Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/388

§ 238 negligibility of skin friction ; and if the whole Memoir be prefaced by the words, “neglecting the influence of skin-friction,” Langley's position would be substantially regularised.

Professor Langley's work has, however, been widely read, and his statements, unqualified as they stand, have been commonly accepted, and it is therefore impossible in a work of this type to be too emphatic in denouncing the errors in question.

It would seem probable that the publication of the “Experiments in Aerodynamics” was unduly hastened; it would otherwise be difficult to account for the repeated misleading citation of Newton (pp. 4, 8, 15, 24, 25, 89, and 105), when a moment's reference to any reliable edition of the Principia would have prevented any such mistake. Newton dealt with a hypothetical medium clearly defined in the enunciation to prop, xxxiv., and not with air at all, and the proposition cited is perfectly sound.

Beyond this the mathematical analysis constituting Appendix “B” is scarcely convincing. Also the calculation forming the second footnote, p. 9, the details of which are not given, is manifestly conducted on insufficient premises. This calculation purports to be a theoretical proof of the negligibility of skin-friction as confirming the supposed experimental result.

So far as the experimental work itself is concerned, apart from inference, it is undoubtedly the most valuable contribution to our knowledge that has so far appeared, with the exception perhaps of the work of Dines already discussed. The general results of Langley's experiments are entirely confirmatory of the theory set forth in the present work, but the experiments suggest that we have in our theory carried the “small angle” hypothesis to about its limit, and that if we have to deal with angles greater than those tabulated in Chap. VIII. some correction or refinement of method may become necessary.

§ 239. The Author's Experiments.—The author has investigated experimentally many of the problems connected with aerial flight. The greater part of these investigations relate to the