Why does an aircraft stall?

Question

Why does an aircraft stall? Why does a speed of <250 Knots cause it to suddenly stall?

Answer 1

I have read the other answers and none actually really hit it completely.
A wing essentially works by creating a pressure differential between the lower and the upper surface. The upper surface is where most of the action occurs, since this is a zone of low pressure. The lower surface by opposition is a zone of higher pressure. If you want to increase lift, you can fly faster (which is usually not an option as drag overcomes you rapidly) or increase the difference of pressure between the lower and the upper surface. But there is a limit to this. You can compress air on the lower surface until it essentially stops, and you can lower the pressure on the upper surface until you nearly reach a vacuum. When the pressure on the upper surface is so low that it might as well be a vacuum, then a lot of funny stuff occurs: the airflow that was supposed to move back relative to the wing gets sucked back, this effectively determines how low the pressure can get. Once you reach that point, the lift goes away, as the lower pressure gets wasted into turbulence and eddies, creating a lots of drag, but not as much lift. We call this detached flow. At this point, it is like there is no air around, from a lift point of view; the air is no longer working for you.
When a wing is nearing stall, there may be little pockets of detached flow at some places on the wing. Usually an airplane is designed to stall some part of the wing first, to get some sort of warning, and also to provide some self recovery mechanism. Jet airliners would have a pre-set twist in the wing, so that the wing root would stall first, while the wing tip are still generating lift. If you have sweepback, losing the lift close to the fuselage means losing it in the front of the plane, so the left-over lift tries to push the nose down (since the tips are in the rear of the wing root near the fuselage), so the plane can recover. Having the wing tip still providing lift also preserve the effectiveness of the ailerons.
You can envision stall like the proverbial straw that breaks the camel back. An airplane slows down? The dynamic pressure (from forward speed) contribution to the lift has to be balanced by an increase in the angle of attack, and thus in the coefficient of lift. But ask too much, and the air "snaps off", just like putting that last ounce of pressure in a rope will make it snap.
The stall speed is thus highly variable, and depends on many factors. The aircraft altitude (higher means less air density, the stall speed will be higher, all other things being equal), the weight of the airplane (the same airplane will stall at a higher speed if it is heavy), maneuver (pulling "g" is equivalent to being heavier), contamination (ice on a wing decreases the effective lift coefficient) and so on.

Answer 2

First of all, let me explain a couple of terms.

Chord: a straight line connecting the leading edge of the wing to the trailing edge.

Angle of attack: the angle between the relative wind and the chord of the wing. (It is NOT the angle between the chord of the wing and the horizon!).

With those two definitions out of the way we can talk about what a stall is. For the wing to generate lift the air must flow smoothly over the surface of the wing. Up to a point, increasing the angle of attack increases lift. There is a point, however, when the angle of attack is so great that the relative wind can't "turn the corner" and stay attached to the wing. When this happens the airflow "separates" from the top of the wing causing a reduction in lift. When this condition gets to the point where the wing can't produce enough lift to support the weight of the airplane the airplane "stalls". The angle at which this occurs is called the critical angle of attack.

So, an airplane stalls whenever it's critical angle of attack is exceeded. There's a number of ways we can do this. The most common are flying too slowly or making a very steep turn while trying to hold altitude.

One quick point I'd like to make. We often talk about the term stall "speed". This term isn't exactly correct because an airplane can stall at any speed. The stall is dependent ONLY on angle of attack. Since most airplanes don't have angle of attack indicators we're forced to use speed as a "poor man's angle of attack indicator". We can do this because in 1G level flight there is a correlation between speed and angle of attack. The slower you go the higher the angle of attack must be to generate the lift necessary to support the weight of the airplane (remember that lift increases with an increase in angle of attack - up to a point). When manufacturers talk about stall speed they load the airplane up to maximum weight and see at what airspeed the critical angle of attack is exceeded. They label this as the "stall speed" of the airplane. Just keep in mind that the actual speed at which an airplane stalls is dependent on flap position, weight, load factor (are you pulling G's?), and how much power you have applied (particularly with multi-engine props).

