Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/201

Rh congealing of portions of the fluid, are known to be unsound, but the results are not without interest.

The next experimental records chronologically are those of Robins (the inventor of the experimental device known as the whirling table), about the middle of the eighteenth century, and Charles Hutton in and about the years 1787–8; whilst among the most recent may be mentioned the systematic researches of S. P. Langley, 1888–90, and the investigations of W. H. Dines of about the same period. An abridged account of the most important of these investigations, with some criticism of the methods and conclusions, is given in a subsequent chapter devoted to experimental aerodynamics.

§ 130. The Normal Plane.—Law of Pressure.—The simplest case of the aeroplane is that in which the direction of motion through the air is at right angles to its surfaces.

Even under these simple conditions the determination of the pressure-velocity law has not been made without some difficulty, and although the approximate form of the expression, $$P$$ varies as $$V^2,$$ was correctly given by Hutton, Smeaton and others more than a century ago, it is only of recent years that the constant connecting the two sides of the equation has been ascertained with any degree of certainty, and that with a possible error of five per cent, or so. Writing the expression in the form—

the value of $$k$$ is variously given by different authorities as from .00166 to .0023 where $$P$$ is in pounds per square foot, and $$V$$ is in feet per second.

The experimental basis of the law of the Normal Plane is twofold; tests of wind pressure at known mean velocity, and experiments on the resistance to motion of planes through still air. At first sight there might appear to be no fundamental distinction between these two methods; the difference might be thought to be merely one of relative motion; owing, however, to certain considerations that require to be taken into account, the