AERODYNAMIC CORRECTIONS FOR THE FLIGHT OF BIRDS AND BATS IN WIND TUNNELS

Few wind tunnel studies of animal flight have controlled or corrected for distortions to behaviour, physiology or flight aerodynamics repres enting the difference between flight in the tunnel and flight in free air. Aerodynamic correction factors are derived based on lifting-line theory and the method of images for an animal trying freely within clo sed- and open-section wind tunnels; the method is very similar to that used to model flight in ground effect, and as in ground effect the co rrections to induced drag may be substantial. These correction factors are used to estimate bound wing circulation, drag and mechanical powe r for comparison with free flight, and to derive testable predictions of optimum flight strategies for an animal in a tunnel. In an open-sec tion tunnel, mechanical power is increased compared to free flight, an d the animal should try at the tunnel centre. In a closed tunnel mecha nical power is usually reduced, and substantial savings are available, particularly at low speeds, if the animal flies close to the tunnel r oof. Anecdotal observations confirm that birds and bats adopt this str ategy. The mechanical power-speed curve in a closed tunnel is flatter than the curve for free flight, and this may explain the flat metaboli c power-speed curves for birds and bats obtained in some measurements.