Gurney flap

The Gurney Flap (or wickerbill) is a small tab projecting from the trailing edge of a wing. Typically it is set at a right angle to the pressure side surface of the airfoil, and projects 1% to 2% of the wing chord. This trailing edge device can improve the performance of a simple airfoil to nearly the same level as a complex high-performance design.

The device operates by increasing pressure on the pressure side, decreasing pressure on the suction side, and helping the boundary layer flow stay attached all the way to the trailing edge on the suction side of the airfoil. Common applications occur in auto racing, helicopter horizontal stabilizers, and aircraft where high lift is essential, such as banner-towing airplanes.

The original application, by automobile racing icon Dan Gurney (who was challenged to do so by fellow 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. The device was installed pointing upwards to increase downforce generated by the wing, improving traction. He field tested it and found it allowed a car to negotiate turns at higher speed, while also achieving higher speed in the straight sections of the track.

The first application of the flap was in 1971, after Gurney retired from driving and began managing his own racing team full-time. His driver, Bobby Unser, had been testing a new Gurney-designed car at Phoenix International Raceway, and was unhappy with the car's performance on the track. 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.) Gurney decided to try adding a "spoiler" to the trailing edge of the rear wing. 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. All that was needed was to balance this by adding additional downforce in front.

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Wikipedia