Recent Contributions to Aviation Problems

Abstract

THE attempts which have hitherto been made to explain the continuous sustentation of tropical and other birds without the performance of muscular work have left many doubtful points requiring to be cleared up. Observers have frequently sought to attribute the phenomena to something acting in violation of the principles of elementary mechanics, and have succeeded in establishing this peculiarity, not perhaps in the way that they intended, but by the chaotic way in which such terms as force, momentum, weight, energy, lift, pull, drag, and gravity are confused by them, and occur indiscriminately mixed up in their writings. In a paper on “Meteorology and the Non-Flapping Flight of Tropical Birds,” published in the Proceedings of the Cambridge Philosophical Society, xxi. 4, Dr. Gilbert T. Walker has now sought to bring sound scientific principles to bear on the numerous observations in India published by Dr. Hankin. From observations of the temperature of the air at Agra at different altitudes and hours of the day, Dr. Walker finds conditions of instability leading to the formation of strong ascending air-currents, and observations in Egypt and various partsof India indicate conditions of “bumpi-ness “in the atmosphere caused by ascending currents, covering the periods employed by birds for “soaring.” Dr. Walker finds that the angle of gliding of the most efficient recent aeroplanes is sufficiently low to satisfy the conditions necessary for continuous sustentation in the presence of these currents, and he examines in detail three possible sources of internal work in the atmosphere, namely, ascending currents, variations of horizontal velocity as functions of time and place co-ordinates, and Lord Rayleigh's hypothesis of variation of horizontal wind velocity as a function of the vertical altitude co-ordinate. He also carefully considers the structure of the birds' wings, in comparison with those of the Handley-Page machine, and the effect of the flexibility of the quills on the aerofoil form in ascending and descending glides. Both from theory and from actual observation, it is found that the ascending air-currents in the higher regions of the atmosphere are greater in diameter than lower down, and from actual observation he contradicts Dr. Hankin's statements according to which birds are seen rising in descending currents. It would thus appear from Dr. Walker's observations that, in the region dealt with by Dr. Hankin, the atmosphere possesses sufficient internal energy to satisfy the conditions of “soarability” required by the latter observer,In regard to the violation of mechanical principles, both hypothetically by birds and actually by writers, we cannot do better than quote Dr. Walker's remark that ... “it is strangely” (asterisk and footnote with references follow) “necessary to insist that it is as impossible to derive energy from a wind that is constant in time and space as it is from a perfect calm.” To theories based on a denial of this principle the late Sir Hiram Maxim used to point out the enormous velocity of the wind due to the earth's rotation and its orbital motion about the sun, and he suggested that if writers believed in these theories, why did not they utilise this energy for the purposes of flight?