Novel Whiffling-Inspired Gapped Wings as Control Surfaces

Abstract

Some bird species fly inverted (“whiffle”) to lose altitude, move laterally, and respond to gusts. Inverted flight twists the primary feathers, creating gaps along the trailing edge and decreasing lift. Whiffling shares parallels to conventional spoilers and ailerons on uncrewed aerial vehicles (UAVs). However, the maneuver had yet to inspire aircraft design. Here, I investigate the capabilities of a novel UAV control surface − a wing with gaps in the trailing edge − using the feather rotation mechanism as a new source of bio-inspiration. Wind tunnel tests revealed that the gapped wings did not reduce lift as much as a spoiler. However, at high angles of attack, a gapped wing produced a greater rolling moment coefficients than a maximum-deflected single aileron. I then simulated a gapped wing with a commercial computational fluid dynamics solver to estimate its work requirements. The gapped wing required higher work than a single aileron at low rolling moment coefficients. However, the gapped wing required less work at higher rolling moment coefficients and ultimately produced a higher maximum rolling moment coefficient. The results also highlighted the fluid dynamics mechanisms underlying the behavior the gapped wings. I additionally compared the gapped wing to a realistic wing with two ailerons. Compared to the two-aileron wing, the gapped wing produced smaller rolling moment coefficients, but favorable yaw at low rolling moment coefficients. A parameter sweep of gap size and configuration further implied that gap geometry could be tailored to maximize rolling moment coefficient per actuation work. Finally, I analytically modelled the rigid body dynamics of aircraft with varying gapped wings. The gaps generally improved the longitudinal modes. They also effectively damped and reduced the amplitude of the gust response. Ultimately, despite several limitations, the gapped wings could be a useful gust mitigation or roll control surface in certain situations.