Theory of Operation
The Gurney flap increases the maximum lift coefficient (CL,max), decreases the angle of attack for zero lift (α0), and increases the nosedown pitching moment (CM), which is consistent with an increase in camber of the airfoil. It also typically increases the drag coefficient (Cd), especially at low angles of attack, although for thick airfoils, a reduction in drag has been reported. A net benefit in overall lift to drag ratio is possible if the flap is sized appropriately based on the boundary layer thickness.
The Gurney flap increases lift by altering the Kutta condition at the trailing edge. The wake behind the flap is a pair of counter-rotating vortices that are alternately shed in a von Kármán vortex street. In addition to these spanwise vortices shed behind the flap, chordwise vortices shed from in front of the flap become important at high angles of attack.
The increased pressure on the lower surface ahead of the flap means the upper surface suction can be reduced while producing the same lift.
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