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

Abstract: Popular Abstract in English Insects, birds, bats and pterosaurs have all evolved the ability to fly, even though it requires a lot of energy. The advantage is obvious: one can move about vertically, to go where others cannot, find and access distant food sources or escape your predators. But flying is also a very fast mode of transportation, and when you count the amount of energy required to get from one place to the other, flight is actually very efficient compared to, for example, running. Not surprisingly, in our quest to learn how to fly ourselves, we took great inspiration from birds. One of the first pioneers was Leonardo Da Vinci, who carefully studied birds and envisioned how to use that knowledge to build flying machines. Later, Otto Lilienthal designed and flew gliders that were based on observations of birds. At the beginning of the 20th century we finally obtained the first self propelled aircraft, an advancement for which we actually had to think beyond the flight of birds: modern aeroplanes do not flap their wings. Whether it concerns a bird, bat, insect or aeroplane, the main flight cost comes from transferring energy to the air. To fly more efficiently, the losses need to be reduced, for example by streamlining of the body and by optimizing wing shape. The drive to make aircraft ever more efficient has produced many useful tools, which can now be applied to once again study the flight of animals. This type of study can improve our understanding about how flight evolved and why flying animals behave the way they do. In this thesis I look at how birds manipulate the flow when they fly, to understand how they use aerodynamics to make their flight more efficient and to shed light on how animal flight might have evolved. For this purpose I studied the wake that is left behind in the air as the bird passes through, somewhat similar to looking at footprints left in the sand. From these wakes I could reconstruct the forces that were acting on the bird I trained a jackdaw, a small member of the crow family, to fly in our wind tunnel in Lund. This wind tunnel can be tilted to an angle so that the bird can glide, without having to flap its wings. In paper I, we measured components of drag (the force that acts to decelerate the bird) and looked at how these varied as the bird changed its posture. At low speeds birds spread their wings fully to produce sufficient lift (the force that lifts the bird in the air), but at higher speeds they gradually pull their wings towards their body, reducing the surface area that is experiencing drag. From our experiments we found that the change in the wing shape reduces this drag even further. We also found that the tail played a more important role than previously thought. At low speeds the bird spreads its tail to generate extra lift, which comes at the cost of additional drag. In paper II, we performed a similar experiment, but now the bird was going through its natural moult. The feathers, that form the wings of birds, are made of dead material, like our hair and nails. Over time the feathers get worn and damaged, so they need to be replaced. Some birds have evolved strategies in which they loose all feathers at once, which makes them flightless for some time. Other birds are so dependent on their capability of flight that they moult one feather at a time. Jackdaws, and most other passerines, do something in between, which means they will not become flightless, but their flight cost goes up substantially. We found that the jackdaw partially compensated for the missing feathers by covering the moult gaps with the adjacent feathers. The bird also temporary lost some weight, compensating for the reduced lifting area. Nonetheless, we found that drag on the wings had increased with up to 50% due to the moult gaps. To describe energy requirements for animal flight, models were adopted from aeronautics. But modern aeroplanes do not flap their wings. So in paper III we developed a model that does take into account how birds flap their wings to propel themselves through the air, and found several inconsistencies with some of the parameters used in the prevailing models. For paper IV we returned to the jackdaw in the wind tunnel, where we now looked at the aerodynamics while it was flying horizontally, flapping its wings. The drag due to lift, the only parameter we could directly measure from these experiments was consistent with predictions by the model of paper III. However, our measurements indicated an imbalance between thrust and drag. If we try to adjust for this force imbalance, we find that energy left in the wake of the bird is according to expectations from the model of paper III. In paper V we took a closer look at the jackdaw, and measured how the air flows around the wing-tips. The outer primary feathers of many birds have special shapes that allow them to deform independently from each other under the aerodynamic loads. These wing slots are clearly visible in large soaring birds circling in thermals, and often considered to improve the efficiency of the wing at producing lift. The jackdaw also has these slotted wing tips, though they are not as large as those found in storks or eagles. In the experiments of paper I and IV we found vortices in the wake (multiple little cyclones), and from the measured air flows around the wing tips we could conclude that these originated from the vertically separated outer primary feathers. These specific wake structures are suggest that slotted wing-tips do indeed function to produce lift more efficiently.