Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/217

Rh assessed; but as the whole range of $$C$$ values lies almost within the admittedly possible allowance for experimental error, our want of knowledge on this point is not so serious as might otherwise appear.

§ 143. Perforated Plates.—Dines has investigated the effect of perforations as affecting the resistance of the normal plane. In one case a plane one foot square was taken and eight circular holes, each one square inch area, were punched, as illustrated in Fig. 90; no difference of pressure could be detected whether any or all of the holes were covered or open. Mr. Dines remarks: “The eight holes together take away more than 5 per cent, of the plate, yet a difference of 1 per cent, in the pressure, had it existed, would certainly have been apparent.” Further experiments were made with two kinds of perforated zinc, the one sample, holes .08 inch diameter 77 per square inch, having only 61 per cent, of the total area, was found to give 91 per cent. of the total pressure; another sample with perforations .22 inch diameter, 11 or 12 per square inch, possessing only about 66 per cent, of the total area, gave 80 per cent, of the pressure on a solid plate.

The curve given in Fig. 91 is deduced principally from Dines' experiments. Abscissae give percentage area removed, and ordinates show the corresponding pressure as a percentage of that on the same area intact. It is supposed that when the percentage of area remaining becomes small, the perforations are of square form as indicated.

The anomalous behaviour of the perforated plate is perfectly explicable on theoretical grounds.

The region in the rear of the plane is occupied by the turbulent “dead-water” at a pressure below that of the undisturbed