Stall stick position
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Stall stick position
I began an emergency manouvre training course in a Pitts S2A yesterday. Now I have been doing aeros for a few years and have never come across this before.
The instructor is convinced (and was very convincing) that in every aircraft there is a stick position where the foil always exceeds the critical AoA and stalls regardless of speed, flaps, gear, Angle of Bank, etc. A particular elevator position always gives that AoA.
Any comments on this theory? I will expand on this if I have failed to be clear enough.
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"eagles may soar but weasils don't get sucked into jet engines..."
http://users.hunterlink.net.au/~dfckm/weasil.htm
The instructor is convinced (and was very convincing) that in every aircraft there is a stick position where the foil always exceeds the critical AoA and stalls regardless of speed, flaps, gear, Angle of Bank, etc. A particular elevator position always gives that AoA.
Any comments on this theory? I will expand on this if I have failed to be clear enough.
------------------
"eagles may soar but weasils don't get sucked into jet engines..."
http://users.hunterlink.net.au/~dfckm/weasil.htm
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Broadly speaking, you can paint a line on the fusalage wall and if the pole is aft of that line at any speed or attitude the wing will stall. You are flying the ideal aircraft to go and prove this in flight. Sadly there seems to be a lowwer level of understanding of aerodynamics these days.
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You're right.. the Pitts is a manouverable little plane isn't it.
We were doing snap rolls (highspeed stall followed by full rudder causing a stalled roll!) it is unreal. and we were doing climbing and descending turns into a stall where one wing stalls first quite violently and a/c spins away out of the climb or into the descent. recovery was quite simple with a little practice. I just hope that it comes back to me if it ever happens for real (touch wood)
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"eagles may soar but weasils don't get sucked into jet engines..."
http://users.hunterlink.net.au/~dfckm/weasil.htm
We were doing snap rolls (highspeed stall followed by full rudder causing a stalled roll!) it is unreal. and we were doing climbing and descending turns into a stall where one wing stalls first quite violently and a/c spins away out of the climb or into the descent. recovery was quite simple with a little practice. I just hope that it comes back to me if it ever happens for real (touch wood)
------------------
"eagles may soar but weasils don't get sucked into jet engines..."
http://users.hunterlink.net.au/~dfckm/weasil.htm
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With all due respect to your esteemed instructor, this is utter balls.
First point is that the aerodynamic stall, and the piloting stall are NOT the same thing. The stuff learned in GS or aerodynamics class is absolutely true about boundary separation at the trailing edge - IN A WIND TUNNEL. From a pilots point of view, the stall is the point where the aircraft does something the pilot didn't command (or fails to do something he did).
To quote a convenient airworthiness standard holding up a table-leg "a stall results as evidenced by a downward pitching motion or downward pitching and rolling motion not immediately controllable to until the longitudinal control reaches the stop" [BCAR S201(a), but JAR-VLA, 22, and 23 use pretty much the same wording I'm certain.]
In something like a Pitts, you've probably got so much elevator authority that the aerodynamic and piloting stall are the same. In plenty of aircraft there is insufficient elevator authority (the CFM Shadow is a classic) to reach an aerodynamic stall and what you get is the classic "mush stall" with the stick on the backstop and the aircraft developing a gentle rate of descent.
But with a reversible control system (e.g. any light aircraft)the elevator authority is a function of CG position, so the stick position to reach the aerodynamic stall on your Pitts will be further back at a further forward CG and visa-versa. I've flown aircraft where at fwd CG you can't reach an aerodynamic stall and only get the "mush", but at aft CG you can get a good solid pitch break, indicative of aerodynamic stall.
For a given wing configuration (i.e.flaps, slats, etc. left alone) an aircraft will always stall at the same AoA regardless of weight and speed, but not at the same stick position. Change the flap / slat settings and the stalling AoA will change too.
I can see however how this could be concluded on the Pitts, it works in a very narrow CG range and has a very powerful elevator - so the stick position to stall probably is damned nigh constant. This is not the case for all aircraft, and only an illusion on the Pitts.
