terms, a stall is defined as when an airfoil
) is no longer producing enough lift to counterract the weight of the airplane
- a wing can simultaneously produce lift and stall! As the most basic example, an aircraft sitting on the ground and not moving is likely not producing much lift, unless there's a hurricane
or a tornado
passing through. It would be considered "stalling."
The most common sense of the word "stall," however, is in reference to when the angle of attack
of a wing is so steep that the wing can no longer support the weight of the aircraft while in flight. This is most commonly caused by travelling slowly (so the aircraft is falling faster than it is flying forward; therefore the angle of attack is greater; therefore stall)
or when a new student decides to buzz his girlfriend's house (he pulls up too quickly; the angle of attack increases dramatically; the wing stalls; the plane crashes into the girlfriend's house)
. It is an important concept to realize that a stall can occur at any airspeed
Because this imperitive fact
is pretty hard to wrap your head around, I'll explain it one more time.
For a wing to produce lift, and to keep the aircraft flying in the air, you need to push the wing through the air at a certain speed and at a certain angle. "Certain angle," you ask? If you mounted the wings on an aircraft facing upwards, and the plane started pulling forward, you would not
get any lift. This is because the wind is hitting the bottom of the wing, and isn't producing any lift. It seems like common sense.
Now put a regular airplane
(with normal straight-forward wings) at 10,000 feet and drop it straight down (while the nose is pointed straight and level). It too does not produce lift, becuase the wind is, again, hitting the bottom of the wing as the airplane does a sort-of belly flop
The perfect angle a wing can be at is usually within 7 degrees of the centerline of the aircraft. If you were to angle the wing up, say, 15 degrees, you would most likely still retain enough lift to keep flying, but it wouldn't be the most effecient. At about 20 degrees angle, however, the aircraft is so ineffecient at producing lift that it won't fly at all.
Now this is where it all ties together. If you take an airplane cruising along at 100 knot
s at 10,000 feet
, and you cut the speed back to about 50 knots... The wings are still producing SOME lift, but the plane starts to descend. Now the wind is hitting the wing at an angle - the aircraft is moving forwards and
downwards. The angle of the wind on the wing is about 15 degrees. If you were to cut the speed back even further, so that the wind angle is about 20 degrees... Just as in the above example, the wing produces no more lift and the aircraft will start to plummet straight down. In addition, an aircraft travelling at 100 knots that suddenly changes pitch
by more than 20 degrees will also lose all lift in the same way.
If you still don't get it, you can always learn how to fly
professionally and they'll learn you good.
Also keep in mind that all the figures I used were for an imaginary aircraft somewhat similar to a Cessna 172
; the real numbers will be somewhat different or even wildly different depending on the aircraft
Now, that big long explanation above is only what a stall does and when it happens, but not what a stall really is
. While flying through the air, a wing has a thin film of compressed air around the surface that doesn't move very much. It's a millimeter
or less thick and is called the Laminar
surface of the wing, or a Laminar air flow
. This laminar layer
is very small but also very smooth - so smooth that it actually improves the wing's surface and reduces drag
. As the Angle of Attack
increases on the wing, there is less air pressure
on the wing - and the laminar layer slowly starts seperating from the wing, starting from the back of the wing and moving forward.
When the laminar layer seperates from the wing's surface, the airflow continues to follow the laminar layer instead of the wing. What this means is that if the laminar layer starts seperating from the wing halfway down the wing, you lose half of your lift
. In the rear half of the wing, between the laminar layer and the wing, the space of air there is now a vaccuum
and it sucks air in from all directions. This causes turbulence
and further reduces lift. Not the big turbulence that makes a plane shake up and down and causes you to lose your lunch, just a tiny bit of turbulence that you as a passenger wouldn't notice.
In fig. 1 below, you can see that a wing travelling straight into the wind has a good laminar layer touching the wing at all points. This wing is producing a lot of lift.
Figure 2 below is supposed to be a slightly angled wing, but I'm no good at ASCII art
. You can see how the laminar layer
starts to seperate because the wing isn't angled exactly into the airflow. The wing still produces lift, just not as much.
In figure 3 you can see that a massively angled wing (either put there by a sudden motion or by lowering the airspeed
) loses it's laminar layer even more than halfway up - this wing would stall. There simply isn't enough airflow
over enough of the wing to produce lift.
Legend: .... = laminar airflow
---- = wing surface
( ( = wind direction
To recover from a stall, it's pretty simple - put the wings back in a position that is into the wind. This is usually most easily accomplished by pointing the nose towards the ground in severe circumstances (such as a spin
) or just pressing forward on the control column to reduce that angle of attack (like fig. 3 above).
If you are approaching stall speed, it's a good idea to avoid turning or using the rudder too much. By turning the aircraft, less lift is devoted to actually keeping the plane in the air and some of that energy is applied into making you turn. This will make you stall faster and may result in a spin
if one wing stalls before the other!