Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/203

Rh air current in the proportion that the sum of the energies bears to the energy of mean velocity.

The validity of the above reasoning is unquestionable if we are dealing with a fluid whose properties are those of the Newtonian medium; the energy of turbulence being represented by a variation in the individual velocities of the particles such as will not affect their mean velocity. It is, however, open to question whether it applies rigidly in the case of a fluid possessed of continuity.

Another standpoint from which we may view the present problem is that the turbulence, by effecting a rapid transference of momentum from one part of the fluid to another, acts in effect to augment the apparent viscosity, and in this way adds to the pressure reaction. In any case experiments made on a fixed plane or other body in moving air, cannot be regarded as valid when the conditions are reversed.

Beyond the above there are certain considerations of a practical nature that tend to further invalidate wind pressure measurements as representing aeroplane resistance. It is probable that the maximum pressure on a plane under given wind conditions is not in proportion to the area exposed, and that a small plane is liable to greater extremes, and where a maximum record is made, the absolute area exposed becomes an important factor in determining the pressure per unit.

§ 132. Still Air Determinations.—Under the conditions of experiment in still air, none of the foregoing considerations apply, and it may be safely asserted that the resistance per unit area is approximately proportional to the square of the velocity and is almost independent of the size of the plane. There is some doubt as to the exactitude of the $$V^2$$ law, as in all similar cases of fluid resistance, and it is likely that this doubt will remain until the methods of experiment have undergone refinement; on the one hand, if there is a departure, existing method is too crude to determine its nature, on the other hand