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.

Exactly. The aerofoil on a Tiger Moth has an MCRIT, but it's of absolutely no concern to anybody - designers, regulator, engineers, pilots.

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

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

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.

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?

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!)

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.

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.

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.

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"

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

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.

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

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.

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?

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.

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?

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

"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.

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.

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.

AnFI, can you point me to an official accident report that has rotor stall as being causative in an accident. Many thanks.

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.

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.

B212 has pre-coning too....

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.

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

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

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.

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?

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

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.

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?

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.

Perhaps AnFi is confusing aerodynamic damping with his UCA.

He's confusing everything with everything....

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

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?

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??