The Gurney Flap
The Gurney flap is a device derived from aeronautical research (mostly at McDonnell-Douglas in the mid 1970s), that has found applications in other fields of aerodynamics.
The device in its simplest form is a vertical lip fixed at the wing trailing-edge, whose length can be as as much as 1% of the chord. The device was initially applied on the DC-10 and its derivative MD-11.
The original experiments showed an increase in lift of as much as 20 % with minimal changes in drag. These results could not be reproduced in all cases, and in fact for some conditions the G-flap does give rise to a drag penalty.
The Gurney flap is device sometimes applied on racing cars wing to produce a little amount of extra lift, in spite of a larger increase in profile drag.
Other applications include rotorcraft: for example the stabilizing rear wing of the helicopter Apache AH-64 has a couple of G-flaps (upward, on the starboard wing; downward on the port wing). The G-flaps improve performance during high-powered climb. The device is applied to other helicopters for stabilization purposes.
The main idea behind the G-flap is to block the flow on one side of the wing in order to increase the overall circulation. The total effect is an increase of pressure on the suction side.
A derivative of the G-flap is the Divergent Trailing Edge (DTE), while a precursor is the split flap (see NACA experiments in the 1940s).
Figure 4: Gurney flap
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