Another very general rule is that the further back you're holding the stick (or wheel) the closer you are to a stall - just something to think about so it doesn't catch you by surprise.

Now we're in a position to answer your question. If you're stalling at 250 knots I imagine you're flying a high performance airplane too slowly during landing (a wing that is designed for high speed doesn't do so well at producing lift at low speed and has a much higher stall speed). I think if you lower flaps and/or slats you should be able to get the stall speed reduced substantially.

If this doesn't answer your question, let us know what kind of plane you're talking about and what position the flaps are in.

Hope that helped.

Answer 3

Airspeed has nothing to do with a stall as Charles G stated.
A stall is simply disrupting the airflow across the wing. Lift is created by the airfoil (wing's shape from the side) creating a high pressure zone below the wing (by allowing the air to flow straighter), and a low pressure zone above the wing (by forcing the air to move up and back down the wing, increasing speed, and decreasing pressure). The two zones want to equallize, so lift occurs.
When the angle of the wing to the relative wind (angle of attack), exceeds it's maximum designed angle, the airflow is disrupted in moving over the wing and lift ceases and your wing is now stalled.
Notice I said wing and not aircraft, some people think that the engine stalls much like a car when it stalls. It is the wing and can be recovered from easily, if the pilot has the proper training.

Answer 4

An aircraft can stall at any airspeed if the critical angle of attack is exceeded. A Sudden or abrupt pull on the stick can increase the load factor on the wings, effectively increasing stall speed.

If the load factor is increased by 2 times (2G's) the wings will have to support 2 times the weight before. Therefore it may stall an aircraft. An other example of a high speed stall is during a steep turn. At 60 degrees of bank there is about 40 knots increase in airspeed. So if you normally stall at 100 knots at level flight, you stall at 140 with bank of 60 degrees.

Answer 5

A stall in an airplane is when the airflow over the top of the wing is too slow, or is disrupted. This means the wing is no longer lifting the plane and the plane falls. And every type of aircraft has a different stall speed. And that speed with also change depending if the flaps are down or not.

Answer 6

to understand why a wing stalls, you have to first understand how a wing generates lift. when you push the wing through the air, it forces air downward, thus using newtons law to push the wing upwards. the more air you can accelerate down, the more lift you can generate. now when you push the wing through the air, you create drag as well as lift. as air flows over the top of the wing, it tends to separate about 40% of the way down the wings chord. this air separation is what creates drag. the more drag you have the more lift you need to maintain flight. now this is where bernoulli's principle comes in, as the angle of attack increases, the point where the airflow separates on the top of the wing starts earlier. this creates more drag, until you overcome the lift created by the wing, and the wing then stall, or quits supporting the airplane. one more thing, the wing does not stall all at once, in most cases, the exception being planes with eliptical wings like the spitfire where the wing was designed to stall across the wing all at once. wings usually stall at the root first as this is the safest place for the average pilot to handle a stall. a tip stall is dangerous because it is unpredictable.

Answer 7

Airplane Stall

Answer 8

Well it's the same thing in a horizontal direction as well. If the aircraft is not flying fast enough, the airflow over the wings will not be enough to keep the airplane aloft and it will stall the same way. A plane merely stalls quicker when climbing because speed is bled off more quickly. It's the same process though.

Answer 9

the aircraft doesnt stall like a car.. in fact a stalled aircraft has nothing to do with the engines losing power.. i think of a stalled aircraft like a car trying to climb up a steep icey hill.. if the angle of the hill is steep enough the car will not beable to climb the hill therefor it will just spin the tires and start sliding down the hill... same thing happens with an airplane.. when all the lift over the wings is depleted, it will begin to fall out of the sky..

Answer 10

First of all, all planes have different stalling speeds. But to answer your question, when the aircraft is moving that slow, there is not enough air-flow over the wings to create lift. You can also stall by flying too high. Air is less dense up there=not enough air over wings=stall. Same sort of principal.