Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/252

§ 162 Let an aeroplane (Fig. 109) be loaded in the manner shown so as to bring its centre of gravity to a position from a quarter to one-third of its width from one of its edges (we may assume it to be a plane of about 4 : 1 ratio in pterygoid aspect), and let it be launched in free flight with the ballasted edge leading to its line of flight. It will then be found that, provided the air is sufficiently calm, the plane will glide after the manner of a bird in passive flight, and will show itself to be possessed of complete stability.

The ballasted aeroplane in free flight may be employed for the determination of aerodynamic data as follows :—

(1) The value of c for planes of different aspect ratio in the expression, $$P_\beta = c\ \beta\ P_{90} .$$

(2) The determination and plotting of the position of the centre of pressure as a function of the angle of inclination for small angles.

(3) The determination of the value of the coefficient of skin-friction, $$\xi$$.

The most satisfactory results can be obtained by employing planes of mica, of only a few thousandths of an inch in thickness, the ballasting being effected by a split lead shot, as shown in the figure. Such planes show a perfection of equilibrium that appears to be unattainable with any other material than mica; it is also important that the ballast should be applied in a compact mass centrally and not distributed along the front edge. In order to improve the “sense of direction” it is found to be advantageous to “dog-ear” the front corners, slightly turning them upwards. It may be further noted that the rectangular form is very advantageous; in general other forms give inferior stability.

The theory of the equilibrium of the ballasted aeroplane belongs more correctly to the domain of aerodonetics, but the