Stalls 101

Jaybird180

Jaybird180

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Jaybird180
Stall question

Does an airplane always stall (1G) at the same airspeed (power-on/off/partial)? I know that it will always stall at the critical AoA, but does this occur at the same speed since AoA is related to airspeed (assuming other constant factors except engine power)? If not, why is stall data published as a set speed (Vs0, Vs1, etc)? If so, then why even mention critical AoA when talking about stalls? Why do manufacturers publish Vs0/Vs1 at a specific speed rather than a range of speeds (depending upon W&B)?

Final question, why do my P-Off stalls occur at a speed lower than the indicator can read reliably (C-172)? I’m expecting full stall break at 33kts Vs0 and would like to do my own experiments but have to wait until the break is just about to occur then quickly snatch the yoke to get the break. What can I do to get a proper stall?
 
Jaybird180 said:
Does an airplane always stall (1G) at the same airspeed (power-on/off/partial)?
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No. The more power, the more thrust you have, and at high angles of attack (where thrust is angled down, not straight aft), the vertical component of the thrust vector supports some of the aircraft's weight. That leaves less weight to be carried by lift, and with less lift required, you can go slower before you reach the critical AoA.

If not, why is stall data published as a set speed (Vs0, Vs1, etc)?
If so, then why even mention critical AoA when talking about stalls? Why do manufacturers publish Vs0/Vs1 at a specific speed rather than a range of speeds (depending upon W&B )?
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Because that's what the FAA requires. OTOH, if they were to publish more comprehensive data, it could become both overwhelming and confusing.

Final question, why do my P-Off stalls occur at a speed lower than the indicator can read reliably (C-172)?
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Because the wing has such good stall resistance that the AoA is so high the flow of air into the pitot tube is distorted by the large angle of the tube to the airflow.

I’m expecting full stall break at 33kts Vs0 and would like to do my own experiments but have to wait until the break is just about to occur then quickly snatch the yoke to get the break. What can I do to get a proper stall?
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Fly something other than a C-172?

BTW, if they installed AoA gauges in all our planes, all this would be much easier to understand.
 
And, for what it's worth, if you are at a speed anywhere in the vicinity of a stall, your eyeballs should be where they should always be, outside! My aircraft has VGs and once had gap strips (and will again, dagnabbit!). I had no idea at what speed it stalled. I really didn't know until I had a CFI do an arrival stall and I got to watch the airspeed. 35 mph indicated on that occasion!

The idea (as I understand it) is to recognize the onset of a stall by the aircraft's behavior, not through the pitot static system.
 
Steingar makes a good point. Your best clue of a developing yaw/slice is visual reference to ground/clouds/etc -- you'll see small yaw rates there long before you'll see them on the instruments. During stalls (and even slow flight at Vmca), stopping those yaws with rudder before they develop further is the easiest way to avoid roll-offs and spins.
 
To add another question here:

The plane will always stall at the critical AoA, but will it stall at an AoA less than the critical? Since a higher weight requires a higher AoA, is it just that it takes a higher speed to maintain the critical AoA?
 
To add another question here:

The plane will always stall at the critical AoA, but will it stall at an AoA less than the critical? Since a higher weight requires a higher AoA, is it just that it takes a higher speed to maintain the critical AoA?
 
jasc15 said:
The plane will always stall at the critical AoA, but will it stall at an AoA less than the critical?
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No. Stall is defined by the disruption of airflow that occurs at a particular AOA.

Since a higher weight requires a higher AoA, is it just that it takes a higher speed to maintain the critical AoA?
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I think you're nibbling around the edge of understanding. Here's the deal...

Lift is directly proportional to the product of lift coefficient (CL, which is directly related to AOA) and the square of velocity (V^2). Stall occurs at the critical AOA. Just before stall, the lift coefficient (CL) reaches its maximum, and then drops off (hence, the stall). As weight goes up, so does the amount of lift required to counter weight. Likewise, so does the speed required to produce that much lift at any given AOA (since CL is constant at any given AOA).

