Angle of Attack: True or False

birdus

birdus

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Jay Williams
True or False: The stick determines elevator position and elevator position determines angle of attack. When the critical angle of attack is exceeded, the wing stalls. So, given a design with enough elevator authority, if the stick is in your lap, then the wing is stalled. If the stick is, say an inch farther forward, the wing will not be stalled. Period.

I had a conversation with a United Airlines captain (737) the other day, and he said I was wrong. As I get older, I tend to be less argumentative, so I didn't pursue it. He said some things about different size loops (don't know what that was about). When he mentioned the Amazon crash off the coast of Texas (the overall conversation was about MCAS), I said that, in a video I had seen (months earlier), it looked like the plane may have been stalled. The descent angle appeared steep and yet the angle between the longitudinal axis of the plane and the ground appeared to be significantly shallower (i.e., stalled). He said something about the descent angle being really steep and the airspeed being really high so it couldn't have been stalled.

I really wanted to say "descent angle and airspeed are irrelevant. If the pilot put the plane into a dive, gained a ton of airspeed, and then put the stick in his gut, then they were stalled." However, I dropped it.

So, are there subtleties I'm not considering? Could the stick not act as an angle-of-attack indicator (if I could position it precisely enough)? I'll try to clarify this post if necessary.

EDIT: Let's stipulate no flaps/slats, calm air, normal CG, and plenty of elevator authority.
 
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birdus said:
True or False: The stick determines elevator position and elevator position determines angle of attack. When the critical angle of attack is exceeded, the wing stalls. So, given a design with enough elevator authority, if the stick is in your lap, then the wing is stalled. If the stick is, say an inch farther forward, the wing will not be stalled. Period.
Click to expand...
Two-thirds of your statement are true. Elevator position is one thing that can cause your wing to exceed the critical angle of attack. Let's say I put my airplane into a fairly steep climb, return the elevator to a neutral position, and pull the throttle. What's going to happen?
 
The stick by itself no. Because elevator efficacy changes with airspeed (and possibly other things like slipstream if the elevator is in the prop wash). Angle of attack is the key, yes. And you can stall at any airspeed if you exceed the critical angle of attack. But it's harder to do when moving faster. Ercoupes were considered stall proof because of limited elevator movement.
 
There are tons of subtleties. Wind shear, for example, can cause angle of attack to drastically increase or decrease. The Ercoupe is based upon limited up-elevator travel, and won't stall in a normal flight regime. But it can stall.
 
You can bet your life on the stick being an AOA control, not an indicator. Consider a power-on stall where the stick position is less aft than with power-off.
 
Kenny Phillips said:
There are tons of subtleties. Wind shear, for example, can cause angle of attack to drastically increase or decrease. The Ercoupe is based upon limited up-elevator travel, and won't stall in a normal flight regime. But it can stall.
Click to expand...

True. I did say "were considered" perhaps I should have said "were advertised"... But they can stall under the right conditions-being critical angle of attack exceeded...
 
birdus said:
True or False: The stick determines elevator position and elevator position determines angle of attack. When the critical angle of attack is exceeded, the wing stalls. So, given a design with enough elevator authority, if the stick is in your lap, then the wing is stalled. If the stick is, say an inch farther forward, the wing will not be stalled. Period.
Click to expand...

You are you saying that its not possible to stall if the stick is not all the way back?
 
dtuuri said:
You can bet your life on the stick being an AOA control, not an indicator. Consider a power-on stall where the stick position is less aft than with power-off.
Click to expand...

:yeahthat:

birdus said:
True or False: The stick determines elevator position and elevator position determines angle of attack. When the critical angle of attack is exceeded, the wing stalls. So, given a design with enough elevator authority, if the stick is in your lap, then the wing is stalled. If the stick is, say an inch farther forward, the wing will not be stalled. Period...
Click to expand...

Not true.

One can have the elevator positioned such that the nose of the airplane is pitched below the horizon, and still have a very high AOA in a steep descent.

