(2) The vital importance of the shape of the plane and of its aspect on the weight supported.
(3) The existence of a critical angle at which the aspect effect undergoes reversal.
(4) The fact that planes can be superposed in flight without sensible diminution of their individual supporting power, provided that they are separated by a certain minimum distance. For planes fifteen inches by four inches in pterygoid aspect the minimum distance between the superposed planes was found to be about four inches, or approximately equal to the “fore-and-aft” dimension.
It is curious that, although Langley in many places elsewhere in the Memoir has pointed out the failure of the sine2 law (the law of the Newtonian medium) as applied to air, he apparently overlooks the fact that the falling plane is on this point actually the experimentum crucis, for it has been shown (§§ 145—50) that if the sine2 law holds good in any fluid or medium, the rate of fall of a horizontal plane will be independent of and unaffected by its horizontal motion.
We find once more, in the chapter dealing with the plane dropper, the assumption that skin friction is negligible, resulting in much false inference. This time the error is tacitly assumed. No further proof is announced. The statements that are affected by this error are sufficiently numerous. The following example will put the reader of the Memoir on his guard:—
“The results of these two series of experiments furnish all that is needed to completely elucidate the proposition that I first illustrated by the suspended plane, namely, that the effort required to support a bird or flying machine in the air is greatest when it is at rest relatively to the air, and diminishes with the horizontal speed which it attains, and to demonstrate and illustrate the truth of the important statement that in actual horizontal flight it costs absolutely less power to maintain a high velocity than a low one.”
