Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/323

Rh indicated by the horizontal line, as confined to a zone in which the inferior limit of the efficiency is 90 per cent, of its maximum. It may be noted that this procedure gives for water a pitch almost exactly equal to the diameter; this is somewhat less than is customary, the generally accepted proportion being, pitch = 1$$\tfrac{1}{4}$$ diameter (approximately). It would appear that designers unconsciously reject the whole of the curve where of less value than about 95 per cent, of the maximum possible. In spite of this fact, the efficiencies so far recorded leave much to be desired, and, apart from practical limitations, would appear to show that there is still considerable room for improvement. The defects in existing practice would seem, according to the present theory, to be found in the want of attention to the requirements of pterygoid section, in the low value of $$n$$ (aspect ratio) commonly adopted (possibly from practical requirements not included in the present hypothesis), and to undue fulness of plan-form towards the blade tips, and consequent excessive frictional loss.

The value of the total efficiency, having selected the blade limits, depends not only upon the efficiency curve, but also on the distribution of the thrust over the length of the blade, or the thrust grading, as it may be conveniently termed. If the range of efficiency were very great, we should have to specify the thrust grading before the total efficiency could be computed; but as the variation does not usually exceed 10 per cent, or so, and as the general character of the grading curve cannot be in doubt, we can arrive immediately at a close approximation.

On the 90 per cent, basis, if the thrust grading were uniform over the length of the blade, the mean efficiency would, for the character of the curve given (Figs. 126, 127), be 96 or 97 per cent, of its maximum. If, as must be the case, the thrust grading fall to zero at the extremities, the efficiency will be increased;