Apologies for being particularly dense, but I'm puzzled by the principles behind the 'stick back' tail stall recovery - how is it that increasing the (conventionally negative) angle of attack increases the (negative) lift of the stalled tailplane? There's been a
previous Pprune thread on this but it doesn't seem to have answered the question; I've found an
FAA guidance note on the topic, and the NASA folk who produced the excellent
video (referenced in the thread on the
Buffalo crash) have also produced a
written report, from which a couple of pertinent paragraphs are:
Reducing thrust was the first part of the procedure because it was increasing thrust that led to the stall event that was encountered. Pulling back on the yoke increased the camber of the tailplane, which provided enough tail download to counteract the nose-down pitching moment and increase the atail. Raising the flaps was initiated by the copilot immediately, but the flaps are hydraulically actuated and movement is rather slow (~ 1º/sec). The major lesson learned to recover from a tail stall was to undo what was just done to cause the event.
It was noted that this tail stall recovery procedure is opposite of the recovery from a wing stall. The reason for the difference is the location of the flow separation. In a wing stall, the flow separates from the upper surface of the wing, therefore reattachment is made by decreasing the wing a. In a tail stall event, the flow separates from the lower surface of the tail and requires a positive increase in tail a to reattach the flow. Because of these differences in the stalling mechanisms and recovery procedures, it was determined that pilots should be made aware of the cues that may occur prior to a tailplane stall. Efforts to increase pilot awareness on this topic are described in the following section.
It's reasonable that to unstall the elevator one will wish to make its angle of attack more shallow (less negative), and that this will correspond to a more nose-up aircraft attitude, hence lower airspeed, reduced flap setting etc. But recovery seems to assume that elevator control remains to some extent effective: pulling back the column, raising the elevator, will increase the angle between the tailplane chord and airflow - ie tailplane alpha will become a larger (but conventionally negative) value. Naively, one would not expect a stalled aerofoil to respond with an increase in (downwards) lift.
A few thoughts...
- The NASA report refers to increasing the tailplane camber, suggesting that the effect is a profile-related increase in the lift of the stalled aerofoil, which is sufficient to change the aircraft attitude and subsequently unstall the tailplane.
- Conventional (wing) stall recovery relies upon a separate aerofoil (the elevator), which we don't have in this case. Pulling back is therefore equivalent to unstalling a wing by deploying flaps.
- There will presumably be an increase in the tailplane drag which, depending upon the aircraft, could help or hinder recovery.
Apologies for the curiosity: I appreciate that what works is more important than why it works, and the NASA video is compelling, so this is entirely academic - but then, so am I!
Many thanks,
Windrusher