Think of the classic stall (& subsequent spin) on the base to final turn. Do you think the "stick" is full back when that happens?
 
The Good news: You can make any airplane un-stallable by limiting up elevator authority.
The Bad news: you won't be able to do a proper flare for landing in such a plane. If you take it to the extreme, you won't be able to climb!

So
CharlieK said:
You are you saying that its not possible to stall if the stick is not all the way back?
Click to expand...
can be true depending on how far back is 'not all the way back'.
Go to altitude and try it sometime...
 
birdus said:
True or False: The stick determines elevator position and elevator position determines angle of attack. When the critical angle of attack is exceeded, the wing stalls.
Click to expand...

For 95% of the flying we do, this is basically true. But I think you're trying to imply a connection which is not right. Your first statement is true and the second statement is true, but the two statements are not connected. First is probably true, have to think harder about "determines angle of attack", there are almost certainly aerobatic situations where it isn't.

One simple example not mentioned is load factor. Do a 45 degree banked turn and your stall speed will increase by a factor of 1.18. That same stick/elevator/angle of attack that would have you blowing the stall horn when level will result in a stall at 45 degrees.
 
The original question is posed in 2 dimensions. Aircraft operate in 3 dimensions.
Angle of attack is the vector of forces in 3 dimensions. That's why a wing moving 200 mph will not necessarily stall in level flight or in a nose high attitude, but will fall out of the sky in a tight turn.
 
CharlieK said:
You are you saying that its not possible to stall if the stick is not all the way back?
Click to expand...

Of course not. Obviously, the designer could've designed in lots of excess elevator authority such that the critical angle of attach could be reached prior to full stick movement being reached.
 
Let's consider another example. Load the plane so that its center-of-gravity (CG) is aft of the CG limits. Depending on how far aft of the limits the CG is located, one may need forward stick to keep the plane flying level. Let the plane slow, in level flight, to below stalling speed in this condition. The plane stalls with the stick forward. One probably can't recover from this stall.
 
The wing can stall a many angles of attack, dependent on the speed of the relative wind, wing configuration (flaps, slats. spoilers) and G loading.

The elevator only changes the resultant angle of incidence relative to the wing.
 
Shepherd said:
The original question is posed in 2 dimensions. Aircraft operate in 3 dimensions.
Angle of attack is the vector of forces in 3 dimensions. That's why a wing moving 200 mph will not necessarily stall in level flight or in a nose high attitude, but will fall out of the sky in a tight turn.
Click to expand...

Irrelevant. Pulling back hard on the stick in straight-and-level flight is no different than pulling back hard on the stick in a bank.
 
birdus said:
True or False: The stick determines elevator position and elevator position determines angle of attack. When the critical angle of attack is exceeded, the wing stalls. So, given a design with enough elevator authority, if the stick is in your lap, then the wing is stalled. If the stick is, say an inch farther forward, the wing will not be stalled. Period.

I had a conversation with a United Airlines captain (737) the other day, and he said I was wrong. As I get older, I tend to be less argumentative, so I didn't pursue it. He said some things about different size loops (don't know what that was about). When he mentioned the Amazon crash off the coast of Texas (the overall conversation was about MCAS), I said that, in a video I had seen (months earlier), it looked like the plane may have been stalled. The descent angle appeared steep and yet the angle between the longitudinal axis of the plane and the ground appeared to be significantly shallower (i.e., stalled). He said something about the descent angle being really steep and the airspeed being really high so it couldn't have been stalled.

I really wanted to say "descent angle and airspeed are irrelevant. If the pilot put the plane into a dive, gained a ton of airspeed, and then put the stick in his gut, then they were stalled." However, I dropped it.

So, are there subtleties I'm not considering? Could the stick not act as an angle-of-attack indicator (if I could position it precisely enough)? I'll try to clarify this post if necessary.
Click to expand...

The stick alone doesn't determine angle of attack, but it does in conjunction with other factors, e.g., center of gravity, airspeed, propwash on the elevator, flap setting, etc.

