Question

Explain and discuss what stall is and what causes an airfoil to stall. Why are commercial aircraft not allowed to fly at conditions near stall?

Answer 1

What is stall?

Stall is an adverse phenomenon in which aircraft wings experience amplified air resistance and declined lift. It can cause an airplane to crash.

Stall occurs when a plane is under very high angle of attack (the angle of attack is the angle between the plane and the direction of flight). It may occur during take-off or landing, when the flight speed is moderately low: at low speed the aerodynamic forces are compatibly smaller, and the only way to get enough lift to carry the weight of the plane is to fly the plane at greater angle of attack.

Due to the stall the wing produces less lift and more drag; the increased drag causes the speed to reduce further so that the wing produces even less lift. In effect, the plane falls out of the air and hits the ground, which happens due to stall.

What causes an airfoil to stall?

It is due to processes within the boundary layer, the layer of retarded air near the surface of the wing (airfoil). In order for a wing to be effective, the air must flow completely around the leading (front) edge of the wing. At too high angle of attack, the air in the boundary layer near the leading edge does not succeed in that and it separates from the wing surface.

When the angle of attack becomes too big, the boundary layer vortices will get separated from the top surface of the wing, and the incoming flow does no longer bend completely around the leading edge. The wing will be stalled, causing a momentous drag.

Why commercial aircrafts are not allowed to fly at conditions near to stall?

When an airplane stalls, it’s no longer able to generate lift. This isn’t due to a mechanical problem, such as engine failure. Rather, airplanes experience stalls when the angle at which they enter the wind current is greater than the critical angle of attack. When this occurs, there’s an inadequate amount of air traveling under the airplane’s wings to keep it up. Consequently, the airplane will drop, thereby reducing its altitude, until the angle of attack is correctly adjusted.

While most stalls are attributed to an angle of attack that’s greater than the critical angle of attack, this phenomenon may also occur if the airplane’s speed is too low. With that said, the pitch of an airplane can also affect whether airspeed will cause a stall. An airplane gaining altitude at a high pitch may stall at a lower airspeed than an airplane flying horizontally at a flat pitch. Hence, commercial aircrafts are not allowed to fly at conditions near to stall.