Gurney flap
Small tab improving airfoil performance, invented by Dan Gurney.
The Gurney flap, also called a wickerbill, is a small tab that sticks out from the trailing edge of a wing. It is usually placed at a right angle to the wing's pressure-side surface and extends about 1% to 2% of the wing's chord. This simple addition can make a basic airfoil perform almost as well as a much more complex high-performance design. It works by raising pressure on the pressure side, lowering pressure on the suction side, and keeping the boundary layer attached along the suction side all the way to the trailing edge. You'll find it in auto racing, on helicopter horizontal stabilizers, and on aircraft that need a lot of lift, like banner-towing planes. It is named after its inventor, American race car driver Dan Gurney.
The idea came from Dan Gurney, who was challenged by fellow racer Bobby Unser. In the early 1970s, Gurney attached a right-angle piece of sheet metal to the top trailing edge of the rear wing on his open-wheel race cars, pointing upward to boost downforce and improve traction. After testing, he found it let the car take turns faster while also reaching higher speeds on straight sections. The first use was in 1971, after Gurney had retired from driving and was running his own team full-time. Unser was testing a new Len Terry car at Phoenix International Raceway but was unhappy with its performance. Gurney needed to restore Unser's confidence before the race. He remembered experiments from the 1950s where racing teams added spoilers to the rear bodywork to cancel lift—at that time, these were seen only as safety devices, not performance enhancers. Gurney decided to try a "spoiler" on the top trailing edge of the rear wing. The part was made and fitted in under an hour, but Unser's test laps were still slow. Unser later told Gurney privately that the new wing was producing so much downforce that the car understeered; the fix was to add downforce at the front. Unser understood the breakthrough immediately and wanted to keep it secret from competitors, including his brother Al. To avoid drawing attention, Gurney left the flaps in plain sight and told curious rivals the blunt trailing edge was just to prevent injury when pushing the car by hand. Some copied the design, and a few tried pointing the flap downward, which made performance worse. Gurney kept using the device in racing for years before its real purpose came out.
- Inventor
- Dan Gurney
- First application year
- 1971
- Typical chord percentage
- 1% to 2%
- Tested flap chord percentage
- 1.25%
- Original application
- top trailing edge of rear wing on open-wheel racing cars
- First helicopter application
- Sikorsky S-76B variant
Lore & Background
The original application was pioneered by American automobile racing icon Dan Gurney, who was challenged to do so by fellow American racer Bobby Unser. It 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, installed pointing upward to increase downforce. Gurney field-tested it and found it allowed a car to negotiate turns at higher speed while also achieving higher speed in straight sections. The first application 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 Len Terry early CAD/CAM designed car at Phoenix International Raceway and was unhappy with its performance. Gurney recalled experiments from the 1950s with spoilers affixed to the rear of bodywork to cancel lift, and decided to add a 'spoiler' to the top 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. Unser later disclosed that lap times were slowed because the wing was producing so much downforce that the car was understeering, and all that was needed was to balance this by adding downforce in front. Unser realized the value immediately and wanted to conceal it from competitors. Gurney left the devices out in the open and deceived inquisitive competitors by telling them the blunted trailing edge was intended to prevent injury when pushing the car by hand. Some copied the design and some attempted to improve upon it by pointing the flap downward, which hurt performance. Gurney used the device in racing for several years before its true purpose became known. Later, he discussed his ideas with aerodynamicist Bob Liebeck of Douglas Aircraft Company, who tested the device, named it the 'Gurney flap,' and confirmed Gurney's field test results using a 1.25% chord flap on a Newman symmetric airfoil. Liebeck's 1976 AIAA paper (76-406) 'On the design of subsonic airfoils for high lift' introduced the concept to the aerodynamics community. Gurney assigned his patent rights to Douglas Aircraft, but the device was not patentable, as it was substantially similar to a movable microflap patented by E. F. Zaparka in 1931, ten days before Gurney was born. Similar devices were also tested by Gruschwitz and Schrenk and presented in Berlin in 1932.
Reader's Guide
The Gurney flap increases the maximum lift coefficient, decreases the angle of attack for zero lift, and increases the nosedown pitching moment, consistent with an increase in camber. It typically increases the drag coefficient, 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 boundary layer thickness. The flap increases lift by altering the Kutta condition at the trailing edge, producing a wake of counter-rotating vortices alternately shed in a von Kármán vortex street. At high angles of attack, chordwise vortices shed from in front of the flap become important. The increased pressure on the lower surface ahead of the flap means the upper surface suction can be reduced while producing the same lift. Gurney flaps have found wide application on helicopter horizontal stabilizers because they operate over a very wide range of both positive and negative angles of attack. At least half of all modern helicopters built in the West have them in one form or another. The flap was first applied to the Sikorsky S-76B variant when flight testing revealed the horizontal stabilizer from the original S-76 not providing sufficient lift; engineers fitted a Gurney flap to the NACA 2412 inverted airfoil to resolve the problem without redesigning the stabilizer. A Gurney flap was also fitted to the Bell JetRanger to correct an angle of incidence problem. The Eurocopter AS355 TwinStar uses a double Gurney flap projecting from both surfaces of the vertical stabilizer to correct a problem with lift reversal in thick airfoil sections at low angles of attack, reducing the control input required to transition from hover to forward flight.
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