

Jelly - I don't think anyone here has a problem with 'blue-sky' thinking - simply with the combative, supercilious and generally unpleasant way that thinking is presented.

AnFI

27th Oct 2016, 06:33

BA
The very last thing I intend is to 'flip you off' (or anything else that sounds like that !
It is difficult to respond suffiviently in detail to such a barrage of counter arguement.
I had hoped that the shorthand reference was enough to identify them since they are quite well known ones, and I don't really want to be led down the path of identifying and debating any more specific accidents. Discussing the Greek case has been awkward enough, and is off topic. (one in a lake, dramatic, one in Afgan, one close to home and the Greek one. That is as far as I want to go with that for allsorts of reasons, AND it is off topic). So I am not meaning to be disrespectful, but I am tending to rush these long and boring answers. (BA please read Crabs noise and tell me whether you think he is helping

Especially true when one oaf continuously delights on deliberately wrecking the debate
(read Crabs last nonsense, someone get this idiot off my back please, are YOU happy to align YOURSELF with his points?)

Jelly
Thanks
It is amazingly hard to counter all these points. I suspect that many who agree have stayed silent enjoying the spectacle of a man on the ropes.
Do you find Crabs input helpful?
Now he says I speeded up the clip ! I guess he is suggesting it looks as fast as I have spent 10000 word defending.

NL
has indeed wisely stayed out of it, and I suspect that his popularity with Crab is more valueable to him than wading throuh this treacle here and saying, "I see the point, nobody ever looked at it like that before, I do/don't find it very interesting". Besides I can't think of anything more unpleasant to engage with that the general level of vitriolic ludite drivel, especially if you don't feel great. Its also awkward to come back and say "now I've measured the speed I see I was wrong about that"
It takes a BIG MAN which surely you are?
If you're going to be wrong I don't mind saying so, I am not defferential, nothing personal great respect for you etc etc etc Maybe flick through the rolladex and throw some more crumbs to the mortals.

[email protected]

27th Oct 2016, 07:28

AnFi - surely you must applaud my method:

I have put forward a hypothesis that is technically feasible, contextually plausible and relevant to the thread. And, I have presented it as fact with no data, proof or evidence of any kind.

I can now sit back and refute any criticism of my hypothesis because I have laid the burden of proof on the critic (you).

I could have included some basic algebra to do with speed. distance and time equations but I could save that to support my hypothesis in a 'mathematical' fashion later.

I am surely your most successful student - I have learned much from you oh master - just none of it about helicopters.

megan

27th Oct 2016, 09:04

Had an email from a reader who provided the following, they assure me the info comes from a reputable source.Load factor is caused by pitch attitude rate and airspeed, so the penalty of waiting a few seconds before pulling out of a dive is that the pitch rate to get the nose level causes a lot more g. The load factor of the rotor reaches a max at about 80% of Vne, if your speed builds too fast you will stall the rotor trying to raise the nose.

When you ask too much from the rotor, it mushes through, and you get an attitude change but your flight path is still unchanged. This means you'll still hit the ground. Stall will limit the load factor, and may cause high vibes and sloppy roll control.It would seem that rotor stall as proposed by AnFI is a fact of life, just not something taught to we stick jockeys. Must admit my ignorance. Bit like the meaning of Va in the FW world until the NY A300 crash I assume.

AnFI

27th Oct 2016, 09:28

Megan
Yes that's right. and you are the thread starter !
Deep respect for you. It takes a straight bat to command respect. You obviously have some moral fibre.
I would have hoped that the logic was sufficient to convey it, without having to resort to 'credentials', the concept of which has been unfasionable since the First World War.
Perhaps you can see why I was so persistent.

Jelly / AC / Punt thanks for the support, it hasn't been easy

Please read Crabs post above, let me know if you want to be part of that.
Total insincere sabotage and harrassment of this thread from the outset.
Anyone siding with him please identify yourself.

Thank you and (Punt) 'Time for Tea' :ok:

chopjock

27th Oct 2016, 10:09

AnFI,
Keep it coming old boy, I agree with almost everything you say on here! :ok:
Especially the single v twin rants!

puntosaurus

27th Oct 2016, 11:34

AnFI i'm delighted you've found further support for your proposition, but I'd like to point out that in the original thread that spawned this one you presented it not as stated above, but as the aerodynamic musings leading to an UCA that megan opened this thread with.

Your aerodynamic musings are still highly contestable but fortunately irrelevant to the core issue.

If you'd said on the original thread, "I think rotor stall is a factor in these accidents", or better still "does anyone think rotor stall is a factor in these accidents" I think you'd have had a higher quality debate.

