§ 17. The thrust of the propeller has evidently to balance the
Aerodynamic resistance = R
The head resistance (including skin friction) = S
Now according to Renard's theorem, the power absorbed by R + S is a minimum when
S = R/3.
Having built a model, then, in which the total resistance
= 4/3R.
This is the thrust which the propeller should be designed to give. Now supposing the propeller's efficiency to be 80 per cent., then P—the minimum propulsion power
= 4/3R × 100/80 × 100/75 × v.
Where 25 per cent. is the slip of the screw, v the velocity of the aeroplane.