Not sure how your true/false question relates to the argument with the UA captain. But if his argument that an airplane in a steep descent with a high airspeed isn't stalled he would be incorrect.
 
IK04 said:
The wing can stall a many angles of attack, dependent on the speed of the relative wind, wing configuration (flaps, slats. spoilers) and G loading.

The elevator only changes the resultant angle of incidence relative to the wing.
Click to expand...

Whhaaaat? The elevator doesn't change the angle of incidence, and the wing only stalls at one angle of attack, the critical angle.
 
birdus said:
So, given a design with enough elevator authority, if the stick is in your lap, then the wing is stalled. If the stick is, say an inch farther forward, the wing will not be stalled. Period.
Click to expand...
What happens when the plane goes inverted?
 
bflynn said:
Do a 45 degree banked turn and your stall speed will increase by a factor of 1.18. That same stick/elevator/angle of attack that would have you blowing the stall horn when level will result in a stall at 45 degrees.
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The elevator position and angle of attack aren't the same in a 45 degree bank, that's the whole point. The aforementioned fact is why the aircraft is closer to a stall.
 
So, you've a$$-raped yourself on the POA. No big deal; I admire your intellectual curiosity and the confidence to pose a thought-out question.
I might humbly add that it wasn't thought-out as much as it needed. This ad-hoc committee has clearly illustrated additional dimensions to your posited thesis: So, learn from it and expand your knowledge. Be blessed.
 
Juliet Hotel said:
What happens when the plane goes inverted?
Click to expand...

If the airfoil is symmetrical, then simply swap "lap" with "panel" and "forward" with "backward." Nothing changes. However, this is a quibble I had hoped to avoid for the sake of this conversation.
 
dmspilot said:
The elevator position and angle of attack aren't the same in a 45 degree bank, that's the whole point. The aforementioned fact is why the aircraft is closer to a stall.
Click to expand...

Wrong. Banking has nothing to do with this conversation. At a given dynamic pressure and angle of attack, the wing exerts some centripetal force on the airplane. Thus, the airspeed at which the stall occurs may be different, but as in my last comment, that has nothing to do with this conversation (I don't think). I'm concerned with stick position, elevator position, and critical angle of attack.
 
birdus said:
Wrong. Banking has nothing to do with this conversation. At a given dynamic pressure and angle of attack, the wing exerts some centripetal force on the airplane. Thus, the airspeed at which the stall occurs may be different, but as in my last comment, that has nothing to do with this conversation (I don't think). I'm concerned with stick position, elevator position, and critical angle of attack.
Click to expand...

What did I say that is wrong?
 
DaleB said:
Two-thirds of your statement are true. Elevator position is one thing that can cause your wing to exceed the critical angle of attack. Let's say I put my airplane into a fairly steep climb, return the elevator to a neutral position, and pull the throttle. What's going to happen?
Click to expand...

The plane's nose will drop (due to the center of gravity being in front of the center of lift) and the plane will gain airspeed.
 
dmspilot said:
What did I say that is wrong?
Click to expand...

"The elevator position and angle of attack aren't the same in a 45 degree bank"

I assumed you meant "The elevator position and angle of attack aren't the same in a 45 degree bank...as in straight and level flight" and they certainly very well may be. You're conflating two unrelated things. That's why it would be better just leave banking out of this conversation. It simply distracts from what we're really talking about for no reason.
 
Bobanna said:
So, you've a$$-raped yourself on the POA. No big deal; I admire your intellectual curiosity and the confidence to pose a thought-out question.
I might humbly add that it wasn't thought-out as much as it needed. This ad-hoc committee has clearly illustrated additional dimensions to your posited thesis: So, learn from it and expand your knowledge. Be blessed.
Click to expand...

