Page:Aerial Flight - Volume 2 - Aerodonetics - Frederick Lanchester - 1908.djvu/377



explanation of the automatic longitudinal stability of an aerodone or aerodrome given by the author in his Patent 8608 of 1897, to which reference has been made in the present work, is as follows:—

"When a machine constructed as hereinbefore described travels through the air with a sufficient velocity its weight is supported dynamically by the reaction of the air on the upper and under wing surfaces, the curved form of section developing a region of pressure beneath and rarefaction above in the same manner as a simple inclined plane, but with the advantage of greatly reduced resistance in the line of motion. Now if the velocity of travel is insufficient the air reaction will be deficient, and if the velocity be excessive the air reaction becomes greater than the weight of the machine, and consequently there is for any particular adjustment of a machine a certain speed at which its weight will just be buoyed up so that (so long as this critical speed be maintained) its centre of gravity will continue to move in a straight line; that is to say at this speed (which has already been referred to as the 'natural velocity') there is no tendency of the course of the machine to change. Now let us suppose that the machine without motor be launched horizontally at its 'natural velocity' then at first no change in its direction of motion takes place, but as its velocity falls owing to the resistance of the air (frictional and otherwise), it gradually assumes a course inclined downwards, and after some oscillation finally settles down