You might like to post the question on the test flying forum, there are one or two people populating that who have done a lot more stall testing than me and might give a different perspective.
G
First point is that the aerodynamic stall, and the piloting stall are NOT the same thing. The stuff learned in GS or aerodynamics class is absolutely true about boundary separation at the trailing edge - IN A WIND TUNNEL. From a pilots point of view, the stall is the point where the aircraft does something the pilot didn't command (or fails to do something he did).
To quote a convenient airworthiness standard holding up a table-leg "a stall results as evidenced by a downward pitching motion or downward pitching and rolling motion not immediately controllable to until the longitudinal control reaches the stop" [BCAR S201(a), but JAR-VLA, 22, and 23 use pretty much the same wording I'm certain.]
In something like a Pitts, you've probably got so much elevator authority that the aerodynamic and piloting stall are the same. In plenty of aircraft there is insufficient elevator authority (the CFM Shadow is a classic) to reach an aerodynamic stall and what you get is the classic "mush stall" with the stick on the backstop and the aircraft developing a gentle rate of descent.
But with a reversible control system (e.g. any light aircraft)the elevator authority is a function of CG position, so the stick position to reach the aerodynamic stall on your Pitts will be further back at a further forward CG and visa-versa. I've flown aircraft where at fwd CG you can't reach an aerodynamic stall and only get the "mush", but at aft CG you can get a good solid pitch break, indicative of aerodynamic stall.
For a given wing configuration (i.e.flaps, slats, etc. left alone) an aircraft will always stall at the same AoA regardless of weight and speed, but not at the same stick position. Change the flap / slat settings and the stalling AoA will change too.
I can see however how this could be concluded on the Pitts, it works in a very narrow CG range and has a very powerful elevator - so the stick position to stall probably is damned nigh constant. This is not the case for all aircraft, and only an illusion on the Pitts.
You might like to post the question on the test flying forum, there are one or two people populating that who have done a lot more stall testing than me and might give a different perspective.
G
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Thanks for your reply Ghengis. Some good food for thought. Might I ask what you do in the scheme of things?
One question
"For a given wing configuration (i.e.flaps, slats, etc. left alone) an aircraft will always stall at the same AoA regardless of weight and speed, but not at the same stick position. Change the flap / slat settings and the stalling AoA will change too."
Can you clarify this statement... I have been always taught quite firmly that the Critical angle of attack never changes. By extending flap you are simply moving the position of the chord line (the line between the centre of curvature at the leading edge and the trailing edge). You are not causing the wing to stall at a different AoA you are only changing the AoA for a given attitude.
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"eagles may soar but weasils don't get sucked into jet engines..." http://users.hunterlink.net.au/~dfckm/weasil.htm
[This message has been edited by weasil (edited 19 December 1999).]
[This message has been edited by weasil (edited 19 December 1999).]
One question
"For a given wing configuration (i.e.flaps, slats, etc. left alone) an aircraft will always stall at the same AoA regardless of weight and speed, but not at the same stick position. Change the flap / slat settings and the stalling AoA will change too."
Can you clarify this statement... I have been always taught quite firmly that the Critical angle of attack never changes. By extending flap you are simply moving the position of the chord line (the line between the centre of curvature at the leading edge and the trailing edge). You are not causing the wing to stall at a different AoA you are only changing the AoA for a given attitude.
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"eagles may soar but weasils don't get sucked into jet engines..." http://users.hunterlink.net.au/~dfckm/weasil.htm
[This message has been edited by weasil (edited 19 December 1999).]
[This message has been edited by weasil (edited 19 December 1999).]
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The lift curve slope (dCL/d alpha) will vary with flap and or slat extension. Stalling AoA may possibly be the same for a given aeroplane with flap/slat as without - but that is unlikely. The lift curve slope is the 'fingerprint' of the wing and is configuration-specific - as is stalling AoA.