This is a bizarre post. I totally love batting things around and learning. If I'm wrong and some folks here help me to learn, then I'll go away very happy. I don't consider that being "a$$ raped." If I did enough research ahead of time ("had a well though-out enough question"), then I wouldn't have posted any question here at all and we wouldn't be having this fun conversation. :)
 
birdus said:
True or False: The stick determines elevator position and elevator position determines angle of attack.
Click to expand...
False
A=B
B=/=C

Stick position helps determine the aircraft pitch, angle of attack is the angle between the wing and the oncoming air. Angle of attack is certainly heavily influenced by the stick but it's not direct as you are implying

Planes like the Cirrus, Grumman, 182 (to an extent) land with a ton of back pressure on the stick. In the Cirrus I often touchdown with the control against the back stops. The wing is not stalled, the wing is not at it's critical angle of attack. In other regimes of flight having the stick against the back stops would indeed lead to a stall, IE, you've exceeded the critical angle of attack
 
birdus said:
"The elevator position and angle of attack aren't the same in a 45 degree bank"

I assumed you meant "The elevator position and angle of attack aren't the same in a 45 degree bank...as in straight and level flight" and they certainly very well may be. You're conflating two unrelated things. That's why it would be better just leave banking out of this conversation. It simply distracts from what we're really talking about for no reason.
Click to expand...

I am not conflating anything. I responded to a post that said the plane is stalled in a 45° bank even if the angle of attack is the same as in straight-and-level-non-stalled flight. Which is ridiculous. The angle of attack is not the same. Then you replied "wrong". So you're trying to say the original poster is right? No he most definitely is not. Yet you selected my post to pick on instead of the incorrect one. How about not taking my post out of context? I quoted two sentences for a reason.
 
birdus said:
if the stick is in your lap, then the wing is stalled. If the stick is, say an inch farther forward, the wing will not be stalled. Period.
Click to expand...
The "period" is the false part.. generally this is true-ish, but there are plenty of ways to stall a wing when the stick is not in your lap, and situations where the stick may be in your lap and the wind is not stalled
 
Things like flap and power will change the pitching moment and airflow over the tail respectively, and thus, change the relation between elevator position and angle of attack which changes the stick position at a stall. Which answers the original question with a: False.
 
GRG55 said:
One can have the elevator positioned such that the nose of the airplane is pitched below the horizon, and still have a very high AOA in a steep descent.
Click to expand...

You're saying the stick can be forward and the wing can be flying at a high angle of attack? Please explain. Also, of course, horizon has nothing to do with this conversation. I'm surprised at how many people here are conflating the horizon or banking with angle of attack.

GRG55 said:
Think of the classic stall (& subsequent spin) on the base to final turn. Do you think the "stick" is full back when that happens?
Click to expand...

I never claimed that the stick had to be full back for a stall to occur. I would claim, however, that if there is ample elevator authority, then a stall is guaranteed at full back stick position, at least if we stipulate calm air (i.e., no gusts at just the right angle).
 
DaleB said:
Let's say I put my airplane into a fairly steep climb, return the elevator to a neutral position, and pull the throttle. What's going to happen?
Click to expand...

You will fly a parabolic trajectory, but you will not stall.
 
IK04 said:
The wing can stall a many angles of attack, dependent on the speed of the relative wind, wing configuration (flaps, slats. spoilers) and G loading.
Click to expand...

How does G loading affect the angle of attack at which a wing stalls?
 
IK04 said:
The wing can stall a many angles of attack, dependent on the speed of the relative wind, wing configuration (flaps, slats. spoilers) and G loading.

The elevator only changes the resultant angle of incidence relative to the wing.
Click to expand...
The wing stalls in a very narrow range of AoA in any given configuration.
 
Tantalum said:
False
A=B
B=/=C

Stick position helps determine the aircraft pitch, angle of attack is the angle between the wing and the oncoming air. Angle of attack is certainly heavily influenced by the stick but it's not direct as you are implying

Planes like the Cirrus, Grumman, 182 (to an extent) land with a ton of back pressure on the stick. In the Cirrus I often touchdown with the control against the back stops. The wing is not stalled, the wing is not at it's critical angle of attack. In other regimes of flight having the stick against the back stops would indeed lead to a stall, IE, you've exceeded the critical angle of attack
Click to expand...

I don't think your explanation really addresses my assertion/question.
 
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