I suspect being liked isn't high up your priority list, but if you want to avoid the bullying and elicit quality input from highly qualified and experienced contributors on this forum (including crab) then you might want to think some more about that.

dClbydalpha

27th Oct 2016, 11:48

Punto, thanks for that I began to wonder whether I was missing the point.

The discussion is not whether you can stall a rotor. There has been lots of discussion here to do with energy management and what happens as blades begin to be worked too hard. All good stuff.

The original proposition is that this always happens at a singular known "ultimate" coning angle. That knowledge of this value would prevent such accidents. This is what I sense most people are uncomfortable with.

[email protected]

27th Oct 2016, 11:54

Is it really 'bullying' to question the proponent of a hypothesis about accuracy, validity of data, logic, mathematical process and relevance of information to that hypothesis?

In scientific circles I believe it is called peer review:ok:

If we can now have a reasoned debate about rotor stall then I am all for it but leave the UCA out there with the scientologists.

AnFi - one more attempt to ellicit facts about your apache speed assessment - if you didn't use accurate reference points, what did you use to ensure accurate timing since that is crucial?

ShyTorque

27th Oct 2016, 12:21

I think you meant to write "shear loads" but in any case, shouldn't that be "bending loads"?
AnFI, I'd be grateful if you could please answer my earlier question.

BTW, my user name is ShyTorque, not Shytalk.

Lonewolf_50

27th Oct 2016, 12:57

Had an email from a reader who provided the following, they assure me the info comes from a reputable source.It would seem that rotor stall as proposed by AnFI is a fact of life, just not something taught to we stick jockeys. Must admit my ignorance. Bit like the meaning of Va in the FW world until the NY A300 crash I assume.
Megan, there is no evidence that the rotors stalled in that crash. Do you know something that we don't? What is obvious is that the rotor head reached a "power required exceeds power available" in terms of thrust vector up not being able to overcome vertical speed down, hence impact with the water. I will suggest to you that the rotor head (Apache does not have a low inertia rotor head, and the engines were still trying to keep it spinning) did not slow down fast enough to stall until after impact (at which point it doesn't matter). Yes, I understand what accelerated stalls are, in both fixed and rotary wing. G loading considered (and we don't know what the G load was) there is still momentum and inertia in play in the brief time between recognition that the maneuver was balled up and the attempt to use all (we presume) power available to stop the rate of descent short of the water.

So while your generic point is that rotors can stall (aerodynamics makes no special allowances for helicopters in that regard) asserting that the rotors did stall is assuming some facts not in evidence. The coning angle is an effect, not a cause, of the rotors changing pitch and responding to loads on the blades/rotating wings. It is a reaction to forces on the blades.

Megan, why is the airspeed around the time of impact of interest? Power margin and airfoil efficiency. Depending on the day, weight, and such from the charts in the back of the flight manual, there's a sweet spot airspeed. Based on similarity to Blackhawk in terms of weight and propulsion, I'll estimate somewhere around 70-75 knots for an Apache on a standard day. That is the knee in the curve analogous to L/D max for a fixed wing. We used to refer to that airspeed as "max conserve" because you had the most additional power available at that airspeed, or more to the point, needed the least amount of power to fly level (and thus burned the least amount of gas).

If the Apache was below that airspeed as the final pitch and pull was attempted, it was already in energy debt. If it was above, (say 90, or 80 knots airspeed (AnFI's estimated are based on ground speed due to measurement method) then there was a little energy "in the bank" for the flare but there's a catch, which is where your concern about rotor loading comes in. Trading that extra energy in the flare/pull gets you some benefit but you are likely passing through that sweet spot airspeed and into the beginning of needing more energy.

At this point in the high demand maneuver, you've got a variety of issues working against you. In overcoming your energy debt with that flare, you've increased drag as you've gotten a bit of lift (the blades bite harder) and your AoA goes up ... but your airspeed is still decreasing which isn't helping, and is indeed hurting.

Whatever angle the blades cone to is a reaction to that bundle of forces and effects on the disc/blades.

Put another way, by the time you are wondering whether or not you have reached a critical coning angle you are already far enough behind the aircraft that you are at the mercy of inertia, and altitude available to trade for more energy ... which thanks to the entry altitude of this maneuver the pilot ran out of in a hurry.

puntosaurus

27th Oct 2016, 14:02

dCl

I'm sorry, it must seem to you that this thread got hijacked, and in a sense it was because I thought the best way to bring it to a satisfactory conclusion was to restate the debate from the original thread.

