Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/413

Rh principles discussed in §§ 182, 183, and 184, may actually result in a diminution in the tangential reaction.

'''§ 247. Author's Experiments. Summary.'—The experiments described in the preceding sections are quite convincing from a qualitative point of view, although quantitatively'' speaking the results are inconclusive.

Any and all of the methods described should be capable of giving results of a reasonable degree of accuracy—far more so than at present achieved—and the results so obtained should be in closer accord, one with another, than the author has so far been able to demonstrate.

The deficiency in the present experiments is chiefly that of apparatus and opportunity. The launching of free flight models requires a suitable apparatus to be designed, by which the initial velocity shall be placed under definite control; beyond this it must be considered quite essential, if reliable results are required, that experiments should be conducted inside a building; the absolute calm necessary for aerodynamic determinations is so rare a phenomenon as to render outdoor experiment almost impossible. It is only those who have watched and waited for a really calm day who can fully appreciate its rarity. In repeating these experiments it would be well to arrange for the use of a large hall, well secured against draughts; the equilibrium of low velocity models, such as it is necessary to employ, is very sensitive; even the previous motion of a person across the line of flight will affect the gliding path. High velocity models, although possessing greater stability, are not well suited to the determination of aerodynamic data.

The author's conclusions as to the value of $$\xi$$ have been given in § 157. Some of the experiments here recorded have been made since these conclusions were formulated, but the differences are not such as to render revision necessary. In brief, it would appear that under all practical conditions the coefficient of skin-friction lies between the values .01 and