9kBud

Check my post.

shdw

Som aerobatic pitch/power configurations. It can be demonstrated in some aircraft by stomping full left, then full right rudder, and alternating. I have also experienced the buffeting from it in a citabria with 40 (maybe 45? been awhile) degrees of flaps when above about 65-70 knots. At low speed it wasn't an issue, but full flaps, nearing the top of the white arc, and a few bumps on final resulted in some tail buffeting.

There is no way for me to know if that was a tail stall. All I can do is speculate and say that the buffets felt on the yoke were similar to those felt on the tail stall demos I had done.

I will agree with rick though, it shouldn't happen in any normal flight situations that I am aware of. The flap addition is the closest to normal and we are talking 40 degrees of flaps on an aircraft designed for >1000' SFTOL.

I don't have time to watch the video now to see and it has been a few months. However, I think they said airspeed plays a factor.

Cubdriver

The NASA video is good as far as it attempts to go into the subject, but it should kept in mind that research on the physics of tail plane stall is incomplete. We do not presently understand the phenomenon very well. The NASA video is a fine attempt to explain things a little bit and it offers us an effective way to recover from recognized tail stall events if we recognize them. However, the video leaves many theoretical issues unanswered even on the basic level.

1. The video correctly shows that increased airspeed aggravates tail plane stall under icing conditions. But you have to ask, why? The opposite would seem more intuitive. Wing stalls are mitigated by increasing the airspeed among other things. The video makes no attempt to explain this contradiction. The idea intuitively would be that slower speeds would lead to horizontal stabilizer stall as the latter tries to generate additional lift through increased angle of (inverted) attack. Additional downward lift comes from additional stick deflection as the pilot pulls back and the elevator deflects upward more as the speed slows. This is true in-as-much as force balance is concerned. In contrast, the video shows that as speed slows the flow attachment tends to improve and the flow tends to stay attached better.

2. The video shows that additional power aggravates a tail stall condition. But again, why? The opposite would seem more intuitive because with wing stalls, adding power delays the critical angle of attack and hence the stall by speeding up the air over the wing, which allows more lift to be had at slower forward airspeeds, and it allows operation at a lower angle of attack. Not so with the tail plane stall, and the video does not offer any explanation. I think it may have to do with the turbulence introduced by prop wash into the airstream compared to that of freestream air.

Several have mentioned the correct generalized tail plane stall recovery procedure, but I take exception to some of the explanations. Namely, if the recovery procedure is to pull the stick back, this is not to lower the angle of attack on the horizontal stabilizer. It is to recover the airplane from a dive after the tail plane stalls. There is no causal relationship to angle of attack there. Pulling the stick back increases the angle off attack at the stabilizer. The confusion may arise because the NASA video does not explain this phenomenon. As pilots we like to do things based on theoretical understanding of why we are doing it. I do not think tail plane icing is a situation you can apply easy logic to at this point. It may be wiser to leave the theory out until more research is performed.

rickair7777

Anything which requires more tail-down force should aggravate the situation.

Prop: More power = more airflow over wing => more wing lift => pitch down moment (CL is behind CG) => horiz stab input required to keep the nose level => more taildown force and higher horiz stab AOA.

Most tail-mounted jets: More power => nose down moment (engines mounted higher than CG) => horiz stab input required to keep the nose level => more taildown force and higher horiz stab AOA.

Cubdriver

Good logic but again, not enough supporting material. More power in a prop plane generally adds a strong nose up moment especially at slower speeds. It also lowers the required angle of attack for the main wings and you can follow consequences, ie. additional tail down-force would not be associated with it. Tail-mounted jets do have a lot of nose down moment associated with their thrust, but tail plane icing is not as much of an issue on bleed-air anti-ice systems used on jets. It's prop airplane problem as far as history is concerned.

shdw

As it should, IMO. It is teaching recognition and recovery to pilots, not theory to engineers.

Remember the job of the horizontal stabilizer? It provides lift in the opposite direction (of the main wing) to counter the rotation caused from main wing lift. Increase speed equals increased main wing lift and the necessity for increased lift on the horizontal tail. Being the area of the wings (an effect within the lift equation) is less on the tail than main wing, a speed increase wouldn't be uniform. The tail might need an increase in AOA along with the speed increase to match the rotation force. If it does, that increased AOA is the answer to your question.

Seems you answered your own question. I would agree with your answer and add that it may vary in aircraft where the thrust line doesn't pass through the CG.

No, there is a direct relationship. Remember, the horizontal stabilizer is an inverted main wing. When you pull back you increase the AOA on the main wing and decrease the AOA on the horizontal stabilizer. Vice versa from forward pressure.

The pull back isn't to save the dive, sure it does that, but it is it reduce the AOA on the tail.



Much of the discussion and many of the questions/comments posed since my original reply could be answered quite simply by knowing two things:

1) If you take the main wing off, flip it upside down, and glue it to the tail you have a horizontal stabilizer.

2) There is no aerodynamic difference in how lift, stall, drag, etc occurs on the main wing versus how it occurs on the tail wing.

You might notice that #2 is the answer to the question I gave in my first reply, "You should wonder, what is the aerodynamic difference between the main wing and horizontal wing?"