You did a fine job of pointing out the missing bits in the original analysis, and AnFI has actually credited you with that, although you need a beady little eye to spot that in his posts. He now talks about a band of UCAs rather than a single UCA, so I think the original argument is dying if not dead.

Therefore, the absolutely fair criticism of AnFI is that his concept of UCA independent of Rrpm was absolutely technically flawed, maybe significantly (see challenge below). But a far more serious criticism is that it was actually completely irrelevant in a situation where Rrpm is largely constant. This is a frequent observation with AnFI posts as Crab points out, the pursuit of some hobbyhorse (eg. UCA, twin reliability) outside any useful context, even though there may be a nugget of truth riding on the hobbyhorse.

Challenge: If AnFi wants to go into battle about UCAs again, then there are two possibilities:

1. either his technical skills are sufficient to model the additional inflow term and throw up some numbers for max coning angles at different Rrpms, or;

2. they are not, in which case he needs to be a bit more gracious to the experts on this forum who might have those skills and ask them a favour without effectively extracting an entire lecture course in aerodynamics for free.

PS to Jellycopter's point, I don't actually think AnFI should be criticised for hypothesising about an UCA and having the courage to present his ideas to this forum, but rather for his rather grudging (it's there, but you've missed out another Rrpm term which still cancels) and dismissive (it's there, but its effect on the UCA is negligible) acknowledgement of the inflow point. If I were an expert I would say stuff you, go stew in your own juice then if that's the reward I get for trying to help. AnFi I think reads that differently, as some kind of paranoid conspiracy of experts to trying to protect their ivory tower from a plucky amateur.

AnFI

27th Oct 2016, 20:23

Sorry ShyTorque, no offence intended with abbreviation of your or anyone’s name.
True, I didn’t come back to you on the difference between Shear Forces (spelt correctly now) and Bending Moments, because one is a product of the other, they are closely related the bending moments create Shear Forces, and the Shear Forces break things. I just really don’t want to get into that, I’m sure you’ll understand.
Maybe a different thread? Or study the relationship yourself?

Lone
That’s a long post and it’s 98% correct, but somethings are a little nuanced in a way I’d like to come back on.

Para 1
Stall, I think we are talking about stall in the sense that for the whole disk the Coefficient of thrust has reach a maximum value, i.e. CtMax, I think most people are happy with that.
“we don’t know what the G load was” true, but my calculation puts it at 2.7g (21deg/s at 90kts is 2.7g), which is probably achieved at about CtMax, and if he had more capacity to pull g then he maybe would have used it. I accept that he (also) probably ran out of energy from the engine (engine power limit) and consumed some RRPM energy as well as Height Energy. According to NL’s excellent graph (from a different type) he had quite a lot of energy available, if (assuming) my speed calculation is close, accounting for the height he had, and the amount of time in which he spent it. 5sec of Max power is equivalent to at least 200ft (Energy), so he spent that height in 2 seconds equivalent to a lot of engine power (maybe 2.5 times), PLUS more than full engine power. One can infer that he was most probably at CtMax.

Para 2
Stall, call it what you like, but I mean running at CtMax. YES the CA is "an effect not a cause”, but it is related by relationship of TRT to Cf (is anyone disputing THAT?) CA=TRT/Cf. So far from being irrelevant, it tells us that for a given RRPM that the TRT is some multiple of Cf (which is a product of Nr^2). The key is even if the blades are going slowly the TRT that can be made is also less, as is the Cf (by the same proportion), so the CA is the same, drooped or not.*

Para 3
Yes also agree, but in this case the helicopter WAS doing about Vy (which as everybody here knows is the place where best use of energy is made to make TRT), so the amount of surplus Thrust that could be made was near it greatest.

Para 4
“AnFI's estimated are based on ground speed” good point, since it is airspeed that is important, and I did not assess the wind speed, so there is room for error there.
“If the Apache was below that airspeed as the final pitch and pull was attempted, it was already in energy debt.“ YES true “IF", but I say it was not below that airspeed (which could of course be wrong, but it is my working figure from my measurement, and if we work with THAT figure then it was not, according to you too). It doesn’t seem like the Airspeed reduced much, and was probably funded by height, but even if it was MY 90kts is an AVERAGE, so regardless of whether it burned KE or not there was quite a lot of energy around (AND he would have been pretty close to the flat bottomed curve of the "sweet spot”) (most surplus from the engine, and height (unmeasured) equivalent to substantial power over a 2 second period). I agree however that he was probably maxed on those too and did in fact also consume energy from the Nr as well (which you point out is not a low inertia rotor (so substantial energy derivable from Nr). He burned a lot of energy, and I think he was at CtMax, all (my) working numbers point at that (particularly 2.7g)

Para 5
Yes in gets hungrier for energy, (and if the Ct is already maxed, then the harder you pull the hungrier it gets, with no increase in TRT, so no additional G) ('Pull more get more party' is over) (at UCA)

Para 6
“Whatever angle the blades cone to is a reaction to that bundle of forces and effects on the disc/blades” Namely those detailed in Para 2 above, which make CA not irrelevant but a direct indicator of how much of the capacity of the Rotor is being used. Please don’t just glaze over at that relationship, it is fundamental. Ca=TRT/Cf

Para 7
Agree completely.

