What causes stalls?

Most wings stall at the root first and then moves to the tip.

The base turn to final is arguably one of the most dangerous turns in aviation. If a pilot goes wide on final some may be tempted to bank more than 20 degrees to correct. The increased bank also increases load factor raising stall speed. If the pilot tries to maintain altitude by pulling back on the yoke/stick in the turn the angle of attack is increased. Done low and slow a stall/spin entry will lead to disaster.
 
Ryanb said:
It's not absolutely wrong. @ktup-flyer stated that if both thrust and lift are taken away, the aircraft will stall (which is true), not one or the other as you made it seem. If both are reduced, the aircraft's AOA will increase trying to maintain sufficient altitude, which will induce a stall.
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Nothing wrong with Ron's post. Point the nose at the Earth directly below and you won't ever stall. ;)

In any case the concept of "taking away lift" is nonsensical in this context. How do you do that? Remove the wings? Baring that the only way I know of to transition to zero lift while in flight is to reduce the AoA to the zero lift point which is as far as you can get from a stall. And the only time thrust or drag has a direct effect on AoA and stalling is when the thrust vector is angled off the horizon and that effect is pretty small relative the lift produced by the wing.
 
Ryanb said:
Yes it's called gliding, but the point was, that if both factors are removed and the AOA is not reduced, a stall will occur.
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Your reply suggests you're confusing AoA with pitch attitude. If you're AoA is less than the critical angle and you reduce lift without changing airspeed you have just reduced the AoA. If you reduce thrust and maintain the exact same AoA there will be on stall. How could there be a stall if the AoA was and still is below the critical angle?
 
citizen5000 said:
Most wings stall at the root first and then moves to the tip.

The base turn to final is arguably one of the most dangerous turns in aviation. If a pilot goes wide on final some may be tempted to bank more than 20 degrees to correct. The increased bank also increases load factor raising stall speed. If the pilot tries to maintain altitude by pulling back on the yoke/stick in the turn the angle of attack is increased. Done low and slow a stall/spin entry will lead to disaster.
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The load factor is only increased if the pilot tries to maintain altitude or slow the descent by pitching up at the same time. Otherwise, the steeper bank will cause the sink rate to be higher (loss of the vertical component of lift) but the load factor shouldn't change much. In a case where load factor really doesn't change the only hazard to a steep bank in the traffic pattern is losing too much altitude, not stalling.
 
This aircraft IS a stall.

TH-LEGACY-IMAGE-ID-470-horse-in-airplane.jpg
 
skier said:
As long as everyone is being pedantic here I want to clairify that in my understanding CG does not affect the stall speed. CG location affects the aircraft's stability. Specfically, a center of gravity too far aft will cause the plane to become unstable while a center of gravity too far forward will result in the pilot running out of elevator travel before fully flaring.
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Stall speed does go up a hair with forward CG. The tail has to push down harder to keep the nose up, and that downforce has to be countered by more lift from the wings. A slight increase, but it's there.
 
bflynn said:
This seems a good argumentative question for the board

We all know that stalls occur when the airfoil exceeds the critical angle of attack. But how does that happen? What affects it? Under what conditions can a stall happen when the airplane is not already at low speed? How can the airplane stall at speeds between Vso and say 1.2 times Vso?
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Taking a crack at the original question:

In mathmatical terms lift, dynamic pressure (indicated airspeed) and AoA are inseparately related as long as the AoA is in the range from the zero lift angle to a little less than the critical angle of attack. IOW if two are fixed you cannot change the third. That relationship is defined by the equation:

Lift = AoA * pressure * K where K is a constant.

Or trading Pressure for IAS^2 (dynamic pressure is proportional to the square of your indicated airspeed):

Lift = AoA * IAS^2 * K

How does this relate to actual flying? Lift has to be equal to the airplane's weight multiplied by the g-load. When you pull back on the yoke or stick the extra weight you feel is caused by an increase in lift which in turn was caused by an increase in the AoA. If the AoA was near the critical AoA (which never changes unless you modify the wing by moving flaps or contaminating the it with ice etc.) that increase in AoA could take you to that critical angle and wallah, you've got a stall.

Looking at the second equation it should be obvious that at lower indicated airspeeds the AoA must increase in order to generate the same amount of lift. And equally obvious is the fact that the slower you go the less "room" is left on the AoA vs lift line between the AoA at that lower speed and the critical AoA. Thus if you are flying so slow that you're only a few degrees shy of the critical AoA you don't have to pull very hard on the yoke or hold your vertical speed while banking more than a little you will exceed the critical AoA and stall the wing.