In addition, the slower you go, the more CL (and thus AOA) you need to maintain enough lift. However, if you're lighter, you don't need as much lift, and that means a lower AOA at the same speed as when you are heavier. So, the lighter you are, the slower you can go before you hit the critical AOA. Likewise, the heavier you are, the sooner you'll reach critical AOA while decelerating.

Got it?
 
488px-LiftCurve.svg.png

From wikipedia http://en.wikipedia.org/wiki/File:LiftCurve.svg
 
Thanks for the chart, Dan -- that provides a good background for the discussion.

And just to make sure...that's a generic chart. Each airfoil's CL vs AOA chart will have a different slope/shape. But it does give you an idea of what the curve looks like and how it all goes together.
 
Jaybird180 said:
If not, why is stall data published as a set speed (Vs0, Vs1, etc)? If so, then why even mention critical AoA when talking about stalls? Why do manufacturers publish Vs0/Vs1 at a specific speed rather than a range of speeds (depending upon W&B)?
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Stall speed is weight dependent as well. With the small spam cans the difference between gross wt stall and empty weight is not nearly as significant...

For a C5M... it could be very significant.

But.. angle of attack is the constant in all this. Unfortunately the average spam can doesnt have an AOA meter.
 
The thing that's usually confusing someone new to this notion is they're still thinking of how airplanes normally fly - upright, level more or less, banked, etc - and not fully connected yet with the fact that the AOA is with respect to the relative wind and not your angle with respect to the ground. Big difference. An airplane can be stalled at any attitude and at any airspeed; i.e. pointed straight at the ground at 120 kts.
 
Ron Levy said:
Got it?
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This is how i understood it, but I wanted to be careful before I say that you can't stall at an AoA less than critical. As long as the wing is below the critical AoA, it will generate lift. I'm an engineer, and I understand fluid mechanics, and some aerodynamics. That being said, there are things I've learned in my flying education that hadn't immediately occurred to me despite my education.
 
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PittsDriver said:
The thing that's usually confusing someone new to this notion is they're still thinking of how airplanes normally fly - upright, level more or less, banked, etc - and not fully connected yet with the fact that the AOA is with respect to the relative wind and not your angle with respect to the ground. Big difference. An airplane can be stalled at any attitude and at any airspeed; i.e. pointed straight at the ground at 120 kts.
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Took me a LONG time to get my head around the bolded concept above.
 
Doggtyred said:
Stall speed is weight dependent as well. With the small spam cans the difference between gross wt stall and empty weight is not nearly as significant...

For a C5M... it could be very significant.
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The change is proportional to the difference in weight, though... So a 152 that is suddenly sans a 167-lb CFI in the other seat would be akin to removing 84,000 pounds from that C-5! (Wow.)

You probably wouldn't notice the difference from the start to the end of the flight due to fuel burn in a 152 much compared to the C-5, though. :hairraise:
 
jasc15 said:
This is how i understood it, but I wanted to be careful before I say that you can't stall at an AoA less than critical. As long as the wing is below the critical AoA, it will generate lift. I'm an engineer, and I understand fluid mechanics, and some aerodynamics. That being said, there are things I've learned in my flying education that hadn't immediately occurred to me despite my education.
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If you want to pick a nit or two, the critical angle of attack as we usually describe it is the value for steady state flow. There are some dynamic effects since it takes some time for the flow pattern (or circulation if you prefer) to develop / collapse.
 
Well, isn't that what the stall is: the formation of circulation and separation in the airflow?
 
If you really want to get into the fine details of aerodynamics, the CL vs AoA curve will also vary with temperature (Reynolds number effects) which could shift the critical angle of attack slightly.
 
jasc15 said:
Well, isn't that what the stall is: the formation of circulation and separation in the airflow?
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A stall is an exceeding the angle of attack for maximum lift. For a real wing that does not stall everywhere along the span at the same time, some flow will have separated before the "stall".

Circulation is one method to model how lift is generated. The formation of circulation would coincide with the start of lift. As the wing stalls, circulation breaks down.
 
