lower surface ahead of the flap means the upper surface suction can be reduced while producing the same lift. Helicopter applications 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 one extreme, in a high-powered climb, the negative angle of attack of the horizontal stabilizer can be as high as −25°; at the other extreme, in autorotation, it may be +15°. As a result, at least half of all modern helicopters built in the West have them in one form or another. The Gurney 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 from scratch. A Gurney flap was also fitted to the Bell JetRanger to correct an angle of incidence problem in the design that was too difficult to correct directly. The Eurocopter AS355 TwinStar helicopter uses a double Gurney flap that projects from both surfaces of the vertical stabilizer. This is used to correct a problem with lift reversal in thick airfoil sections at low angles of attack. The double Gurney flap reduces the control input required to make the transition from hover to forward flight. See also Lift (force) References External links the original 1935 Zaparka patent Auto racing equipment Aircraft wing components 1971 introductions Aircraft wing design