The aerodynamics of a spinning shell

The paper to which this abstract is meant to serve as a non-technical introduction contains the results of work undertaken at the request of the Ordnance Committee and is published with their permission. (1)The Problems of Experimental External Ballistics .—A spinning shell moving through the air is subjected in general to a complex reaction, which is determined primarily by its motion relative to the air. But a shell has an axis of symmetry, and when this axis of symmetry, its axis of rotation, and the direction of motion of its centre of gravity through the air all coincide the reaction naturally assumes its simplest form. When a shell is moving in such a manner it may be said to be moving “nose-on” through the air, and it is a matter of experience that, by giving a shell a suitable axial spin, it may be made to move practically nose-on through the air over very considerable distances. Under such conditions the symmetry clearly demands that the reaction of the air shall reduce to a single force, acting in the direction of motion reversed, which therefore merely tends to destroy the linear velocity of the shell, and a couple about the axis of symmetry, which tends to destroy the axial spin. The latter is comparatively unimportant, and it appears that, so long as the motion remains practically nose-on, the problem of determining the motion of the shell is reduced to one of particle dynamics, whose solution is now classical.