Influence of miniflaps on sailplane flight characteristics

Abstract

The effect of miniflaps for increasing the L/D ratio and the lift coefficient has been studied on airliners as well as on UAV-s and wind turbines. For sailplanes the lift when Cl > 1.0 is of main interest. As the maximum wing loading of racing sailplanes reaches 60–62 kg/m², it is necessary to achieve a high Cl max (1.7–1.8) in thermals. In this case the decrease in TAS caused by a high Cl max even compensates for the drop of the L/D ratio to a certain extent, as the climb speed will increase when the spiral flight radius diminishes in thermals. To bring the L/D to Cl > 1.0, a 2% chord miniflap at a 30° deflection angle was attached to the trailing edge of a Jantar-Standard 3 type sailplane wing (airfoil NN-8). In flight tests it was found that the miniflap increased the sailplane‘s Cl max to 1.35–1.66, i.e. by 23% (Re 1.0–0.92×10⁶). At the same time the L/D ratio Cl increased by over 1.0. Especially good L/D improvement was noted with Cl at 1.13–1.19. In thermal Cl of 1.57–1.65 the roll control was good. At lower Cl < 1.0 values, the miniflap reduced the L/D ratio in comparison with a normal configuration.

Keyword : miniflap, airfoil, sailplane, L/D ratio