Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/386

§ 235 diagram given, so that the efficiency will vary over the length of the blade from 70.4 per cent, to about 40 per cent.; if we take the mean as a rough approximation of the efficiency value to be expected, we have 55 per cent. The actual efficiency obtained was 52 per cent., which is quite as near as could be anticipated.

Again, as to the number of blades, Langley found that two blades gave a better result that any greater number of blades. Now the rule laid down in § 218 can hardly be relied on in the present case: the design of this propeller is abnormal. We may fall back on § 211. In the propeller under discussion the thickness of the peripteral zone (§ 210) will be evidently nearly as great, if not quite as great, as the pitch, consequently we must be approaching the point at which one blade will interfere with itself, and two blades will certainly overlap to some extent. It is consequently quite evident that any increase on the number must be detrimental. Thus we again find substantial confirmation of the peripteral theory. The concluding words of the chapter on the Dynamometer Chronograph are singularly to the point in view of the conclusion in § 211 on the comparison in theory of the aerial and marine propeller. Professor Langley says:—

“... It may be said that, notwithstanding the great difference between the character of the media, one being a light and very compressible, and the other a heavy and very incompressible, fluid, these observations have indicated that there is a very considerable analogy between the best form of aerial and of marine propeller.”

'''§ 236. Langley's Experiments. The “Counterpoised Eccentric Plane.”'''—An apparatus devised for determining the variations in the positions in the centre of pressure, for varying angles of inclination of a plane to its line of flight.

This appliance follows on established lines, the point of suspension of the plane being fixed for each trial and the angle of equilibrium being experimentally recorded. The result of these