Aerodynamics of bird flight

Aerodynamics of bird flight

Abstract. Unlike airplanes birds must have either flapping or oscillating wings (the hummingbird). Only such wings can produce both lift and thrust - two sine qua non attributes of flying. The bird wings have several possibilities how to obtain the same functions as airplane wings. All are realized by the system of flight feathers. Birds have also the capabilities of adjusting the shape of the wing according to what the immediate flight situation demands, as well as of responding almost immediately to conditions the flow environment dictates, such as wind gusts, object avoidance, target tracking, etc. In bird aerodynamics also the tail plays an important role. To fly, wings impart downward momentum to the surrounding air and obtain lift by reaction. How this is achieved under various flight situations (cruise flight, hovering, landing, etc.), and what the role is of the wing-generated vortices in producing lift and thrust is discussed. The issue of studying bird flight experimentally from in vivo or in vitro experiments is also briefly discussed.

One Introduction

People have always been fascinated by the perfection of the bird flight, and in the past centuries many enthusiasts attempted to copy it, unfortunately, in most cases with no success. About the end of the fifteenth century Leonardo da Vinci systematically studied the flapping bird wings and observed and described very carefully the bird's flight.

The first successfully man-operated airborne vehicle by Otto Lilienthal (about the end of the nineteenth century) had fixed wings. However, Lilienthal also studied in detail the bird wing structure and discovered the importance of cambered profiled wings. Horatio F. Phillips about the same time had investigated the aerodynamics of curved double-surfaced airfoils, and in eighteen eighty-four had it even patented. All this knowledge was applied by Wright brothers, who in nineteen o four performed a first successful flight with a heavier-than-air-airplane of their own design. Their airplane had also fixed wings, and, fixed wings had all other man designed and operated airplanes in the whole century which followed. It was a magnificent century of hectic research and development of new aircrafts, and at its end people proudly and immodestly claimed that in flight abilities their airplanes had reached almost a bird-like perfection.

It is quite interesting that during the twentieth century people scarcely ever sought inspiration in the animal world. Rather, they used the most sophisticated instrumentation, available mainly towards the end of the century, to realize that what they have achieved, birds have possessed for millions of years.

However, towards the end of the last century a new interest has appeared in small autonomous or remotely controlled flying vehicles. The so called micro-air- vehicles (MAV) were expected to be used in aerial reconnaisance in open as well as confined spaces, in monitoring polluted areas which people cannot enter, and (last but not least!) in many military applications. Soon it has become clear that if these vehicles should also be able to carry a reasonable payload, they would have to have either rotating or moving wings. Since the MAVs operate at about the same Reynolds numbers as birds, bats and larger insects, it has recently opened a new interrest in aerodynamics and flight mechanics of all of them. In this paper, however, we will deal only with avian flight, i.e., with the flight of birds.

Two Bird wings

To fly, birds must have wings. Unlike airplanes they must have moving wings, i.e., they must have either flapping or, in one exceptional case (the hummingbird), even oscillating wings. Only such wings can produce both lift and thrust - two sine qua non attributes of flying.

First airplane designers, like Otto Lilienthal and Horatio Phillips, believed that it is enough to copy the bird wing shape and the wing profile. Unfortunately, the bird wing is not a simple curved compact surface like any of the vintage airplane wings.

The basic structure of a bird's wing resembles the human hand, only proportions of the bones are different, as much as they are different in every bird species. The hand section of the wing provides the main dynamic control for the bird. It represents about eighty percent of the wing length in small birds, who have to manoeuvre in constraint environment. In larger birds, using the wings mainly for soaring, gliding or slow flapping, the hand section is much smaller, and the wing is ruled by the arm bones extending from forty to sixty percent of the wing span.

Primaries are connected to the bird's fingers. Primaries are the longest and narrowest of the outer feathers (the remiges), and they can be individually rotated. These feathers are the main source of thrust, mostly generated on the downstroke of flapping flight. On the upstroke the primaries are separated and rotated, reducing air resistance while still helping to provide some thrust. Remiges on the wingtips of large soaring birds like condors or vultures also allow for spreading the feathers, reducing thus the creation of wingtip vortices.

Secondaries are connected to the ulna. They remain close together in flight (they cannot be individually separated like the primaries) and help to provide lift by creating the airfoil shape of the bird's wing. Secondaries are usually shorter and broader than primaries.

The alula feathers are not flight feathers in the strict sense; however, they are very useful in slow flight. Attached to the bird's "thumb", they lie normally flush against the leading edge of the wing and detach only at higher angles of attack creating a gap between the alula and the rest of the wing (compare with slats on airplane wings). Birds can thus avoid stalling at low speeds or at landing.

The actual shape of the bird wing is made of two organized sets of feathers - in the first set are the flight feathers anchored in the digits (primaries) and the ulna (secondaries). There are also three sets of the so called coverts which act as a protective cover for all or part of the folded primaries and secondaries.

The vanes of each feather have hooklets that lock the feathers together, giving thus the wing - if locked - the necessary strength to withstand the lift force and to maintain its shape. Each feather has a bigger side and a lesser side. Moreover, the shaft bends slightly off its longitudinal axis with the lesser side to the front and the bigger side to the rear of the feather. The feather anatomy enables rotation of the feather in its follicle. During the up-stroke the bigger side is pressed down, opening thus the wing like a jalousie and allowing air to slip through the wing. This considerably reduces the upward resistance of the wing during upstroke.

The large flight feathers at the wing tip (primaries, outer remiges) contribute greatly to the production of lift. These feathers form the tip slots which can considerably reduce the induced drag by acting as winglets, i.e., by making wings effectively non-planar and eliminating thus the intensity of the tip vortex by spreading the vorticity vertically.

The flight feathers together with the covert feathers are responsible for morphing the wing shape. The many degrees of freedom of the wrist and elbow bones and all these feathers make the bird wing highly flexible in changing the wing chord and span, and in the spanwise twisting and bending of the wing, thus helping to maintain attached flow and reduce induced drag at the wing tip.