Thanks to the pioneer work of Lanchester and Prandtl everyone seriously interested in aeronautics has nowadays sufficiently clear ideas about the power and the associated drag which are essential for the support of an aeroplane in flight. This induced drag, as it is now called, can be calculated with an accuracy which is ample for most practical purposes, and even with the very clean aeroplanes of the present time it contributes, at and above cruising speed, a minor part only to the total drag. We are equally familiar with the idea that the remainder of the drag—the drag involved in getting the aeroplane through the air as opposed to holding it up—can be reduced, by careful design, nearly to the mere skin-friction of the air rubbing over exposed surfaces; the reason being, of course, that the pressures on the surface of a well streamlined aeroplane are so distributed that their horizontal components form an almost completely balanced system.

An account is given of developments during the last three years in the production of light magnesium alloys suitable for aircraft construction. The heat-treatment of castings and the hardening by ageing of forged alloys are considered as well as the copditions affecting forging at various temperatures. The connection between these properties and the crystalline structure of magnesium is discussed.

In the present paper it is demonstrated that variations in the Reynolds number as due to altitude are unimportant, provided that NR at sea level is in the region of 107.

That under these conditions the law of velocity in relation to altitude, is implicitly contained in the expression W= KLρV2 x area, in which ρ and V only are variables; i.e., the velocity of flight must vary in the inverse ratio of .

That the velocity limit due to the elasticity of air is lower at high altitude, for the velocity of sound is lower, the optimum flight velocity is higher. The facts concerning this are set forth.