Page:Aerial Flight - Volume 2 - Aerodonetics - Frederick Lanchester - 1908.djvu/431

Rh angle of inclination of the plane of rotation, and any influence acting to retard the precession has the opposite effect.

Now a displacement forward of the centre of pressure, as in the case of the disc or a simple aeroplane, manifestly tends to hurry the precession, for it is the equivalent of a force acting parallel to the pendulum rod but some little distance in advance of it in its motion; hence the effect of such displacement is to reduce the inclination of the plane of rotation. This is actually found to take place when a boomerang is thrown; it becomes during its flight less and less inclined, that is to say, the axis of rotation becomes more and more nearly vertical, until after a complete loop has been traversed the boomerang will sometimes describe one or more simple oscillations of the kind already described with reference to Fig. 196. A diagram of a typical flight path is given in Fig. 198.

Fig. 198.

For the same reason, in order to obtain the longest possible flight, the initial angle should be very steep; in some cases, in fact, the boomerang is thrown in a nearly vertical plane. The extent to which the disc effect is felt depends primarily upon the form of the boomerang; if the limbs are broad and the proportions we generally "chubby," the torque about the transverse axis will be relatively considerable. When, on the contrary, the limbs of the boomerang are thin and spider-like, the disc effect will not be so noticeable.