Cubdriver

Wups, didn't mean to hit your edit button there. You do not have to calculate anything, you can get all the flight dynamics info you need by using the data dump feature of X-Plane. It won't include anything having to do with icing but it would show that tail plane angle of attack goes up with angle of attack and flaps, and down with airspeed.

shdw

Ok maybe I need to add something else to my list of 2 things before:

#3 Center of pressure: The point on the chord line where lift force is being applied. This point will shift. With its shift results in a change in arm between the CP and the CG. Less of an arm equates to less rotational moment for the tail to counter once rotation stops and pitch is constant again.

That said, higher main wing AOA means a more forward center of pressure. The result is less rotational moment. Less rotational moment means less work for the horizontal stabilizer, not more, and a reduced AOA.

The reverse occurs with a low angle of attack. A more aft CP, greater arm, greater rotational force, and more lift needed from the tail (higher AOA).

Singlecoil

Exactly correct.

Cubdriver

Sorry I am still not with you so let's keep working on this. I made the following study using X-Plane software. The first two screen shots are of a regional airliner prop-plane at low angle of attack ("alpha"). Flaps are down, airplane is holding altitude more or less. Check out the relatively low angle of attack for the main wing (0.4), then check out the relatively low angle of attack for the tail plane as well (-3.6). The tail plane angle of attack is the last one on the lower left-hand side of the screen. The first picture is of the cockpit and the second is of the outside of the aircraft at exactly the same microsecond just for comparison sake:







Next, the same airplane is slowed to a near stall (100 or so). The flaps are down as above, and the speed has dropped to about 100 mph. The angle of attack of the main wing is much higher as one would expect. But note that the angle of attack on the tail plane is to the absolute limits (-15 degrees). The tail plane cannot generate enough negative lift to keep the airplane nose up at this point, and the airplane has begun to mush.

Cubdriver

Outside view of the last configuration (3 pictures per post limit). Again, the flaps are down and the airplane is going very slow, almost stalling.



The conclusion one can draw from all this is that a high positive angle of attack ("alpha") on the main wing (8.7 here) corresponds to a high (negative) angle of attack at the tail (-15 here). Do you agree? Is my study flawed? Would the angle of attack on the tail increase, or rather would it go back down with an increase in speed and corresponding drop in angle of attack on the main wing?

bcrosier

It seems to me, that by pulling back to recover from the tail stall, you are in fact altering the effective camber of the horizontal stab, very much like extending plain flaps on a regular wing. In doing so, you are ending up with an airfoil which can produce more lift at a higher angle of attack - precisely what you need to counter the loss of tail down force and resulting nose down pitch.

I also don't think anyone has really addressed the issues of why this phenomenon is so frequently associated with flap extension: You are altering the airflow around the main wing, and increasing the downwash the wing is generating. This increased downwash results in an increased angle of attack on the horizontal stabilizer. IF the stab is operating very close to it's critical angle of attack (for example, if it is contaminated with ice), this increase will result in the horizontal stabilizer stalling.

Cubdriver

The points you make are all covered in the NASA video on tail plane stalls and they are in agreement with the video as well. We are trying to agree on why the tail stall recovery procedure involves a stick-back response and slowing down and less power rather than the more intuitive response of pushing forward just like for a main wing stall recovery, adding power and speeding up. Saying the tail plane operates upside down is not a satisfactory answer. None of us disagrees that recovery should be stick-back etc., we just disagree on the theory. Actually, my point is only that the theory on tail plane icing is as-of-yet unclear even with NASA, and that anyone who sounds sure on the topic is mostly guessing.

shdw

You must have high power in this situation. Recovery from a tail stall should have lower power settings. We did agree on this:

Sounds to me, from the NASA video, that you should use power as need to avoid striking the ground. Anyways, retry your tests with a very low power setting, allowing to give altitude for reduced engine power.

Edit: Oh, to see proof in the "aerodynamics" you must take engine power away from the equation. Then we are talking glider tail stalls, which I believe is more common. I was taught tail stalls in a glider and demonstrated them and their recovery: stick to your stomach.

Singlecoil

No, we're not guessing. Many of us are almost quoting the flight training handbook verbatim. Thanks for posting the screen grabs from X-plane, that looks like a cool program. I disagree with your analysis of the data presented, however. What you are stating is horizontal stabilizer angle of attack, looks to me like the datum for elevator deflection, not stabilizer alpha.
Look at the lower box on the right side of the screen in the two slides. There are data there for the negative lift on the horizontal stabilizer. You will notice that there is more negative lift in the first scenario (high speed) you presented than in the second. 969 compared to 743. The faster the plane goes, the more negative lift is required. Why is that? It is because at higher speeds the vertical component of the total force on the airfoil (lift) moves further aft of the center of gravity as the wing angle of attack is decreased, thus requiring a higher tail down force.
I don't see any values for horizontal stabilizer angle of attack on your screen grabs.

R Banger

I don't know if this was added already as I just skimmed the previous posts.

Another thing to consider with tail stalls is that they can occur when the aircraft is dirty and not coordinated. An example of this is in a forward slip. From my experience in a high wing aircraft like a C-172 if the flaps are fully extended and you are in a forward slip you are more prone to encounter a tail stall. This is because the airflow over the tail is disrupted from the flaps being down and the aircraft being uncoordinated. The relative wind can be blocked by the high mounted wings and even the fuselage when slipping. This is even more noticeable in the older C-172N models that allow you to bring flaps down to 40 degrees. This is important to know because usually if you are slipping it is for landing purposes and are close to the ground, a tail stall in this situation can prove hazardous. A recovery from a tail stalled condition requires pulling back on the yoke to decrease AoA on the elevator, and clean up if necessary.