A stall is what happens to a wing when it produces less lift with an increase in the angle of attack (AOA). First, lets go back and have a quick review of lift. Bernoulli's theorem states that as the velocity of a mass of air increases, it's pressure decreases. The wing is shaped so as to accelerate the air passing over the top of it and create a low-pressure area above the wing. As the AOA increases, the air flow is accelerated more, the pressure above the wing gets lower and the wing produces more lift. Unfortunately, you never get anything for nothing and there are two problems with increasing the AOA. First, the drag increases (but that is another story and I will talk about it latter) and secondly as the AOA increases, the air flow eventually will break away from the surface of the wing and become turbulent, the low pressure area diminishes and the wing is stalled.
Lets look at the graph below showing the amount of lift produced by a typical wing at various angles of attacks.
There are a couple of interesting things about this graph. First of all, it is a straight line. There is a liner increase in lift as the AOA increases. Secondly, there is some lift produced at 0 AOA. This is because the wing is curved; a flat plate would produce no lift at 0 AOA. Our airfoil stalls at 15 AOA and notice that there is still lift produced pass the stall, only the amount of lift now decreases with an increase in AOA. Now the one important lesson from this graph is this: A wing always stalls at the same Angle of Attack, not at the same speed. You can stall a wing at any speed simply by pitching up very fast. People always talk about the stalling speed of an airplane as if it was a fixed value. Lets look at the lift formula and see where this misconception comes from.
|Lift = CL * ½ P * V2 * S|
|CL = Shape of Airfoil and the Angle of Attack|
|P = Air Density|
|V = Airspeed|
|S = Surface area of the wing|
Since the shape of the airfoil, air density and surface area are all
constants; changes in lift must come from a combination of AOA and airspeed. To maintain
level flight, a decrease in speed must be offset by an increase in AOA. The slowest speed
that an airplane can fly is the speed that when combined with an AOA of just below the
stalling angle, generates the required amount of lift. Now lets say that we fixed the AOA
at a value of just below the stall (14.90 in our pretend airfoil) and only varied speed in
order to generate the lift required to counter the planes weight in level flight. If the
weight increases, more lift is required and more speed is required. Another important
lesson: The stalling speed of an airplane increases as the weight of the plane increases.
When you take off with a full tank of gas, your stalling speed is higher than when you
land with an almost empty tank.
The stalling speed of an airplane is also higher in a turn, let me explain. As you go around a corner you pull G forces. Remember as a kid in the playground, there was the spinning ride and the faster it went around, the harder it was to hold on to until you were eventually thrown off. As you went around in circles, your G loading increased and your weight also increased. Sitting on the ground, a plane experiences one force of gravity. When a plane is banking in a turn it's G loading increases. In a 60 angle of bank turn a plane is pulling 2 G's and in a 75 angle of bank turn the plane has 4G's. For the techno whizzes out there the formula for G loading is "G = 1 / cos (angle of bank)". Now if the G loading on an airplane increases, its weight increases and its stall speed increases. Therefore we arrive at our last important lesson: The stalling speed of an airplane increases as the angle of bank increases.
Lets talk about the movement of the Center of Pressure (COP) before and after the stall. The Center of Pressure is the theoretical point on the wing in which all of the lift is centered. It is used for balance and also to understand the pitching moments of the plane. As a wing approached the stalling angle, the Center of Pressure moves forward causing an upward pitching moment. Past the stalling AOA, the Center of Pressure moves backward causing a nose down pitching moment.
Finally, let's talk about a spin. A spin is nothing more than a stall with rotation. You enter a spin by stalling during a turn or by applying rudder at the moment of stall. A plane in a spin will fall vertically in a slight nose down attitude while rotating about its vertical axis. The important thing to remember in a spin is that the wings are stalled and the ailerons are useless. Spin recovery is fairly simple as long as you stay calm and do everything in the correct order.