Tailplane stall recovery
 

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:

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...

1. 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.
2. 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.
3. 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

The tail is conventionally producing negative lift. Think of it as an "upside down" wing. If you were just into a wing stall, and lowered flap, the increased camber effects the airflow of the air ahead of the wing, and the stall is recovered. Same for the tail, just upside down.

Okay, happy with that - but I guess it prompts me to clarify what's meant by the stall in this case (I'll assume that we're looking at the tailplane upside-down, so that lift and angle of attack are positive):

1. the tailplane is stalled if the elevator deflection is fixed - ie lift decreases rather than increases if the angle between airflow and chord is increased (which, from this point of view, makes the stick-fixed aircraft unstable in attitude, as observed); but nonetheless
2. the lift increases as usual with elevator deflection. The increase in camber therefore wins over the increased effective angle of attack.

I guess I'm comparing this with the usual tale about ailerons being ineffective at the stall...

Windrusher

Checkboard I'm not necessarily sure that's the case.

Imagine using ailerons in a stall situation (a BIG no no!) you merely deepen the stall on the wing with the downward moving aileron by increasing the AoA, this being one of the reasons that you may get a wing drop. I would suggest that using flaps in the case of a wing stall you would deepen the stall on both wings.

The standard stall recovery is to reduce the angle of attack of the wing, to unstall it. To use flaps or ailerons has the opposite and completely unwanted effect.

As I understand it a tailplane stall causes stick force reversal and leads to the elevator being forced into the nose down position, or vice versa, and the reason a hefty pull is required is to overcome this force (which can be huge).

Tailplane stall recovery
 

Part of the issue with the tailplane stall is 'snatch' of the control surface, more accuaratly called a 'reversal of the hinge moment.' The ice contamination at or near the leading edge results in a 'bubble' of separated flow on the underside (vacuum or lifting side) of the tailplane. As the AOA (of the tail) increases, the bubble grows in length, with the re-attached flow point moving further aft on the tailplane chord, until it encompasses the control surface. Then the control surface is 'sucked' into the bubble and the flow fully separates. This 'snatch' of the elevator will typically pull the control column right out the pilot's hands and slam it into the full nose down position.

Part of the reason for the 'apply up elevator' step in the tailplane stall recovery procedure is to move the elevator back out of the 'snatched' full down position, which helps get the airflow to re-attach.

The same condition can happen on the ailerons when operating behind a contaminated leading edge. This aileron snatch is theorized to be the dynamic that caused the loss of roll control in the Roselawn accident with the ATR.

The aileron snatch phenomenon is more likely as the main wing approaches the stalling AOA (which is much less than normal due to the icing contamination of the leading edge.)

I assume this "snatch" only applies to non powered flight controls.

Just watched the movie from the first post. It answered my question for me.

Aileron snatch
 

You are correct; control surface hinge moment reversal is cited in the literature as not an issue in aircraft with powered controls.

Of course, that does not offer immunity from ICTS.

Quite an interesting video on that subjet :

Tailplane Icing

Regards,

og

Chesty Morgan -

Don't know what airplane you're referring to, but every transport catagory airplane I've ever flown requires flap extension as part of a clean stall recovery.

However there must come a point where the aerodynamic forces at the elevator will be stronger than the hydraulic powered controls (as per the well known servo transparency / jack stall in the rotary world) - has anyone any idea what sort of forces we are talking about in this fixed wing scenario?

DC-ATE, Q400, BAe 146 and E195. None of these require flap extension to recover from stall situations. Even after pusher activation.

In fact the E195 doesn't have a pusher only a shaker. The recovery from that is to apply full power and maintain the pitch attitude.

The only way to unstall a wing is to reduce the AoA of the wing not increase it by using flaps and or ailerons as this only causes the wing to enter deeper into the stall.:=

Chesty Morgan -
Well, that might be true in those "little" machines you're talking about, but not the stuff I've flown.

Well I'd have thought the only way to unstall any wing is to reduce the AoA.

I'd be interested if you could expand a bit on your comments and which types you're referring to.

Chesty Morgan -
Lockheed Constellation, Douglas DC-6/7, Boeing 737 (-200), and Douglas DC-8 (-50, -61, -62, -71). Flew the Lockheed Lodestar as well, but have no recollection of the stall recovery procedure any more!

Well obviously those old fashioned things you flew have different characteristics to the "Little" modern things I fly!;):}

Chesty Morgan -
Well, those "old fashioned" things took pretty good care of me for thirty years and I never bent a piece of metal, so something must've been right compared to all the problems I see and read about now-a-days.

Good luck to you.

737s do not require any flap selection for stall recovery anymore, at least not 300 to 900s.

Well, I flew the 300 (but it was in the last Century!!) and from the Clean Stall we selected flaps. The incriment changed through the years, however. Time has a way of changing things I guess.

Can I just comment that you are not pulling back on the elevator in a tail stall to try to unstall the tailplane - you're puling back on the elevator to try to get some nose-up aircraft motion, because with the tail stalled the aircraft will be nosing over like mad if you don't.

To unstall the tail you need to reduce the AoA at the tail - which is achieved primarily by selecting wing flaps up and reducing the downwash over the tail.

The normal effect of flaps on an airfoil is to reduce the AoA at which it will stall (you're making the airfoil work harder, so you'd expect to have it "give up" sooner) - although the maximum lift for the flapped airfoil does increase. I would therefore be shocked if any of the cases cited used flaps ALONE to recover; lowering the AoA has to be part of stall recovery.

What lowering the flaps will do, in combination with a lowered AoA, is enable you to generate more lift once you're unstalled, so you'd need to lose less altitude to recover back to a safe flying speed. I don't know, but I'd think that's the reason for flaps selection on some types - to make the recovery better.