Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/219

Rh plane itself that prevents the dead-water from being washed bodily away. Consequently the vacuum on the rear of the plane is increased to just the same extent as the pressure on the front is diminished, both quantities being measured by their integration over the respective faces of the plane; that is to say, the existence of a perforation has no influence on the total reaction on the plane.

When perforations are made of great size in proportion to the dimensions of the plane, we can conceive of the efflux stream passing en masse through the dead-water without parting with the whole of its momentum, so that in such a case the plane will be relieved of a portion of its resistance. The same may be supposed to happen if the perforations become sufficiently numerous.

.—In the present chapter the discussion is based principally on the result of Mr. Dines' investigations, the value of $$C$$ for the plane of compact form being taken at .66. The author has been influenced in this partly by the fact that in all probability Dines' results are nearer the truth than those of Professor Langley, but more particularly by the consideration that when instituting a comparison it is safer to confine one's attention to the work of a single investigator, and Langley's experiments with the normal plane were not carried far enough to give the information required.

For general employment in the subsequent volume (“Aerodromics”) the value of $$C$$ is taken as .7, which is the result given by Langley for a plane of square form and corresponds with the result given by Dines for a plane 4 × 1.

In adopting this value it has been borne in mind that it is desirable to have a general average figure that can be used with safety without specifying the exact form of the plane, and, taking $$C$$ as .7, it will not matter seriously whether Langley's or Dines' result should ultimately prove to be the nearer to the truth.