Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/331

Rh in terms of the radius, we shall have for $$n =$$ 3, radius $$\times$$ 2.8, which, multiplied by $$\tfrac{180}{\pi} ,$$ gives 160 degrees apart, or two blades nearest whole number, and for $$n =$$ 8 we get 2.1, which gives 120 degrees, or three blades almost exactly.

In view of experience, it is evident that these results are lower than necessary; it is found advantageous to employ more blades than here stated. This discrepancy is perfectly explicable, for the assumptions we have made all tend towards a minimum value. The desirability of keeping the propeller circle as small as possible is probably responsible for the employment of a fourth blade in the marine propeller. Four blades doubtless give rise to some slight interference and loss of power, but not sufficient to be seriously detrimental.

If we take three blades as a standard for the marine propeller where $$n =$$ 3, the corresponding value when $$n =$$ 8 (a probably impracticable proportion) would be four blades almost exactly. Carrying the matter further on the same basis, if we design an aerial propeller, discarding below 95 per cent, of maximum, the blade length will be approximately 1.2 times the radius (at 45 degrees), so that the proportional number will be 5 blades for $$n =$$ 3, and 6$$\tfrac{1}{2}$$ blades for $$n =$$ 8; that is to say, six blades can be carried. If we lower the discard point to 90 per cent, the conditions as to number of blades will become approximately the same as for the marine propeller discarding from the higher percentage; extending the comparison, it would be very difficult to distinguish the one propeller from the other, both being fashioned in accordance with the present theory for the same $$n$$ value.

It is probable that, owing to the much lighter pressures required, it will be practicable to employ greater values of $$n$$ in the aerial propeller than in the marine propeller: an aspect ratio of 6 or 8 does not appear to present any difficulty. This being the case, it is highly probable that the aerial propeller in practice may become almost as economical as, if not more