Effect of Spanwise Trailing Edge Gaps on Aerodynamic Performance

Many waterfowl have been observed whiffling in order to rapidly descend. Whiffling accomplishes this by taking advantage of the nature of feathers to quickly lose altitude at a steep angle of descent. Understanding the aerodynamic effects of whiffling may inform new technologies that enable fixed wing aircraft to steeply descend in a controlled fashion. To investigate this phenomenon, several symmetric wings with varying amounts of fixed-width gaps in the trailing edge were tested at a Reynolds number of 2.29∙10^5. We found that increasing the number of gaps decreased the lift force at all angles of attack. At low angles of attack, increasing the number of gaps increased the drag force. However, at higher angles of attack, the wings with more gaps produce less drag force than the wings with fewer gaps. With respect to the lift-to-drag ratio, the 0-gap (ungapped) wing performs better than the gapped wings at low angles of attack. However, at higher angles of attack, the 0-gap wing performance degrades to that of the gapped wings. The lift-to-drag ratios of the gapped wings remain approximately constant as the angle of attack increases. We show that trailing edge gaps would enable a wing to produce less lift and greater drag than an ungapped wing at the same low angle of attack, thus enabling a steeper descent at a slower horizontal speed. Our results prove promising for applying active trailing edge gaps to a morphing wing actuation system to enable fixed wing aircraft to rapidly descend.