What causes stalls?

bflynn

bflynn

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Brian Flynn
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?
 
Airflow separation. You should be able to google up some videos of wing stall tests that use tufts of yarn to illustrate airflow. I've seen several from the development of vortex generator applications.
 
bflynn said:
We all know that stalls occur when the airfoil exceeds the critical angle of attack. But how does that happen?
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Normally the pilot causes it to happen.

bflynn said:
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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Stall speed increases in proportion to load factor.

Student pilot stuff, there are even questions on the written test about it; not sure how there is any room for argument.
 
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We got to test different airfoils in a small wind tunnel for one of my thermofluids labs when I was getting my engineering degree. Pretty cool to see the airflow separation at the critical angle of attack
 
Lack of interest on the part of the wing.
It develops too much "attitude", and thinks "screw this" and decides not to cooperate.
Given enough time, it gets over it's little snit, gets with the program, and starts flying right again.
Sometimes the behavior is so egregious it becomes self destructive, with serious, or even fatal, results.
 
bflynn said:
Under what conditions can a stall happen when the airplane is not already at low speed?
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Briskly apply full up elevator at any speed below Va (adjusted for weight), and the airplane will stall.

Briskly apply a vertical gust of the appropriate velocity below rough air speed (adjusted for weight), and the airplane will stall.
 
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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Running the treadmill backward without an aoa
 
When I take people flying for their first GA flight, I explain stalls as the point where the airplane transforms into a brick, and starts falling.

From an aerodynamic point of view, I would simply describe a stall as the point where the wing geometry as it relates to the relative airflow is such that it nonlinear produces enough lift to keep the airplane flying.
 
dmspilot said:
Normally the pilot causes it to happen.

Stall speed increases in proportion to load factor.

Student pilot stuff, there are even questions on the written test about it; not sure how there is any room for argument.
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Absolutely should be student pilot stuff. And yet loss of control remains the number one cause of GA accidents. I would wager that not a single one of those who had an accident intended to do it. None of them walked up to their airplane and said "You know, today I think I'll try out that loss of control thing, just for kicks".

My thought is that there are really just two causes
- flying too slow so that the wing develops an attitude problem and says "forget this flying stuff, I'm out of here"
- load factor surprises a pilot who is already flying slow.

Anything else?

Yes, I know that you can intentionally stall an airplane at almost any speed, but I think those are rarely surprises.
 
One of the major causes of low altitude stall/spin accidents are from "buzzing". A pilot attempts to show off to people on the ground, flies low, pulls too many G's. & the rest is history.

Other common ones: Divided attention during the approach & taking off beyond the density altitude of the aircraft. It will fly in ground effect giving a false sense that it's going to climb, then stalls out.

Yes, the major cause is the nut behind the wheel.
 
Stewartb said:
Airflow separation. You should be able to google up some videos of wing stall tests that use tufts of yarn to illustrate airflow. I've seen several from the development of vortex generator applications.
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Yup, there is some good stuff out there.

In a nutshell, around half the wing's lift comes from suction on the top surface. Separate the flow, and the suction is on other parcels of air rather than the wing, so it doesn't help you. A stalled wing does NOT fall freely; it has some lift. Just not as much as you might like. And separation in general doesn't occur everywhere at once, so some stalls are "deeper" than others.
 
Well

Weight
CG
G loading
Contamination
Bank angle
Etc
 
Exceeding critical angle of attack causes airflow separation and loss of lift which can occur at any speed.

Stall speed increases in proportion to the square root of the load factor. This means that an airplane with an unaccelerated stalling speed of 50 knots can be stalled at 100 knots by inducing a load factor of 4 G's. A steep base turn to final may produce excess load factors and stall speed resulting in a wing stall close to the ground.
 
Well, of course, as you say, when the critical AOA is exceeded. The quick answer is ANYTHING that results in the critical AOA being exceeded.

More specifically, and generally speaking, the otherwise laminar flow across the wing becomes more and more disturbed until, at the CAOA, it can tolerate no more loss of the flow. This is called the coanda effect and makes for interesting reading.

tex
 
Go do coordinated maneuvers at MCA. Get into a 30* turn and raise the nose a little while stepping on the rudder a little. Really easy to do when distracted, right? What happens? You've landed a thousand times. You can do it in your sleep, right?

What might lead a pilot into that scenario? Gusts? Wake turbulence? I barking dog or crying kid in the back? A passenger who grabs or steps on a control without thinking in order to adjust their posture? Any combination of these? Stuff happens during accidents that most of us can't know or understand.
 
Ya know... I've stayed out of the "AoA" running gag, 'cause I didn't get in at the start and it was competently handled by others. :) But...

IMHO, a lot of what makes flying difficult to understand at first is that what we WANT is a direct indication of angle of attack. The wing stalls at a particular AoA.

But, since we don't have an AoA indication, we have to infer it from other indications. IAS, "load factor", etc.
 
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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Stewartb nailed it. In normal flight the boundary layer (that invisible sheet of air closest to the skin of the upper surface of the wing) adheres to the wing surface. As the angle of attack nears the critical angle the boundary layer begins to separate from the wing surface at the trailing edge. As the situation gets worse the separation point moves forward until it reaches the center of lift, at which time lift goes away and the wing is stalled. Many desiigns have stall strips attached to the leading edge to control where the stall begins; others have vortex generators on the upper surface of the leading edge for much the same purpose.

www.youtube.com/watch?v=f8Soy6C9Ynw

Bob
 
Stewartb said:
Go do coordinated maneuvers at MCA. Get into a 30* turn and raise the nose a little while stepping on the rudder a little. Really easy to do when distracted, right? What happens? You've landed a thousand times. You can do it in your sleep, right?