I think Ron already mentioned that when the wing stalls it doesn't stop producing lift completely, in fact it doesn't even drop the lift in half. What happens at the stall break is that the reduction in lift causes the airplane to accelerate vertically (this is what you feel when "the bottom drops out") and that in turn magically reduces the AoA just enough that even though the wing remains stalled, the AoA is actually just a tiny bit higher than the critical angle and all it takes to terminate the stall is a slight additional decrease in AoA.

All that assumes that you've increased the AoA gradually enough that you haven't pitched up appreciably. If you start way below the critical and pull back hard ("whip stall") your angular momentum on the pitch axis makes the AoA go much higher than the critical AoA and you need a much greater change in pitch attitude to get back below the critical angle.

AoA is hard to visualize because you can't see or sense the direction or magnitude of the relative wind. If you get a chance to fly an airplane with an AoA indicator try some maneuvers, changes in airspeed and g-load, plus a few different stalls to see what the wing is really doing.
 
citizen5000 said:
Most wings stall at the root first and then moves to the tip.

The base turn to final is arguably one of the most dangerous turns in aviation. If a pilot goes wide on final some may be tempted to bank more than 20 degrees to correct. The increased bank also increases load factor raising stall speed. If the pilot tries to maintain altitude by pulling back on the yoke/stick in the turn the angle of attack is increased. Done low and slow a stall/spin entry will lead to disaster.
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There's much more to it than that. Look at this:

lesson9figure02.gif


A 20 degree bank increases the stall only a tiny bit, 3 or 4 percent, maybe. A 30-degree bank increases it by about 8 percent. So if you're at 1.5 Vso on base, you have 50% to play with. A long ways from that 4 or 8%.

The base-to-final turn kills when pilots cross-control the airplane, trying to rudder it around without banking further. They have to use opposite aileron to stop secondary roll, and with the inside aileron down, the outboard section of that wing is at a higher AoA than the outside wing, and if the airplane gets anywhere near the stall, it will spin because that inside wing will stall first. Add in the helical differential between the inside and outside wings in a descending turn, even though it's a small fraction of a degree, and the inclination to spin is enhanced.
 
skier said:
As long as everyone is being pedantic here I want to clairify that in my understanding CG does not affect the stall speed. CG location affects the aircraft's stability. Specfically, a center of gravity too far aft will cause the plane to become unstable while a center of gravity too far forward will result in the pilot running out of elevator travel before fully flaring.
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CG also affects the airspeed at which the stall occurs. In a C172RG there is a 6 knot difference between most forward and most rearward with zero flaps and a 3 knot difference with 30 degrees flaps
 
Pulling up in a turn increases the load on the wing. Also interested in this. There's some good postings on the BackCountry Pilot site about canyon turns, avoiding stalls in tight turns at high DA.
 
citizen5000 said:
Most wings stall at the root first and then moves to the tip.

The base turn to final is arguably one of the most dangerous turns in aviation. If a pilot goes wide on final some may be tempted to bank more than 20 degrees to correct. The increased bank also increases load factor raising stall speed. If the pilot tries to maintain altitude by pulling back on the yoke/stick in the turn the angle of attack is increased. Done low and slow a stall/spin entry will lead to disaster.
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The base to final turn

WLzq_f-maxage-0.gif
 
citizen5000 said:
The base turn to final is arguably one of the most dangerous turns in aviation. If a pilot goes wide on final some may be tempted to bank more than 20 degrees to correct. The increased bank also increases load factor raising stall speed. If the pilot tries to maintain altitude by pulling back on the yoke/stick in the turn the angle of attack is increased. Done low and slow a stall/spin entry will lead to disaster.
Click to expand...

If you are squeamish about banking more than 20° that's probably why you overshot final in the first place. In fact it is apprehension of steeper bank angles that causes the pilot to skid the airplane with inside rudder that causes the stall/spin in the first place.

In many airplanes even a 45° bank does not increase stall speed above a typical approach speed.
 
