Inherent Stability Modes of Low-Aspect-Ratio Wings

The development of micro aerial vehicles has been hindered by a poor understanding of the flight dynamics associated with the unique aerodynamic regime. This study experimentally estimates the aerodynamic damping derivatives of flat-plate wings with aspect ratios less than 3 at a Reynolds number of 7.5×104; when combined with previously published results detailing the lateral and longitudinal static loads, a dynamic model is developed for low-aspect-ratio wings. The initial-condition response of the linear equations of motion shows that the loading created by roll stall results in purely aerodynamic lateral modes which, unlike conventional aircraft, are not attributed to geometric features, such as the vertical tail; this response was favorably compared with the integration of the full nonlinear equations. The mode is manifested by divergent, high-amplitude perturbations in sideslip, bank angle, and roll rate; furthermore, it is seen to be affected by angle of attack variations, which significantly alter the instantaneous value of the roll stability derivative Lβ. If the input frequency of the angle of attack oscillations is close to the natural frequency of the pure lateral mode, the bank angle is seen to drift away from its equilibrium value due to an attenuated restoring roll moment. This represents a previously unconsidered stability mode, referred to as roll-resonance, which couples the lateral and longitudinal stability axes for small perturbations from equilibrium flight conditions.