And sinc
jasc15 said:
This is how i understood it, but I wanted to be careful before I say that you can't stall at an AoA less than critical. As long as the wing is below the critical AoA, it will generate lift. I'm an engineer, and I understand fluid mechanics, and some aerodynamics. That being said, there are things I've learned in my flying education that hadn't immediately occurred to me despite my education.
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One small but important part of what occurs during a stall is that once you reach the critical AoA (where a further increase in AoA means less lift at the same airspeed) the fact that the airplane wing begins to descend (or increases the descent rate) causes the AoA to increase further if the pitch attitude is held constant. This is most of what generates the "bottom falling out" feeling that some folks mistakenly attribute to a sudden total loss of lift. And once you're AoA is (slightly) higher than the critical AoA, in order to terminate the stalled condition you have to increase the airspeed (dynamic pressure) to something greater and/or reduce the wing loading more than needed to prevent a stall in the first place.
 
gismo said:
And sinc

One small but important part of what occurs during a stall is that once you reach the critical AoA (where a further increase in AoA means less lift at the same airspeed) the fact that the airplane wing begins to descend (or increases the descent rate) causes the AoA to increase further if the pitch attitude is held constant. This is most of what generates the "bottom falling out" feeling that some folks mistakenly attribute to a sudden total loss of lift. And once you're AoA is (slightly) higher than the critical AoA, in order to terminate the stalled condition you have to increase the airspeed (dynamic pressure) to something greater and/or reduce the wing loading more than needed to prevent a stall in the first place.
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Doesn't the CP shifting aft cause the front of the wing to "fall" thus causing the "drop" sensation?
 
nosehair said:
What do you mean by this? Are you trying to 'model' a checkride demonstration? Are you trying to duplicate a textbook example?
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Textbook demonstrated stall.

While I'm at it, what am I hearing about an accelerated stall and a negative G stall? Can someone explain those and how you make them happen?
 
Jaybird180 said:
While I'm at it, what am I hearing about an accelerated stall and a negative G stall? Can someone explain those and how you make them happen?
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Accelerated stall simply means stalling the airplane at greater than 1G. During training, this is typically done by stalling the airplane while in a level turn, since bank angle necessitates a load greater than 1G. Recovery is no different, just relax the elevator and continue flying. But you could also do an accelerated stall at any attitude, including recovering from a normal stall. Your CFI might call that a secondary stall, and those are typically accelerated. Do not do accelerated stalls above Va. The point of accelerated stalls is to illustrate the fact that the plane can stall at any airspeed (up to Va). Above Va, you might actually break something with excessive G before you actually exceed critical AOA and stall the airplane.

Negative G stall is no different from a positive G stall, it's just that you'll have to push forward to stall the airplane rather than pulling back. Highly unlikely you'll be doing this in a non-aerobatic airplane that lacks inverted fuel and oil. Negative G stalls are typically associated with aerobatic spin training. In theory, you could do a negative G stall in a 172 without doing aerobatics by pitching up to 30 degrees, waiting for the airspeed to drop below Va, and applying full down elevator. In reality, I have a feeling some trainer types may not actually have enough down elevator to produce a stall. But in any case, I don't know many people who would want to do this in a non-aerobatic airplane since it will throw loose items around the cockpit, cause the engine to momentarily quit, and probably dump some oil onto the belly of the plane. I'd save those for aerobatic airplanes.
 