GlennAB1 said:
Factor in air density.
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luvflyin said:
I wouldn't think that would make a difference. Unless your getting your airspeed indication from something other than an air pressure device like a pitot tube.
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Factor it into what? As luv implied, air density affects the stall speed in TAS, but not IAS/CAS, since the latter already is affected by air density as equally as the wing is affected by air density.
 
dmspilot said:
Factor it into what? As luv implied, air density affects the stall speed in TAS, but not IAS/CAS, since the latter already is affected by air density as equally as the wing is affected by air density.
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Factor, as in: taking into account, not necessarily: factor in mathematically.
The fact that most of you will never command an aircraft where it is a factor, doesn't mean it isn't a factor in aviation.
 
GlennAB1 said:
Factor, as in: taking into account, not necessarily: factor in mathematically.
The fact that most of you will never command an aircraft where it is a factor, doesn't mean it isn't a factor in aviation.
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That doesn't answer the question. Air density is a factor in aviation. I think everyone knows that. Can you be more specific or are you being vague on purpose?
 
So, what you are saying is: every response in this thread is a factor and everbody knows it, so why post any of it. No one mentioned air density, so I did, and its true. There are plenty of references to be found if you're so inclined.
The fact that high performance aircraft have equipment that takes altitude/density into consideration to reduce error and pilot workload doesn't change the fact that it's a factor.
 
GlennAB1 said:
So, what you are saying is: every response in this thread is a factor and everbody knows it, so why post any of it. No one mentioned air density, so I did, and its true. There are plenty of references to be found if you're so inclined.
The fact that high performance aircraft have equipment that takes altitude/density into consideration to reduce error and pilot workload doesn't change the fact that it's a factor.
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No, I am asking a question. You said air density is a factor. I asked what it is a factor in, and your answer was that air density is a factor in aviation. The question is how does this relate to stalls, which is the topic of the thread? What equipment are you talking about that uses air density to "reduce error and pilot workload" and how does it do that? And how does THAT relate to stalls?
 
dmspilot said:
If you are squeamish about banking more than 20° that's probably why you overshot final in the first place. In fact it is apprehension of steeper bank angles that causes the pilot to skid the airplane with inside rudder that causes the stall/spin in the first place.

In many airplanes even a 45° bank does not increase stall speed above a typical approach speed.
Click to expand...

If you need more than 20 degrees in the base-to-final turn then you may be too close to the runway on downwind.
 
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dmspilot said:
No, I am asking a question. You said air density is a factor. I asked what it is a factor in, and your answer was that air density is a factor in aviation. The question is how does this relate to stalls, which is the topic of the thread? What equipment are you talking about that uses air density to "reduce error and pilot workload" and how does it do that? And how does THAT relate to stalls?
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Air data computer, stall warning computer, they take raw information and compute, providing accurate information to the flight deck that otherwise would take time, and distract from the primary responsibility of flying the aircraft.
 
citizen5000 said:
If you need more than 20 degrees in the base-to-final turn then you may be too close to the runway on downwind.
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I think 30 degrees allows for a tighter and more squared pattern and is more practical.
But to each his/her own, I suppose.
 
luvflyin said:
CG also affects the airspeed at which the stall occurs. In a C172RG there is a 6 knot difference between most forward and most rearward with zero flaps and a 3 knot difference with 30 degrees flaps
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Interesting. It's always good to learn something new. Thanks!
 
GlennAB1 said:
Air data computer, stall warning computer, they take raw information and compute, providing accurate information to the flight deck that otherwise would take time, and distract from the primary responsibility of flying the aircraft.
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A computed stall warning is a real bad idea.

Every one I've ever seen has been some sort of direct AoA measurement, sometimes rudimentary. The conventional light aircraft warnings measure flow velocity on the leading edge, essentially a stagnation point measurement closely related to AoA. Some larger aircraft use vanes or dual port probes.
 
GlennAB1 said:
Factor, as in: taking into account, not necessarily: factor in mathematically.
The fact that most of you will never command an aircraft where it is a factor, doesn't mean it isn't a factor in aviation.
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Hmm. I've heard of this thing called compressibility I think it was. At real high speeds things change from what we know down lower and slower. Is that, or something like that what you were talking about?
 
The Air Data Computer (ADC) measures pitot and static pressures and converts them to digital signals for use with a glass panel. It can also do things such as calculate TAS which of course is dependent upon air density. However, I can't think of a way this helps with stall awareness or prevention.

This stall warning computer apparently measures AOA, aircraft configuration, and altitude and airspeed, possibly because of what one source calls Mach Number Effect which can reduce the stall AOA due to shockwaves.
 
