Birds in the Flow: Flight Mechanics, Wake Dynamics and Flight Performance

Abstract: I share my fascination of bird flight with many others, and here I summarise my thesis on the subject. This thesis emphasises the mechanics of bird flight by focusing on flight mechanics, wake dynamics and flight performance. Gliding flight performance of a jackdaw was measured in a wind tunnel and I conclude that the current model for predicting optimum wingspan as a function of forward gliding speed is inadequate. As an alternative model I present a new set of equations. Two papers on the barn swallow concern changes in wingbeat kinematics and tail posture with flight speed. The tail is used extensively at low flight speeds, but the current approach to consider the tail a delta wing does not match the observed changes in tail posture and orientation. An analysis of head-on photographs of songbirds revealed that passerines have a larger projected body frontal area in relation to body mass than waterfowl and raptors, a result affecting the associated body drag coefficient. We have developed a novel method for making quantitative and qualitative measurements of the wake behind a bird flying in the wind tunnel. For the first time we can visualise the wake behind a bird, a thrush nightingale, over it's entire natural range of flight speeds. As part of the same series of measurements we studied the wingbeat kinematics under similar conditions. The main conclusions are: (i) The wake is complex and cannot be categorized as either of the two previously described types (closed vortex loops and undulating wingtip vortices of constant circulation). The wake is mostly of an intermediate type but at the extreme low speed and high speed, respectively, approximations to the closed loop and constant circulation models can be made. (ii) Enough vorticity to support the bird's weight was found, hence resolving the long-standing wake momentum paradox. (iii) The concept of discrete ‘gaits’ must be refuted for bird flight because the measured wake properties change continuously. (iv) A simple model to predict the wake topology for the entire range of flight speeds is suggested. (v) Wingbeat kinematics and wake topology have common denominators. To describe the hunting behaviour and flight performance of an aerial predator we tracked Eleonora’s falcons in free flight off the coast at a breeding colony on a small island in Italy. The Eleonora's falcon shows impressive flight performance and proves to be well adapted for hunting birds on the wing. On the basis of aerodynamic theory and the morphological adaptations of the prey and predator, we analysed theoretically the outcome of three different attack-escape situations.