Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/332

§ 211 economical than, its marine prototype. The whole question depends upon whether the gliding angle for the proportions of blade employed in the two types is in favour of the one or the other.

If the aspect ratio employed is as high as that here suggested, and if it is found advantageous to discard from as high a point as 95 per cent., it may with some confidence be predicted that it will be found advantageous to employ as many as six blades.

'''§ 212. Blade Length. Conjugate Limits.'''—In §§ 206 and 211 the limits of the blade length have been assumed as determined by the rejection of those portions of the efficiency curve falling below some stated percentage of the maximum, but it has not been demonstrated that this course results in the highest efficiency over the whole blade.

It might, with some show of plausibility, be argued that since the outer portion of the blade has more work to perform, the region of maximum efficiency should be nearer the point of the blade than given by the method suggested, and that therefore the diameter in terms of the pitch should be less than that deduced. Let us investigate the point in question.

We will suppose that, with a certain stated pitch and therefore an efficiency curve of defined scale, we have (in accordance