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jasc15 said:
This is how i understood it, but I wanted to be careful before I say that you can't stall at an AoA less than critical. As long as the wing is below the critical AoA, it will generate lift.
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It still generates some lift above critical AoA (at least for a little bit above) -- see the curve, above. However, since the CL is lower, and speed is no higher (and, due to drag, probably lower), it's not enough to support the aircraft's weight, so down you go.
 
dmccormack said:
Doesn't the CP shifting aft cause the front of the wing to "fall" thus causing the "drop" sensation?
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That's part of what precipitates the pitch down but the "drop" is the result of the wing generating less lift than needed to support the weight of the plane. The point I was trying to make is that lift doesn't go away completely the instant the critical AoA is exceeded (the top of the curve is fairly rounded) and if you could mange to keep the AoA barely above critical, there wouldn't be much of a falling sensation. Instead the initial/slight "drop" becomes magnified as that initial slight drop increases the AoA, causing a further loss of lift, which increase the AoA more...
 
dmccormack said:
Doesn't the CP shifting aft cause the front of the wing to "fall" thus causing the "drop" sensation?
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~~~~ I thought the CP shifted forward w/ an increase in AOA? not aft?

Does it suddenly shift aft at or just above the critical AOA?
 
whifferdill said:
Accelerated stall simply means stalling the airplane at greater than 1G. During training, this is typically done by stalling the airplane while in a level turn, since bank angle necessitates a load greater than 1G. Recovery is no different, just relax the elevator and continue flying. But you could also do an accelerated stall at any attitude, including recovering from a normal stall. Your CFI might call that a secondary stall, and those are typically accelerated. Do not do accelerated stalls above Va. The point of accelerated stalls is to illustrate the fact that the plane can stall at any airspeed (up to Va). Above Va, you might actually break something with excessive G before you actually exceed critical AOA and stall the airplane.

Negative G stall is no different from a positive G stall, it's just that you'll have to push forward to stall the airplane rather than pulling back. Highly unlikely you'll be doing this in a non-aerobatic airplane that lacks inverted fuel and oil. Negative G stalls are typically associated with aerobatic spin training. In theory, you could do a negative G stall in a 172 without doing aerobatics by pitching up to 30 degrees, waiting for the airspeed to drop below Va, and applying full down elevator. In reality, I have a feeling some trainer types may not actually have enough down elevator to produce a stall. But in any case, I don't know many people who would want to do this in a non-aerobatic airplane since it will throw loose items around the cockpit, cause the engine to momentarily quit, and probably dump some oil onto the belly of the plane. I'd save those for aerobatic airplanes.
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That... Was a GREAT answer. :yes: :thumbsup:
 
In theory, you could do a negative G stall in a 172 without doing aerobatics by pitching up to 30 degrees, waiting for the airspeed to drop below Va, and applying full down elevator.
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I read one of Bill Thomas' books where he described doing this in (I think it was) a C-150. He said he went to zero airspeed without stalling and won a bet with someone. I guess he didn't go to down elevator quick enough to stall.

Reading some of the posts here, I am a little confused what some of the questions are. It looks like some others may be too. Can you be a little more specific in what you are asking? Not an instructor, but when you stall in a power-off or power-on stall and the nose drops, that's not from the wing but from the tail per aircraft design. Maybe (at least I think) I am not understanding exactly what you are asking.
 
somorris said:
Not an instructor, but when you stall in a power-off or power-on stall and the nose drops, that's not from the wing but from the tail per aircraft design.
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Actually, it is from the loss of lift from the wing. If the tail stalled first, you really wouldn't like what happened next.
 
whifferdill said:
snip... Do not do accelerated stalls above Va. The point of accelerated stalls is to illustrate the fact that the plane can stall at any airspeed (up to Va). Above Va, you might actually break something with excessive G before you actually exceed critical AOA and stall the airplane.,,,snip
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I'm glad someone pointed this out! Good answer, whifferdill!
 
somorris said:
I read one of Bill Thomas' books where he described doing this in (I think it was) a C-150. He said he went to zero airspeed without stalling and won a bet with someone. I guess he didn't go to down elevator quick enough to stall.
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Yep, he would NOT have been able to achieve zero indicated airspeed if he had stalled the airplane going up. He probably just pulled into a very steep climb, neutralized the elevator, and let the airplane follow a natural parabolic trajector with zero indicated at the top...or just did it the way I always do - with a hammerhead. :)
 