What might lead a pilot into that scenario? Gusts? Wake turbulence? I barking dog or crying kid in the back? A passenger who grabs or steps on a control without thinking in order to adjust their posture? Any combination of these? Stuff happens during accidents that most of us can't know or understand.
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Do you fly around the pattern at MCA?

I really don't like hearing the stall warning at any point before the flare. I generally fly the pattern at least as fast as 1.3*Vs0.

It's a nice demonstration, but to explain an incipient spin in the pattern, it would have to be fast enough to avoid the stall warning, not at MCA.
 
bflynn said:
And yet loss of control remains the number one cause of GA accidents.
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Let's not succumb to Maui's propaganda. "Loss of control" is a broad category, and includes many types of accidents other than stalls and spins.
 
bobmrg said:
... Many designs have stall strips attached to the leading edge to control where the stall begins ...
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That's exactly what they do, but I think their purpose is to cause that portion of the wing to stall earlier than it otherwise might,leaving the wing near the ailerons unstalled.
 
dmspilot said:
Let's not succumb to Maui's propaganda. "Loss of control" is a broad category, and includes many types of accidents other than stalls and spins.
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Which also includes spatial disorientation. An AoA indicator isn't going to save you in that case.
 
"That's exactly what they do, but I think their purpose is to cause that portion of the wing to stall earlier than it otherwise might,leaving the wing near the ailerons unstalled."

Their PURPOSE is to maintain attachment of the airflow to the top of the wing at higher AOA. That allows a higher controllable AOA (slower minimum speed), better aileron control at slow speeds above the stall, and coincidentally, at "normal" pattern speeds the airplane usually has a flatter attitude. I like VGs. My new plane's wings have slats and VGs. Better yet. This wing can fly at very slow speeds and the airplane turned by skidding it with rudder and there's no stall. Not that I prefer to fly that way but the safety factor for slow ops is much improved over conventional wings. Very cool stuff.

Washout is what makes the wing stall differentially.
 
MAKG1 said:
Do you fly around the pattern at MCA?

I really don't like hearing the stall warning at any point before the flare. I generally fly the pattern at least as fast as 1.3*Vs0.

It's a nice demonstration, but to explain an incipient spin in the pattern, it would have to be fast enough to avoid the stall warning, not at MCA.
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Do I? Sometimes, yes. If you want to operate into really short and tight spots the airplane has to be slowed. As long as there's altitude to trade for airspeed it's no big deal. The bigger stall threat comes when trying to get out of those spots. For new guys? That's the difference between power off and power on stalls. Power off are easy to manage. Power on when close to the ground and climbing over obstacles? Not as forgiving.
 
airdale said:
Stick and Rudder: An Explanation of the Art of Flying 1st Edition
by Wolfgang Langewiesche
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So much this. Sure you can find the scientific explanation elsewhere, but what this book does differently is it helps you to build an intuitive mental model of how a wing flies and stalls. I didn't realize how much I was confusing attitude with angle of attack until I started reading this book.
 
Take away thrust and lift or add weight and drag. You'll eventually stall.
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I think the think that always required me to contemplate the scientific-side of the stall was related to load factor. It's easy to see how getting slow and having less airflow over the wings can cause a stall (i.e. slow flight, approach/departure stalls). However, I was always much less confident in knowing how hard I could pull on the elevator in a 45/60-degree bank before I got close to the critical AOA that would cause a stall, which is generally likened with to the "overshooting the base-to-final turn" scenario. Practicing stalls while straight-n-level is easy, and the recovery is relatively simple as well. I don't know of anyone who practices steep-turn accelerated entry to stalls, aside from aerobatic pilots, which makes recognition and avoidance much more difficult without AoA or even G-load instruments as a visual reminder.
 
SoonerAviator said:
I don't know of anyone who practices steep-turn accelerated entry to stalls, aside from aerobatic pilots, which makes recognition and avoidance much more difficult without AoA or even G-load instruments as a visual reminder.
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I know I did not. But my examiner for my private asked me to do an accelerated stall on my check ride.
 
To me the interesting question is what events precipitate a pilot stalling their aircraft. I understand what a stall is, and I suspect the vast majority of pilots do, too. But what I don't understand it how pilots get themselves tripped up to let it happen. Obviously, something is occurring that some pilots are not aware of that is catching them by surprise. What is it? Sure, overloading the wings by pulling too hard when turning base to final due to overshooting is one way. But it can't be the only one.
 
ktup-flyer said:
Take away thrust and lift or add weight and drag. You'll eventually stall.
View attachment 52222
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ABSOLUTELY WRONG. Take away thrust and you'll slow down. Take away lift and you'll descend. Neither in itself is stall.

A stall is defined as the point where an increase in angle of attack no longer results in an increase in lift. Another fallacy is all lift goes away during a stall. Again, that's not true either. If it were true, you'd not be able to spin, you'd just drop like a rock. In fact the list vs. angle of attack curve looks pretty symmetrical about the critical angle. The big problem is drag is greatly increasing when you get the AOA up that high as well).

Another fallacy is airspeed determines the stall. Not true. Only angle of attack matters. An airplane can be sitting at zero airspeed and not stalled and it can be stalled at very high airspeeds. The so-called stall speed is the point where if you hold that speed, you'll reach the stall AOA trying to maintain sufficient lift for level flight.
 
flyingron said:
ABSOLUTELY WRONG. Take away thrust and you'll slow down. Take away lift and you'll descend. Neither in itself is stall.
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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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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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Precisely, it is not the speed or any of the forces that make the stall, it's increasing the AOA that causes the stall.
Why stall "speeds" are important is they are a yardstick that says "you'll stall inceasing AOA trying to maintain altitude at a speed less than this."
 
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