Page:Aerial Flight - Volume 2 - Aerodonetics - Frederick Lanchester - 1908.djvu/378

App. II. to such an inclination as will by its descent enable it to be supplied with the energy necessary for its propulsion, the steepness of the descent under given conditions depends upon the rate of dissipation of energy by the resistance of the machine; this angle may be termed the angle of 'recuperation.' If now we suppose the machine to be launched at a speed considerably above its natural velocity, then in the first instance it takes an upward course of gradually increasing inclination till its excess of kinetic energy has been absorbed as potential, the inclination of its course then gradually diminishes till it again becomes horizontal, when, its velocity having become considerably deficient, its course takes a downward trend, and thus it proceeds to describe curves in the air of approximately trochoidal form of gradually diminishing amplitude, slowly settling down to its angle of 'recuperation.' If the launching velocity be excessive (greater than one and a half times the natural velocity in most cases) the machine runs serious risk of being capsized, and the limit of longitudinal stability may be said to be reached somewhere about this point."

"The natural velocity of a machine may be modified at will by altering the relative angle between the wings and tail-plane, thus causing the former to meet the air at a greater or less angle."

"If in the course of its evolutions a machine (constructed as hereinbefore described) heel over sideways one way or the other, or if a rolling motion be set up, the first effect is for the machine to begin to slide down, so to speak, in the direction in which it is for the time being inclined, this motion is very quickly arrested, however, by the resistance of the ' fins ' whose centre of pressure is arranged above the centre of gravity of the machine, and equilibrium is thereby restored. A similar result might be brought about by inclining the wings or the tips of the wings upwards to the right and left; but an arrangement of fins is specially valuable owing to its damping action on any side oscillations that may be set up."

"It is easy to understand when a motor is employed for the