Yoke deflection, load factor & stall

Richard_SWave said:
Ok, I thought The AoA between both wings during a coordinated turn is not equal, which causes a wing-drop during a coordinated turning stall is enough to induce an incipient spin.

Actually I've done few spins during my PPL training, but far not addressed so detailed. Actually it was more my theoretical lack of understanding here, practicing them it would be hard to figure out exactly what causes what.
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Which airplane for the spin training?
 
flyingron said:
Nope. A whip stall would cause the nose to continue to rise and increase AOA.
It's not what you described in your message.
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I asked questions about whip stalls years ago here. Interesting subject. Since that time, I've accidentally done a few and didn't like being on the end of the whip.
 
Ok, back to this assertion, which I agree with as to effect, but not to the principle cause:

Dan Thomas said:
Other way around. A skidding stall drops the inside wing.

In a descent, the inside wing is already at a higher AOA even when the airplane is coordinated. Skidding it increases that AoA difference, making the inside wing stall much sooner than the outside wing and leading to the spin.

Go to this thread http://www.pilotsofamerica.com/forum/showthread.php?t=40070&highlight=AoA+table&page=4 and scroll down to post #93 to see the pictures.

Dan
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Dan Thomas said:
The outer wing in a climbing turn is at a higher angle of attack. This isn't intuitive, any more than the higher AoA on the inside wing in a descending turn. ...

Dan
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​
Clearly, this is of more interest in Canada (http://www.avcanada.ca/forums2/viewtopic.php?f=3&t=50570) than in the US, which piques my curiosity all the more: Are Canadians smarter? :eek:
EDIT: Reading their forum, they are more polite.

Following the links, I came across this table which, assuming the math is correct, shows only one-thirty-thousandths of a degree difference (.030) between wingtip AoA (in a climb vs. descent too) at 500 fpm and 100 kts:
Granted, it would be more in magnitude near the stalling speed, but then it would be half as much, too, because the average difference isn't calculated at the wingtips.

I doubt pilots are skillful enough, or the airplanes are constructed precisely enough, or the airframes are rigid enough to demonstrate the difference in stalling behavior attributed to this theory. It may be true, but is there much more? Methinks there is.

dtuuri
 
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dtuuri said:
Following the links, I came across this table which, assuming the math is correct, shows only one-thirty-thousandths of a degree difference (.030) between wingtip AoA (in a climb vs. descent too) at 500 fpm and 100 kts:Granted, it would be more in magnitude near the stalling speed, but then it would be half as much, too, because the average difference isn't calculated at the wingtips.

I doubt pilots are skillful enough, or the airplanes are constructed precisely enough, or the airframes are rigid enough to demonstrate the difference in stalling behavior attributed to this theory. It may be true, but is there much more? Methinks there is.

dtuuri
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I didn't check the accuracy of your source but I ran the numbers several years ago and came to the same basic conclusion as long as the bank is fairly shallow (the difference in turn radii increases at least exponentially with bank angle). Also worth noting, there's potentially a degree or two of difference between the wings angle of incidence so it's quite possible that the inside wing actually has less AoA than the outside one depending on the direction of the turn.

But probably most important fact here is that the actual aerodynamics in a turn are fairly complex when you include the nuances of aileron position, dihedral, vertical CG, wing washout, prop slipstream, and numerous other factors. When you try to explain a situation as complex as this with simple analogies and models, you are pretty much guaranteed to make some incorrect assumptions that affect the result. And IME the errors in that result will be overwhelmed by the inaccuracies in the actual control of the airplane combined with air movement (e.g. turbulence, rising air, etc) so it's really kinda moot beyond the entertainment value. If you understand the effects of the controls and their limitations (e.g. the typical inability to raise a stalled wing with aileron) you can recover from any stall (in an airplane designed to recover normally) by making appropriate movements of the controls and that (how to react with proper control movement) is pretty much what you really need to know when learning to fly.
 
dtuuri said:
Following the links, I came across this table which, assuming the math is correct, shows only one-thirty-thousandths of a degree difference (.030) between wingtip AoA (in a climb vs. descent too) at 500 fpm and 100 kts:

I doubt pilots are skillful enough, or the airplanes are constructed precisely enough, or the airframes are rigid enough to demonstrate the difference in stalling behavior attributed to this theory.
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At 100 knots, and what bank angle? At 100 knots the radius of the turn, which is what's important here, is large unless we're steeply banked. Turning final at 65 knots and 30° bank is a different story. And at 100 knots a 500 fpm descent is also rather shallow compared to the same rate at 65 knots.

I did note, I think, in the explanation of those pictures, that the AoA differences are exaggerated so as to be visible and because it's difficult to create such a demonstrator that's 20 feet across. The principle applies: there is an AoA difference in a turn when climbing or descending, and a skidding turn is asking for trouble in that descending turn because it makes the difference larger.

I used to teach the various stall/spin scenarios. A skidding-turn stall was a very intense experience for a student, especially in a less-forgiving airplane such as a Champ or Citabria, and it made them wiser and much more careful.

Dan
 
Dan Thomas said:
At 100 knots, and what bank angle?
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According to the post from which the table in the PDF is linked it's a standard rate turn.


Dan Thomas said:
And at 100 knots a 500 fpm descent is also rather shallow compared to the same rate at 65 knots.
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Good point. More detail is required. Also, the steeper the bank, the less difference in radius; at 90° they're the same.

Dan Thomas said:
I did note, I think, in the explanation of those pictures, that the AoA differences are exaggerated so as to be visible and because it's difficult to create such a demonstrator that's 20 feet across. The principle applies: there is an AoA difference in a turn when climbing or descending, and a skidding turn is asking for trouble in that descending turn because it makes the difference larger.
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Yes, but your model compares the aft edge of one wing to the leading edge of the other--not a fair comparison, since the relative wind comes more from above the leading edge and more from below the trailing edge when in a turn. It's only tangent at the CG.

Dan Thomas said:
I used to teach the various stall/spin scenarios. A skidding-turn stall was a very intense experience for a student, especially in a less-forgiving airplane such as a Champ or Citabria, and it made them wiser and much more careful.
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Me too, but I never needed your explanation to explain why the plane behaves as it does. My logic came from Kirshner's Advanced Pilot's Manual. There, a diagram indicated a side-slip induces turbulence that initiates the partially blanketed wing to stall first. Either way, we agree that it does, but I'd like to know for sure exactly why it happens. If you're right, I'd like better evidence, that's all. :)

dtuuri
 
dtuuri said:
Yes, but your model compares the aft edge of one wing to the leading edge of the other--not a fair comparison, since the relative wind comes more from above the leading edge and more from below the trailing edge when in a turn. It's only tangent at the CG.
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I'm not sure what you're getting at, there. Does anyone else?

The steel rails represent relative wind, and the airplane's wings represent airplane wings. A helical flight path results in an AoA difference between left and right wings. You could fool with the positioning of the wing on the rail. but it doesn't change the geometry of the difference in the descent angles of the different radii.

More opinions:
http://www.pprune.org/archive/index.php/t-393855.html

http://www.avcanada.ca/forums2/viewtopic.php?f=3&t=50570

http://www.langleyflyingschool.com/Pages/CPGS 4 Aerodynamics and Theory of Flight Part 1.html

http://www.caa.govt.nz/fig/basic-concepts/medium-climbing-and-descending-turns.html

Dan
 
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