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, littleworkhasbeendone 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 λ 0.75, 0.5, and 0.25 were placed in the wind tunnel at a Reynolds numbers of 7.5 × 104. Angle-of-attack sweeps were performed at sideslip angles of β 0, −5, −10, −15, 20 and −35 deg, and the side force, yawmoment, and roll moment weremeasured. Although the side force and yawmoment coefficients (CSF andCn) were typically negligible, the roll moment coefficient Cl 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 amicro aerial vehicle in equilibrium flight conditionsmay experiencemagnitudes ofCl;β at the upper limit of, or evenabove, the range of−0.10 ≤ Cl;β ≤ 0 considered to represent goodhandlingqualities in aircraft.