Page:Aerial Flight - Volume 2 - Aerodonetics - Frederick Lanchester - 1908.djvu/433

Rh flight of 500 feet traversed, we have, if W be the weight, energy = 100 W. Now, if this be taken wholly from the energy of translation, and if, as is sometimes the case, the velocity be approximately nil when the boomerang alights, the initial velocity will be that corresponding to a fall of 100 feet, or say 80 ft./sec, and the time of flight will be between six and seven seconds. As a matter of fact, the time for a flight of this length is usually about eight seconds, so that probably the velocity of projection is somewhat lower and some of the energy is derived from the rotation.

The author does not believe, in the case of a good boomerang, that much of the energy comes from the rotation, for after a successful flight the boomerang commonly comes to earth spinning briskly. In some cases it has even been observed that the speed of revolution is greater at the end of a flight than at the beginning, both as a matter of simple observation and when counted against a stop-watch.

The relative absorption in flight of the energy of translation and that of rotation depends upon the form of the boomerang. Thus if it be quite flat it is evident that both sources of energy will be utilised, whereas if it be of appropriate screw form the rotation may be maintained by the pressure reaction, when the energy of translation will alone be drawn upon. Again, if the said screw form be exaggerated the energy of rotation may actually be increased at the expense of that of translation. If, on the contrary, the screw form be of opposite "hand," i.e., of cork-screwlike form, the energy of rotation may be drawn upon to any desired extent.

It is not always easy to appreciate from mere inspection of a boomerang whether it is, in effect, of right-handed or of left-handed screw form; thus the flat or under face may be markedly a right-hand screw and yet the whole form be in effect left-handed. The reason for this is to be found in the explanation given in Vol. I., § 80, of the aerial tourbillion; the flow round the convex leading edge of the boomerang ejects the dead water in that region and causes a suction by the centrifugal force of the adjacent