Gurney flap

[5] Common applications occur in auto racing, helicopter horizontal stabilizers, and aircraft where high lift is essential, such as banner-towing airplanes.

[7][8] The original application, pioneered by American automobile racing icon Dan Gurney (who was challenged to do so by fellow American racer Bobby Unser), was a right-angle piece of sheet metal, rigidly fixed to the top trailing edge of the rear wing on his open-wheel racing cars of the early 1970s.

Gurney needed to do something to restore his driver's confidence before the race and recalled experiments conducted in the 1950s by certain racing teams with spoilers affixed to the rear of the bodywork to cancel lift (at that level of development, the spoilers were not thought of as potential performance enhancers, merely devices to cancel out destabilizing and potentially deadly aerodynamic lift).

[11] The device was fabricated and fitted in under an hour, but Unser's test laps with the modified wing turned in equally poor times.

When Unser was able to speak to Gurney in confidence, he disclosed that the lap times with the new wing were slowed because it was now producing so much downforce that the car was understeering.

[13] To conceal his true intent, Gurney deceived inquisitive competitors by telling them the blunted trailing edge was intended to prevent injury and damage when pushing the car by hand.

[25] The Gurney flap was first applied to the Sikorsky S-76B variant,[14] when flight testing revealed the horizontal stabilizer from the original S-76 not providing sufficient lift.

[14] The double Gurney flap reduces the control input required to make the transition from hover to forward flight.

A Gurney flap shown on the underside of a Newman airfoil [ 1 ]
The "variable-lift airfoil" shown in Figure 1 of the 1935 E. F. Zaparka patent U.S. patent RE19412 . It is a movable microflap, similar to the fixed Gurney flap.
A Gurney flap on the trailing edge of the rear wing of a Porsche 962
Double Gurney flaps on a Bell 222U helicopter