Calling Nick Lappos - Blade Stall

[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