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I'm an airworthiness & flight-test Engineer by profession, I survey, test and approve / disapprove new and modified aircraft designs from drawing board to C of A. In between I do about 70 hrs pa of either private flying or low grade airworthiness test flying as either pilot or observer.
Critical AoA doesn't change by and large (actually it does at very low or very high airspeeds, but most aircraft never reach either end of the scale) with either loading or airspeed unless you have a particularly flexible wing. However, by deploying flaps or slats you change the aerofoil section, and almost inevitably the critical AoA with it.
G
Critical AoA doesn't change by and large (actually it does at very low or very high airspeeds, but most aircraft never reach either end of the scale) with either loading or airspeed unless you have a particularly flexible wing. However, by deploying flaps or slats you change the aerofoil section, and almost inevitably the critical AoA with it.
G
Does the aircraft stall at the same stick position?
The factors affecting the stall speed by changing the critical angle of attack are:
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
The factors affecting the stall speed by changing the critical angle of attack are:
- Flap configuration
- Ice/damage to the wing surface
- 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
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Good explanation!! But I do remember being taught when flying the Gnat that control column position (mil-spec. QFIs never call it a 'stick'!!) was a very good indicator of the incipient stall at low speeds. However, that was in an aircraft with a very unusual and highly complicated longitudinal control system!!.
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Interesting but: Don't you think that most pilots fly on stick pressures rather than positions. True on aerobatics, true on slow flight. Am I wrong? In case I'm not, why bother with the stick position hypothesis.
The whole thing about certification authorities is they want to see a nice, stiff, stick force gradient so you can't apply enough of a g load when you're fast to reach stall AOA. Again--pressures are part of the approved design. Reason being that we rarely are conscious (rarely need to be, shouldn't be) of stick position.
Don't mean to be grumpy. All the best.
The whole thing about certification authorities is they want to see a nice, stiff, stick force gradient so you can't apply enough of a g load when you're fast to reach stall AOA. Again--pressures are part of the approved design. Reason being that we rarely are conscious (rarely need to be, shouldn't be) of stick position.
Don't mean to be grumpy. All the best.
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Couldn't agree more Bizjet; by and large stick forces are far more important than stick displacements.
However, you still need reasonable stick displacements with most control system designs otherwise you start to encounter fine-control problems as soon as normal wear and tear cause a bit of freeplay in the system.
However, most pilots will be aware if there is a lot of back-stick, so it does no harm to identify that as one of the stall warning clues.
G
However, you still need reasonable stick displacements with most control system designs otherwise you start to encounter fine-control problems as soon as normal wear and tear cause a bit of freeplay in the system.
However, most pilots will be aware if there is a lot of back-stick, so it does no harm to identify that as one of the stall warning clues.
G
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Weasil
I think that for the Pitts, your instructor is right. I had to do Cof A test flights this summer on three of those naughty biplanes and they have enough control authority to make this happen. (Like any good aerobatic machine) There is a word about stalling on my website www.tsr3.freeserve.co.uk at the bottom of the first page.
I think that for the Pitts, your instructor is right. I had to do Cof A test flights this summer on three of those naughty biplanes and they have enough control authority to make this happen. (Like any good aerobatic machine) There is a word about stalling on my website www.tsr3.freeserve.co.uk at the bottom of the first page.
For a Civil type with a good force gradient, stick (I'm sure everybody knows what I mean ;-) ) pressure provides a fairly good situational awareness (except the A320?? ), but in an aircraft with minimal longitudinal stability (like the Pitts) there can be very little force gradient, especially at low g/airspeed. I think that stick position is more definitive.
Stick force is too trim/airspeed/artificial feel dependant.
[This message has been edited by Checkboard (edited 22 December 1999).]
), but in an aircraft with minimal longitudinal stability (like the Pitts) there can be very little force gradient, especially at low g/airspeed. I think that stick position is more definitive.Stick force is too trim/airspeed/artificial feel dependant.
[This message has been edited by Checkboard (edited 22 December 1999).]