Calling Nick Lappos - Blade Stall

[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.