In the wake of a bird - Quantifying aerodynamic performance of avian flight

Sammanfattning: Flight is an expensive form of locomotion, but also offers the ability to travel great distances at relatively low cost. Birds have developed various adaptations for optimizing flight performance. The main aim of this thesis is to identify how morphological variables in birds affect aerodynamic drag and flight performance. To this end I studied the aerodynamic performance of a jackdaw (Corvus monedula) that was trained to fly in a wind tunnel. We performed experiments with the bird gliding in the wind tunnel at different glide angles. From the measured wakes we computed the components of drag, and related these forces to the variation in gliding posture. Induced drag was mainly affected by wingspan as expected, but contrary to expectations, the use of the tail at low speed negatively affected the span efficiency. The tail also strongly increased the measured body drag coefficient. In a second experiment we examined the aerodynamic effects of moult gaps during several stages of the annual wing moult and found that span efficiency reduced and the wing profile drag coefficient increased. The bird partially mitigated the negative effects by adjustments in wing posture to minimize the gaps and by weight loss to reduce wing loading. In powered flight, birds flap their wings to produce thrust, which complicates the aerodynamic analysis. We developed a mathematical model that relates the aerodynamic power in flapping flight to the drag in gliding flight. This model showed that the effect of reciprocating wings on the induced power is commonly underestimated in simplified animal flight models. To test the new power model, we performed experiments with the jackdaw in forward flapping flight, and we computed forces and aerodynamic power from the measured wakes. We found the induced drag coefficient to be in line with that predicted from the flapping model, but we found an imbalance in thrust and drag. Adjusting our power measurement for the residual drag, resulted in reasonable agreement between the data and the prediction from the power model. In a final experiment we looked at the slotted wing-tips of the jackdaw, where we measured the flow at the vertically separated outer primaries. We found multiple vortex cores in the tip vortex originating from the separated feathers. The multiple cores are indicative for increased span efficiency. We also found these structures in flapping flight. Many non-gliding birds feature these wingtip slots and we consider the possibility they may have evolved to improve efficiency in powered flight.