Roll Stall for Low-Aspect-Ratio Wings

The longitudinal aerodynamics of micro aerial vehicles, although not fully understood, have been the subject of several studies in recent years; however, little work has been done to investigate the lateral loading. In this experiment, flat-plate wings with rectangular planforms of aspect ratios AR = 0.75, 1, 1.5, and 3 and tapered planforms of lambda = 0.75, 0.5, and 0.25 were placed in the wind tunnel at a Reynolds numbers of 7.5 x 10(4). Angle-of-attack sweeps were performed at sideslip angles of beta = 0, -5, -10, -15, 20 and -35 deg, and the side force, yaw moment, and roll moment were measured. Although the side force and yaw moment coefficients (C-SF and C-n) were typically negligible, the roll moment coefficient C-l was found to increase linearly with angle of attack before stalling in a manner reminiscent of a lift curve. This "roll stall", which has not previously been observed for micro-aerial-vehicle-type wings, is attributed to the upstream tip vortex creating an additional lift component due to its impact on the spanwise variation of effective angle of attack in addition to the force generated by its own low-pressure core. As the downstream tip vortex is convected away from the wing, a net moment is created. Computations of the roll-stability derivative indicate that a micro aerial vehicle in equilibrium flight conditions may experience magnitudes of C-l,C-beta at the upper limit of, or even above, the range of -0.10 <= C-l,C-beta <= 0 considered to represent good handling qualities in aircraft.