Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/443



Authorities are generally agreed at the present time that one at least of the varieties of soaring practised by the larger birds involves the abstraction of energy from the wind fluctuation, that is to say, the soaring bird can derive the power required for its flight from the energy of turbulence of the wind (comp. §§ 37, 131).

It is clear that a bird having no horizontal force applied to it from without (in contradistinction to a kite which is connected to the earth by a string), is unable to effect any change in the total (horizontal) momentum of the air that comes within its grasp; consequently it cannot raise or lower the mean velocity of the wind, although it may be able to cause some parts to move faster and some more slowly.

It is evident that if a bird can, by altering its angle and altitude, so manipulate the wind coming within its grasp, that the portions that are moving in excess of the mean velocity have their velocity reduced, and those that are moving at less than the mean velocity are accelerated, the total energy of the