MAKG1 said:
A computed stall warning is a real bad idea.
Every one I've ever seen has been some sort of direct AoA measurement, sometimes rudimentary. The conventional light aircraft warnings measure flow velocity on the leading edge, essentially a stagnation point measurement closely related to AoA. Some larger aircraft use vanes or dual port probes.
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All of the aircraft I've worked on, the stall warning fed into some computer, be it the Gulfstream 159 with the wing lift trasducer and flap position fed into the lift computer, or the F4 where AOA fed into the central air data computer along with pitot-static and other inputs, like transport aircraft.

My statement was "Factor in air density."

Argue with this guy:
"TAS where the stall occurs increases with altitude because of the lower air density"
Found here:
http://www.experimentalaircraft.info/flight-planning/aircraft-stall-speed-1.php

The graph in the link below shows the stall speed increase due to altitude for the specific aircraft and possibly why computed data is so important.
https://en.wikipedia.org/wiki/Coffin_corner_(aviation)
 
GlennAB1 said:
All of the aircraft I've worked on, the stall warning fed into some computer, be it the Gulfstream 159 with the wing lift trasducer and flap position fed into the lift computer, or the F4 where AOA fed into the central air data computer along with pitot-static and other inputs, like transport aircraft.

My statement was "Factor in air density."

Argue with this guy:
"TAS where the stall occurs increases with altitude because of the lower air density"
Found here:
http://www.experimentalaircraft.info/flight-planning/aircraft-stall-speed-1.php

The graph in the link below shows the stall speed increase due to altitude for the specific aircraft and possibly why computed data is so important.
https://en.wikipedia.org/wiki/Coffin_corner_(aviation)
Click to expand...

You set your speeds by TAS? Really?

When you use CAS like you're supposed to, the density effect goes away.

It's one of those things where the literal statement is true, but the application is off the deep end.
 
citizen5000 said:
If you need more than 20 degrees in the base-to-final turn then you may be too close to the runway on downwind.
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You must be flying a huge pattern.

Guessing you're not a fan of rolling out from base to final into a slip ether?
 
LDJones said:
Answer to OP: Pilots. Pilots cause stalls. Period.
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Usually when I let the clutch out too fast. ;-)

citizen5000 said:
If you need more than 20 degrees in the base-to-final turn then you may be too close to the runway on downwind.
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Key word being MAY.

James331 said:
You must be flying a huge pattern.

Guessing you're not a fan of rolling out from base to final into a slip ether?
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Be nice now. LOL. Down boy. Sit!

exncsurfer said:
DPE's and CFI's usually cause stalls, 'give me a power off stall, straight ahead'.

:)
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Engine failures too. So annoying. LOL.
 
GlennAB1 said:
My statement was "Factor in air density."
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Your statement was vague and without context.

GlennAB1 said:
Argue with this guy:
"TAS where the stall occurs increases with altitude because of the lower air density"
Click to expand...
Already addressed in post 96. There is no argument against the quoted statement, but it is of dubious relevance to the conversation.

You want to say that air density is important because if you have an air data computer that presents to you TAS that your stall speed in TAS will change. You would have to convert that back to IAS. Who does that? On aircraft equipped with an ADC, the TAS would be presented in addition to, not instead of, IAS. You're not going to convert from IAS to TAS and then try to convert that back to IAS to figure out what your stall speed is. Are you trolling? I think so.
 
I think @GlennAB1 has a point. I would say an approach to turn stall would be much more likely to happen on a high DA day simply because you may be expecting more performance. For anyone who's maneuvered up in the flight levels, its obvious that it is way easier to stall the aircraft. You want the aircraft to do something, you expect it to do something but it just won't do it. That situation in an overshoot at very high DA could give the sensation of speed, but lack the performance.
 
Cooter said:
I think @GlennAB1 has a point. I would say an approach to turn stall would be much more likely to happen on a high DA day simply because you may be expecting more performance. For anyone who's maneuvered up in the flight levels, its obvious that it is way easier to stall the aircraft. You want the aircraft to do something, you expect it to do something but it just won't do it. That situation in an overshoot at very high DA could give the sensation of speed, but lack the performance.
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IAS should keep this from happening. Where DA might bite you in your example might be from ground reference issues, no? Say your IAS is normal but it looks like you're too fast out the window due to higher ground speed due to DA. ?
 
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