Experimental investigation of Gurney flaps

The aerodynamics of a Gurney-flap-equipped airfoil has been explored by means of low-speed wind-tunnel experiments performed at a chord Reynolds number of 1.0 × 10⁶. Various chordwise locations and sizes of Gurney flaps were tested. Surface-pressure distributions and the wake momentum deficit were measured and used to determine lift, pitching moment, and drag. Compared with the clean airfoil, the measured maximum lift coefficient can be increased by nearly 30 % with these simple devices. The amount of lift increase has a nearly linear dependency on the chordwise location and size of the Gurney flap. Minimum drag is primarily affected by the flap size and, to a lesser extent, by the chordwise location. The Gurney flap increases in maximum lift are obtained by increasing the lower-surface pressures over the aft part of the airfoil. At the same time, the magnitude of pressure peak on the upper surface near the leading edge is reduced such that the upper-surface pressures over the middle parts ofthe airfoil are reduced and the separation point is moved aft by the reduced pressure-recovery gradients. As expected, this increases the aft loading and results in an increased nose-down pitching moment. As the angle of attack is decreased, the influence of a Gurney flap extending from the lower surface likewise decreases as the flap is increasingly immersed in the thickening boundary layer. A Gurney flap mounted to the upper surface behaves in the opposite way: increasing the negative lift at low angles of attack and having less and less influence as the angle of attack is increased. Although Gurney flaps result in significantly higher drags for airfoils with extensive runs of laminar flow, this disadvantage disappears as the amount of turbulent boundary-layer flow increases, as is the case with fixed transition near the leading edge of the airfoil.