A Gurney flap is a fitting which can enhance the performance of an airfoil like a wing or so-called spoiler on a vehicle. It can improve the ratio between lift and drag to increase efficiency and performance. Such fittings can be retroactively added to improve performance on existing equipment, in addition to being engineered into the design of an airfoil. The characteristics of the flap can vary, depending on the application.
Flaps change the way air behaves as it moves around and across an airfoil. In wind tunnels with smoke to make patterns of air movement visible, the Gurney flap can demonstrate what it does as an airfoil moves through space. The equipment can increase pressure on one side of an airfoil and control the boundary flow of air as it moves across the surface to improve the way it performs. A small vortex swirls around the flap to change the quality of air movement.
This innovation is frequently credited to Dan Gurney, who worked with racing cars in the 1970s. Gurney wanted to find a way to improve performance in a cost effective way and found that mounting a strip perpendicular to the airfoil in his cars could have startling results. His vehicles were able to achieve higher rates of speed, particularly around corners, an area where speed is commonly lost in racing because drivers need to slow down for safety. The Gurney flap quickly became a hit, and was added to other vehicle designs as racers caught on to the concept.
The same basic design also works for some aircraft, including planes and helicopters. Many helicopters utilize the gurney flap to effectively control their lift, which can vary considerably during flight due to the way the craft moves. The flaps make the helicopter more stable and more efficient, offering a number of benefits to pilots. A number of helicopter designers and developers regularly use this equipment in their projects, and conduct research into other measures to improve aerodynamics.
Gurney flaps stand out visually because they are perpendicular, rather than being streamlined into the design as is the case with other aerodynamic features. They may be fixed in place, as seen with many designs. Some retract, much like the massive hydraulic flaps used on commercial aircraft during takeoff and landing. The Gurney flap allows more control over lift and speed for pilots and drivers as they handle high speed, performance planes, helicopters, and cars.