69.4 Jan. 7 How swifts control their glide performance with morphing wings LENTINK, D.*; MÜLLER, U.K. ; STAMHUIS, E.J. ; DE KAT, R.; VAN GESTEL, W. ; HENNINGSSON, A.; HEDENSTRÖM, A.; VAN LEEUWEN, J.L.; Wageningen University, Wageningen, The Netherlands; Wageningen University, Wageningen, The Netherlands; Groningen University, Groningen, The Netherlands; Delft University of Technology, Delft, The Netherlands; Wageningen University, Wageningen, The Netherlands; Lund University, Lund, Sweden; Lund University, Lund, Sweden; Wageningen University, Wageningen, The Netherlands email@example.com
During gliding flight, birds continually change the shape and size of their wings. Wing geometry has a profound effect on aerodynamic performance. Here we show how morphing enlarges the performance envelope of swift wings, affording swifts effective control of their gliding flight. To quantify the effect of morphing, we measured lift and drag forces using a wind tunnel for a range of wing geometries and glide speeds that extend well beyond the birds behavioural envelope. We formulated six figures of merit related to flight costs and agility to evaluate variable wing geometry. We show that sweeping the wings back by up to 50 degrees can alter flight cost related figures of merit by 30 to 265%, and agility by 125 to 320%. Straight gliding favours extended wings at low glide speeds and swept wings at high speeds, whereas turning favours extended wings at all speeds. However, all fast gliding flight generates excessive wing loads that can only be accommodated by high sweep. Our semi empirical glide model predicts the most cost effective glides at speeds between 8-10 m/s whereas agility peaks at 15 and 25 m/s. Swifts in fact roost at 8-10 m/s, thus our model accurately predicts minimal energy loss during resting behaviour. We conclude that morphing wings show special promise for innovative agile and efficient bird sized air vehicles.