Punto
“AnFI has actually credited you [dc/da] with that, although you need a beady little eye to spot that in his posts. He now talks about a band of UCAs rather than a single UCA”
and challenge.

Yes I did, but I qualified it (as you rightly said) as being insignificant.
The reason I say it is insignificant is that it might be more significant in the illustrative thought experiment I presented of loading a hovering helicopter with progressively more weight as Nr is increased, such that CtMax is maintained, since indeed the IndFlow would change (and worsen the situation wrt CtMax), however this is much less relevant at speed where the increased Induced Flow with weight is much less significant. I said it was baby out with the bath water, not a First Order effect.
AND in any case and regardless, when CtMax is reached that also would be CAmax (UCA) (at a constant RRPM). Agree?
AND furthermore, I think the onus is on dc/da to quantify HIS point. I was asked to quantify mine and give assumptions, which I did, I am entitled to make my assumptions. I make the assumption that Increased Induced flow is not relevant, and under that assumption I presume dc/da agrees with me? QUESTION

The ‘family of UCAs' is in part developed through the discussion here, but also falls out of the maths too. e.g. it is dependant on density (linear), and it also is a function of the Ct fall off that NL gives with speed (in one of his excellent graphs).

“even though there may be a nugget of truth riding on the hobbyhorse.” quite a substantial nugget !

Ref ‘the Challenge’ from Punto I asked dc/da multiple times to quantify the term. It’s HIS objection, the onus was on HIM to substantiate it. It MIGHT be significant, quantify it, show us how significant it is. Is it the baby or the bathwater, is it a 'hair on the egg’? Or is it indeed a substantive point? PARTICULARLY at speed.

Does he agree that under the assumption of that effect not being significant the main point holds ‘true’?

The piece where he points to an Ω term on the bottom is (I say) wrong and highly distorts the significance of Ω.
I quote the piece here and explain why (I say) it is wrong below that.

dc/da
"Unlike a fixed wing, a rotary wing is rotary. Therefore the "v" that any aerofoil element experiences relies on rrpm and the element's distance from the hub let's say Ωr . However the downwash the element is seeing is independent let's call it u.
Therefore
α = arctan(u/ Ωr)
Assuming a small angle
α = (u/ Ωr)
And substitute

κ = ρ.A. (u/ Ωr).a /2m.

As you can now see, there is a term for rotor speed in your equation. Q.E.D. "

why that's wrong
Well the α term should say (P-u/ Ωr) where P is pitch
I don’t think you can leave out the Pitch of the blade.

So by substitution we have
κ = ρ.A. (P-u/ Ωr).a /2m
would expand out to
κ = (ρ.A. P.a /2m) - (ρ.A.(u/Ωr)/2m)
so the "κ = ρ.A. (u/ Ωr).a /2m" overstates the dependancy of κ on Ω, which is too naughty to say QED, and relegates it to an examination of the baby by looking at the bathwater.
AND in any case THAT also assumes that P is not varied, which is NOT a fair assumption.
AND furthermore where u is negligible as in the case of a helicopter pulling out of a dive at 90kts, then the term almost disappears.
AND furthermore in the ‘thought experiment’ of increasing the Nr and increasing the weight correspondingly the u value WOULD NOT be independent of Ω, but it would be dependant on it, thus reducing the assumed error further (to zero, by cancelling)

So while I do freely admit the proposed relationship MAY NOT be EXACT, I do say that any difference is just bathwater and not a substantive point. Quantifying the difference might be quite complex and subtle, but at least we can say it is comparatively small.
So I say that my point (whatever it was) still holds ‘in essence’.
AND the onus would be on dc/da to substantiate his objection otherwise.

So with respect to the Challenge Punto
1 I’d say that I have covered why that term is NOT substantial.
So the answer is that UCA it does not (substantively) vary with RRPM, which is the point

2 If someone contends that it does vary with RRPM THEY should qualify it, and we’ll see if it is substantive or irrelevant.

Sorry if I did not answer anyone’s important point. “the pig is getting tired"