What rotorfossil said
, plus:
"The ... Gurney flap ... works 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. In short, the increased pressure on the
lower surface ahead of the flap means the upper surface suction can be
reduced while producing the same lift over a very wide range of positive and
negative angles of attack. For example, at one extreme, in a high-powered
climb, the negative angle of attack of a horizontal stabilizer can be as high as
-25°. In autorotation, it may be +15°.
On the 206, the Gurney flap corrects an angle of incidence problem in the
design that was too difficult to correct directly. Another reason is to induce
turbulence in autorotation, to spoil the lift and make its effect as small as
possible, otherwise the fuselage attitude would be too extreme."
Phil