WHY DO BIRDS HAVE TAILS - THE TAIL AS A DRAG-REDUCING FLAP, AND TRIM CONTROL

Birds have tails, bats do not. Does this fundamental difference in fli ght morphology reveal a difference in flight capability, and if so are birds or bats better fliers? I use Munk's stagger theorem, and Prandt l's relation for the induced drag of a biplane to show that for a give n lift, and given wingspan, the induced drag of the wing-tail combinat ion is lower than the induced drag of a wing alone. However the same r eduction in induced drag could be achieved by slightly increasing the wingspan. While increasing the wingspan reduces induced drag, it can a lso increase profile drag and wing inertia. Induced drag is dominant a t low speeds and during turns. Profile drag dominates at high speeds. The tail allows birds to have the wings needed for efficient cruising and high speed flight (when the tail can be furled giving little drag) , at the same time the tail can be spread at low speeds or during turn s to reduce induced drag. The tail can play a role in maintaining stab ility and balance, and it appears that the stability of birds is tailo red so that the tail is required to generate lift at low speeds, when the interaction between the wings and the tail can also most effective ly reduce induced drag. (C) 1996 Academic Press Limited