Theory of Flight - The stall
Welcome to the stall page, one of the most talked about things in aviation - The stall. This page is aimed at introducing you to the basic characteristics of a stall, what causes them and how to overcome them. There are so many different view and opinions on what a stall actually is. I will just explain the basics of it. Hope you learn something new from this page!
What causes a stall?
An aircraft is considered to have stalled when the airflow no longer follows the shape of the wing and separates completely from the upper surface of the wing. This causes a significant reduction in the coefficient of lift. Stalling amongst aircraft typically happens at the angle of 15°.
Once the aircraft has reached the angle of 15° it stalls. Before the aircraft reaches this critical angle of attack (AOA) the aircraft will experience the onset effects of a stall. Turbulent air will be forming above the wing as it starts the separate altogether from the wing. This turbulent air above the wings will cause buffeting on the aircraft. Usually pilots can easily identify this sort of buffeting and immediately drop the nose. If not and the aircraft reaches the AOA of 15° then the stall has happened and the air separates from the upper part of the wing altogether.
Ok so getting a little bit more technical now, what exactly happens during a stall? As the AOA is increased the centre of pressure(cp) gradually moves forwards. At the stalling angle of 15° the cp moves aft which isn't very good. In order to get the cp forward again, the nose has to come down in order to start producing lift again. In other words, the AOA must be decreased. To achieve this we need to re-energise the boundary layer and start producing laminar flow again. The boundary layer is the layer of air directly in contact with the aircrafts wing surfaces with some degree of friction being created.To increase the boundary layer, the AOA must be decreased, or an enormous amount of thrust must be generated to force the air over the wing.
The angle of attack is the angle between the chord line and the reletive air flow. The most efficient angle of attack where the aerofoil section wing is viewed to be where the lift force exceeds the drag forces by the largest multiple. This angle is between 3° and 4°. At 15° the aircraft will stall, lift is destroyd and drag will increase.
How to overcome stalls?
Once a stall has occured then your first thing to do is reduce the AOA. With the AOA currently at 15° the aircraft cannot sustain flight since lift has been destroyed. So you need to point the nose down to decrease the AOA. By doing this you are re-energising the boundary layer and getting air flow over the wings again to produce lift and reduce drag.