The action of an air-screw may be likened to a bolt turning in a nut (the screw being the bolt and the air the nut), the difference being that whereas one turn of a bolt with, say, a Whitworth pitch of 14 threads per inch in a nut is bound to advance a distance equal to the pitch = ¹/₁₄ in., an air-screw may only advance 75 per cent. of its theoretical pitch, owing to the yielding nature of the air. This loss in efficiency is called “slip,” and is usually expressed as a percentage of the theoretical pitch. Thus a screw with a theoretical pitch of 4 ft., which possesses 75 per cent. efficiency, has an effective pitch of 3 ft. That is to say, each turn of the screw will take the aeroplane forward 3 ft. If, however, the screw were working in a solid, it would advance its theoretical pitch = 4 ft. A greater efficiency is obtainable with screws working in water, owing to the difference in density of the two media, namely, air is to water as 800: 1. Probably no air-screw has yet exceeded 80 per cent. efficiency, 70 per cent. being a fair average.
It may, perhaps, not be amiss to outline some of the factors involved in the design of an air-screw. Having decided on the diameter of it, the proportions of the block from which the screw is to be carved are required. It is a very good rule to make the pitch from one and a half to twice the diameter for single-screw machines, and from two and a half to three times the diameter for twin-screw machines. It is possible to use much longer-pitched screws with twin-screw machines (it being understood that the screws revolve in opposite directions), since the torque, or tendency of the screw to capsize the machine in the opposite direction to which it revolves, will be balanced. For the purposes of this chapter, however, it is presupposed that a screw is required for a single-screw machine, and a diameter of 12 in. has been decided on. One and a half times 12 in. gives 18 in. as the pitch. Remembering the formula for pitch,
| (thickness of block) | ||
| P = 3¹/₇ | × D × | ———————— , |
| (width of block) |
where P = pitch, D = diameter of screw, and using a ratio of width of blade to diameter of screw of 6: 1 (which gives 2 in. as the width of block) gives
| 22 | 12 | (thickness of block) | |||
| 18 = | —— | × | —— | × | ———————— , |
| 7 | 1 | 2 |
| 61 | ||||
| from which thickness of block = | ·954 = | — | approx. | |
| 64 |
Fig. 39
Fig. 40