Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/384

§ 234 Let us see what Langley says on the subject. We find (p. 63) the story of the plotting told m a few words:—

“... These values have been plotted in Fig. 11, and a smooth curve has been drawn to represent them as a whole. For angles below 10 degrees the curve, however, instead of following the measured pressure, is directed to the origin, so that the results will show a zero horizontal pressure for a zero angle of inclination.”

It may be remarked parenthetically that here the complete assumption has been made of that which it should have been the function of the experiment to prove. The author of the Memoir continues:—

“This, of course, must be the case for a plane of no thickness, and cannot be true for any planes of finite thickness with square edges, though it may be and is sensibly so with those whose edges are rounded to a so-called fair form. Now the actual planes of the experiments presented a squarely cut end surface one-eighth of an inch 3.2mm. thick, and for low angles of inclination this end surface is practically normal to the wind. Both the computed pressures for such an area and the actually measured pressures, when the plane is set at degree, indicate conclusively that a large portion of the pressures measured at the soaring speeds of 2 degrees, 3 degrees, and 5 degrees, is end pressure, and if this be deducted the remaining pressure agrees well with the position of the curve. The observed pressures, therefore, when these features are understood, become quite consistent. The curve represents the result obtained from these observations for the horizontal pressure on a plane with fair' shaped edges at soaring speeds.”

The above argument appears to the present author to be excusable as an attempt to explain why the results of one experiment or series of experiments might differ from some other experiment or established fact, but it does not constitute a demonstration that skin friction is negligible. The fallacy of an argument on these lines has been already pointed out in