Cubdriver

Nope, opposite is true. Backward stick movement deflects the elevator upwards. The horizontal stabilizer is normally pulling down, and it pulls down a lot more in slow flight. Elevator up deflection increases the camber of the bottom side of the stabilizer. More camber means more downward lift, faster flow over the bottom, and a higher angle of attack quickly ensues for both the main wing and the stabilizer. Pulling back more on the stick adds more to the angle of attack factor on both items. The logical solution would be to move the stick forward again, but we know this is against the recovery procedure NASA found actually works in testing with ice shapes. My argument is simply that we do not have enough data in the NASA video to be able to say what makes the recovery procedure work as it does, and there are apparent contradictions in the theory as it stands.

Singlecoil

Increasing airspeed in level flight causes the center of pressure to move aft on the wing due to the lower angle of attack. This moves the center of pressure further away from the center of gravity, requiring a larger tail down force from the horizontal stabilizer to keep the nose from dropping. Therefore, the angle of attack on the tail increases as airspeed increases. Remember, in small planes like we are talking about here, the wings and tail are welded in position, inversely mounted to each other. As you increase the angle of attack on one, you decrease the angle of attack on the other.

Again, if you are lowering the angle of attack on the wing, you are automatically increasing it on the tail.

The tail is an upside down airfoil. When you pull the stick back, the elevator deflects upward, increasing the chord line of the upside down airfoil, and increasing it's downward lift. If the tail is near a stalled angle of attack, deflecting the elevator upward by pulling back on the stick will act on the tail as flaps do on the wing. It changes the chord line of the airfoil and allows it to operate at a higher angle of attack without stalling. Pulling back deflects the elevator upward, increasing tail downward lift, pushing forward deflects the elevator downward, decreasing downward lift. Think of the elevator as a combination flap and spoiler for the tail. Deflected upward it increases tail down force, deflected downward it decreases tail down force by change the chord line of the airfoil.

In jets the stabilizer moves when you trim, unlike in small general aviation aircraft and most turboprops.

Cubdriver

This is not correct. The lift vector of a wing is always perpendicular to the chord line and almost perpendicular to the relative wind. It points more backward when the angle of attack is higher and more forward or up when the angle of attack is low. The center of lift goes back dramatically with the addition of flaps also. You may be thinking about wing moment which moves slightly backward during lower angles of attack, but it is a small move. It is safer to say that wing moments do not change with angle of attack for most wings and be clear I am speaking of wing moments isolated from anything else such as the CG of the airplane. In contrast, there is large shift backward in the angle of the lift vector and the center of lift with added flaps as speed slows down, see NASA video at about 7:10. This causes nose down force the tail must counter.

Correct, except be clear on whether you mean center of lift or center of pressure. The center of pressure moves forward with increase in angle of attack as the air speeds up more and more to make it around the front of the wing to the top. In contrast, the lift vector moves rearward as the wing rotates through angles of attack. Lowering flaps and slowing down are strong contributors to this.


No, it's the other way around. Slower flight requires more down force from the tail plane as the angle of attack increases on the main wing.

Not so- the NASA video clearly shows that angle of attack increases for both surfaces during slower flight and while adding flaps.

This is all correct but any flying surface that is beyond its critical angle of attack stalls as a result. We raise the critical angle of attack of a tail plane by pulling back on the stick indeed, but at some point it can be exceeded especially when the flow is disturbed by icing. The method to recover any stalled surface back to flying is to lower the angle of attack and demand less lift from it. The angle of attack a tail plane sees is reduced by reducing the downwash angle (ie. flap reduction) and by lowering the nose of the airplane. Lowering the nose is best accomplished by stick forward movement IF the tail plane is flying. Since it is not and you have no control over pitch other than what is built into the airplane, you have to wait until the tail plane naturally sees an angle at which it can keep the flow attached again. What is surprising about this is that NASA finds that stick forward does not help to lower the angle of attack and re-attach the flow.

shdw

Cub, this is his supporting evidence. We know, in aerodynamics, that a thrust line above the CG would introduce a nose down.

The rest we also know to be true if we apply our knowledge of basic lift and drag on these two wings. Particularly how AOA plays a key roll throughout this process.

If it helps to get started, here is a list of how you could go about proving this mathematically:

1. Assign pos/neg value to lift and AOA accordingly.
2. Consider all the forces that could cause rotation about the lateral axis.
3. Consider the tails job to counter that and how it varies based on CP.
4. Leave CP constant, as I am sure you know it is a calculations nightmare for only a small percent error.
5. Lift will not equal weight during any rotation about the lateral axis.
6. Calculate the total lift in any given scenario.
7. Calculate the main wing's rotational force about the lateral.
8. Calculate the lift required, and in what direction, to counter the rotational force from the main wing.


This list is in no way complete and likely includes things you already know. However, it can hopefully get you started if you want to prove it to yourself that we already know this.

What aerodynamics cannot answer, yet, is icings role in the puzzle. The rest is fundamental aerodynamics, we've known it for years and your training level can easily perform the calculations.

Repeat these, adding your own steps of as needed, for any scenario rich gave. You will find that, for the most part, his statements are accurate.