Flight Test Studies of the Formation and Dissipation of Trailing Vortices

A flight test program has been conducted to measure experimentally parameters which describe the characteristics of vortex instability and behavior. Three basic atmospheric flight conditions were investigated: steady level flight in calm air, steady level flight in light gusting air and light winds, and unsteady flight produced by control surface oscillations in calm air. A DeHavilland Beaver DHC-2 airplane and a Beechcraft T-34B airplane were used in the investigation. Smoke grenades were located near the wing tips of each airplane such that the vortices could be seeded with smoke and thus made visible. This made it possible to take measurements from photographs of the visible vortices. The experimental results show wavelengths of vortex instabilities, dissipation time of trailing vortices, effects of atmospheric current or gust, and the effect of control surface oscillation. These results provide additional experimental verification of the existence of vortex wake instability predicted by theory and show that small oscillations in pitch at a critical frequency accelerate the dissipation of high-intensity vortices.