Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/250

§ 161 treatment of the whole motion of the fluid, in which the pressure reaction should appear as an integration; the present theory may be said to be based on the assumption that this integration of the whole motion of the fluid may be fairly represented on the hypothesis of a finite layer uniformly acted upon.

While pointing out the imperfect nature of the hypothesis at present adopted, it is perhaps fair to say that its defects are comparable to those of the Rankine-Froude method of dealing with the problem of propulsion, and in common with that method it may be found to perform all that is practically required.

At present there are some difficulties, as will appear when the method is more fully discussed; these difficulties relate principally to the application of the somewhat unreliable data at present available—in particular, estimates of the value of $$\kappa$$ from existing data can be little more than guess-work, and it is questionable whether experiments conducted with merely a pair of planes are sufficient; in all probability the true value can only be obtained when a veritable screen of planes is employed.

It would appear highly probable that a separation that might be sufficient to prevent loss of pressure where two planes only are superposed would prove quite insufficient if a greater number of planes were involved, for, according to § 122 (Fig. 73), the