Does the aircraft stall at the same stick position?
The factors affecting the stall speed by changing the critical angle of attack are:
- Flap configuration
- Ice/damage to the wing surface
and the factors affecting the stalling speed, without changing the critical angle of attack are:
- Power
- Weight
- Centre of Gravity position
- Load Factor
Now the question concerns a Pitts special, in aerobatics, and you instructor was really referring to the stick position in the straight and level stall (initiating a spin), compared to a 2-3G stall (flick/snap roll) to a 4g stall (say, entering a loop) being the same.
Considering the above, flap configuration doesn't matter (as the Pitts doesn't have flaps), hopefully the wings haven't suffered damage and you are not performing your routine in icing conditions! As all of these change the critical angle of attack the stick position is definitely changed.
I think that (for an individual sortie) the CofG and the weight are pretty constant so in this context the question really is: Does the stick position change
significantly with power and load factor in a stall?
Now the stick position refers directly (via cable and push rod) to the elevator angle. This question is a little like the statement "You don't trim the aircraft to an attitude, you trim it to an airspeed."
If you set up a particular airspeed, say 80 knots, in S&L, trim then reduce the power the aircraft will start descending (in trim) at
around 80 knots. The key is the "around" bit. As the power is reduced the power/drag force couple has been changed and the aircraft is descending so the Lift/weight force couple has changed (as the lift required in a descent is less) so the elevator (which acts to balance these force couples) needs to be in a slightly deferent position to maintain 80 knots exactly. If you try this in the air, it should prove to you that the elevator (and stick position) change with power application - i.e. a stall with no power and a stall with power will require a different stick position.
Is the elevator position required to produce the critical angle of attack at 100 knots (4g) the same as it is at 50 knots (1g)? The Lift varies with V², as does the Drag and as can be seen from the pic, these two force couples roughly oppose each other, the force produced by the tailplane (moving the tailplane down) will also vary with V² , as will the "weather cocking" effect trying to force the tail plane up. In practice it seems that these effects all
roughly cancel out. The stick position is
roughly the same at the critical angle of attack with changes in load factor.
Was you instructor literally correct in teaching this? No - but instructing isn't about being literally correct all the time, it is about achieving results. I used to teach the same concept when I was teaching aeros (but a little more accurately *wink*
) and the effect is that the power, weight, CofG etc., don't change much so the stick position is
roughly in the same position for the stall. The purpose of the instruction is to make a student aware of how close the aircraft is to the stall, when that student has probably only ever seen S&L stalls before, and is hung up on "stalling speed". It is a situational awareness thing, and also a handy technique when teaching how to accelerate the flick roll - but that is another story