Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/310



§ 197. Introductory.—The employment of the Newtonian method (§ 2) in the theory of propulsion has been already mentioned (§ 8). The application of this method, which constitutes the foundation of modern theory, owes its development principally to the work of Rankine and W. Froude.

In the general theory of propulsion we are not concerned with the machinery of propulsion, i.e., the form of propeller—paddle, screw, jet, or other known or unknown mechanism; we merely take account of the fact that forces are exerted between the propelled body and certain parts of the fluid, and investigate the conditions that obtain and the proportion of power that may be utilised and lost. The theory of propulsion on this broad basis is the common foundation of propeller theory generally, and the conditions deduced from the Newtonian principle are essential to every form of propeller. It is convenient, in the initial consideration of the problem, to introduce the notion of action at a distance, and to suppose the propulsive forces to consist of repulsions (or attractions), acting in the direction of motion between the propelled body and the particles of the fluid.

§ 198. The Newtonian Method as applied by Rankine and Froude.—It is supposed in the first instance that the fluid on which the propeller operates is at rest at the time the propulsive forces commence to act; this condition is intended to exclude