nosehair said:
What do you mean by this? Are you trying to 'model' a checkride demonstration? Are you trying to duplicate a textbook example?
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Apropos a "proper" stall, my FBO has 2 Cherokees, which are almost identical (one has a slightly more powerful engine). The w/b charts are very similar. One performs stalls easily, noses over (also tends to drop left wing, which needs to be caught early). The other one I can never get to do proper stalls, either power on or power off. It just starts sinking, and will sink as long as back pressure is maintained. The only way to make a semblance of crisp stall is to dive and zoom. I don't think examiner is going to appreciate the technique if I do that. I'm kinda starting to suspect that it's somehow tail-heavy or something. But on the other hand, it's quite simmetric and does not bank a lot in a stall. This used to bother me quite a bit, but my instructor rated my stalls as acceptabe, and I decided to explain it all to examiner if I get the "reluctant" plane for checkride.
 
zaitcev said:
Apropos a "proper" stall, my FBO has 2 Cherokees, which are almost identical (one has a slightly more powerful engine). The w/b charts are very similar. One performs stalls easily, noses over (also tends to drop left wing, which needs to be caught early). The other one I can never get to do proper stalls, either power on or power off. It just starts sinking, and will sink as long as back pressure is maintained. The only way to make a semblance of crisp stall is to dive and zoom. I don't think examiner is going to appreciate the technique if I do that. I'm kinda starting to suspect that it's somehow tail-heavy or something. But on the other hand, it's quite simmetric and does not bank a lot in a stall. This used to bother me quite a bit, but my instructor rated my stalls as acceptabe, and I decided to explain it all to examiner if I get the "reluctant" plane for checkride.
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PA28's don't tend to have a "break" on the stall - In fact, I would suggest that something is wrong with the one that does... Maybe mis-rigged somehow (which might also explain the wing drop).

On a checkride, don't worry about getting a break on the stall - If you're in the Cherokee, when you get the yoke all the way back, that's all it's going to do - Say something to that effect, and recover. Done. I would consider the stall to have happened when you begin losing altitude. An examiner has seen enough stalls in PA28's that they'll know how it behaves, and know it won't break. No big deal.
 
zaitcev said:
The other one I can never get to do proper stalls, either power on or power off. It just starts sinking, and will sink as long as back pressure is maintained. The only way to make a semblance of crisp stall is to dive and zoom. I don't think examiner is going to appreciate the technique if I do that.
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I agree, since that is verging on a "whipstall," which I believe is prohibited in the Cherokee AFM.

I'm kinda starting to suspect that it's somehow tail-heavy or something.
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More likely nose-heavy, resulting in having inadequate elevator (well, "stabilator") authority to get to the true stall point.

...my instructor rated my stalls as acceptabe, and I decided to explain it all to examiner if I get the "reluctant" plane for checkride.
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Don't bother with the explanation unless the examiner says something. Just get that yoke all the way back, hold it as much of a stall as you can get, and recover. All should be fine.
 
flyingcheesehead said:
PA28's don't tend to have a "break" on the stall - In fact, I would suggest that something is wrong with the one that does... Maybe mis-rigged somehow (which might also explain the wing drop).

On a checkride, don't worry about getting a break on the stall - If you're in the Cherokee, when you get the yoke all the way back, that's all it's going to do - Say something to that effect, and recover. Done. I would consider the stall to have happened when you begin losing altitude. An examiner has seen enough stalls in PA28's that they'll know how it behaves, and know it won't break. No big deal.
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So true -- older C172s can be the same. My Chief barely breaks -- just mushes. 36' wings lifting 1150 lbs will do that.

Somewhere along the line I read/heard that the C152 was designed with a more pronounced break for training value. OWT or true?
 
Jeanie said:
~~~~ I thought the CP shifted forward w/ an increase in AOA? not aft?

Does it suddenly shift aft at or just above the critical AOA?
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The AoA does shift forward as angle of attack increases, as the boundary layer breaks up toward the trailing edge and lift starts to disappear over the aft area of the wing. But in the stall itself, the whole top surface becomes turbulent and the CP moves aft again.

Dan
 
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