Question

If an airfoil experiences boundary layer separation, what specific change occurs in the pressure distribution that causes the stall?

Accepted Answer

Boundary layer separation causes stall by creating a dramatic loss of suction pressure on the upper surface of the airfoil. To understand this, first consider that lift is primarily generated by low pressure created over the top of the wing as air flows smoothly across it. The boundary layer is a thin layer of air immediately touching the wing surface that stays attached due to friction and viscosity, which is the resistance of a fluid to flow. When an airfoil reaches a critical angle of attack, the air no longer has enough kinetic energy to travel against the rising pressure gradient toward the trailing edge, causing the airflow to detach from the surface. Once this separation occurs, the smooth, low-pressure flow is replaced by a chaotic, stagnant wake of recirculating air. Because this separated air cannot maintain the high-velocity, low-pressure condition required for lift, the pressure on the upper surface rises significantly, moving toward ambient levels. This collapse of the low-pressure zone results in a sudden loss of lift and a massive increase in pressure drag, which is the resistance force acting opposite to the direction of motion. The redistribution of pressure, specifically the loss of the suction peak near the leading edge and the pressure rise across the top of the wing, is the direct physical mechanism that defines the stall.

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If an airfoil experiences boundary layer separation, what specific change occurs in the pressure distribution that causes the stall?