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Gull wing morphing allows active control of trade-offs in efficiency, maneuverability and stability

University of British Columbia

Birds flying in turbulent conditions demonstrate impressive flight stability and control. This versatility is hypothesized to derive from dynamic wing shape changes, an ability termed wing morphing. Bird wings can morph passively through inertial or aerodynamic loading of flexible components or actively when birds stimulate their network of intrinsic wing muscles. The majority of active wing morphing is actuated through the wrist or elbow joints. Wrist flexion improves high-speed and turning performance, but little is known about the morphology or aerodynamic consequences of morphing the elbow joint. Here we show that gulls gliding in unsteady environments reduce their passive stability by actively reducing their elbow angle. We first photographed gulls in gliding flight to quantify their wing shapes. We next used cadavers to determine the viable range of elbow angles and isolate the subset that was used by gliding gulls. The behavioral observations and cadaver manipulations revealed an in vivo gliding elbow angle range of 90°-154° and that there is a significant reduction of the elbow angle used by gulls as local wind speeds and gusts increase. Next, wings were prepared and dried across the full range of elbow angles and tested in a wind tunnel at varied turbulence intensities. These force measurements revealed that the lower elbow angles used by gliding gulls had improved aerodynamic efficiency but reduced passive pitch stability. Moreover, we found that the in vivo elbow range captures the majority of the available aerodynamic variation. Collectively, our results indicate a coupling in efficiency and stability in avian gliding and that wing morphing allows gulls to modulate aerodynamic trade-offs which may allow for a steadier flight path in an unsteady environment.

Text

2019-03-31

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/64987

Zoology

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/