Calling Nick Lappos - Blade Stall

[[email protected]]

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 pilots

annoyingly 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]

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.

[Bell_ringer]

Originally Posted by Frying Pan

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

[AnFI]

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]

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

[megan]

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

[AnFI]

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]

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:

[Ascend Charlie]

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]]

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]

"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]

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]

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.


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.

[[email protected]]

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

[NickLappos]

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]

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

[212man]

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

[Bell_ringer]

Originally Posted by 212man

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

In this instance a butter knife.

[AnFI]

2 Second loop of G Apache

http://ytcropper.com/cropped/xO57ff80c3359db

and some reasonable calculations

[AnFI]

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