Stall (fluid dynamics)

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack exceeds its critical value. The critical angle of attack is typically about 15°, but it may vary significantly depending on the fluid, foil – including its shape, size, and finish – and Reynolds number.

Stalls in fixed-wing aircraft are often experienced as a sudden reduction in lift as the airflow separates from the upper surface, which may be due to an increase in the wing's angle of attack past the critical angle or a decrease in the critical angle of attack. The former may be due to slowing down (below stall speed), the latter by accretion of ice on the wings. A stall does not mean that the engine(s) have stopped working, or that the aircraft has stopped moving—the effect is the same even in an unpowered glider aircraft.

Because stalls are most commonly discussed in connection with aviation, this article discusses stalls as they relate mainly to aircraft, in particular fixed-wing aircraft. The principles of stall discussed here translate to foils in other fluids as well.

Formal definition

A stall is a condition in aerodynamics and aviation such that if the angle of attack on an aircraft increases beyond a certain point, then lift begins to decrease. The angle at which this occurs is called the critical angle of attack. If the angle of attack increases beyond the critical value, the lift decreases and the aircraft descends, further increasing the angle of attack and causing further loss of lift.