CHAPTER V
Practical Construction:
Bending Air-screws
Great diversity of practice exists with regard to the construction of model air-screws, some aero-modellists favouring small diameter with long pitch, others long diameter and short pitch, and still others who adhere to either bentwood or carved screws in either of the above forms. Generally speaking, a screw with a large diameter in proportion to short span has a short pitch, say one and a quarter times the diameter, while those having a short diameter in relation to span should have a fairly long pitch, from one and a half to twice the diameter. It is a useful rule to make the diameter approximately one-third the span of the machine for either single-screw or twin-screw machines. This relation seems to give a very small effect on lateral stability, whereas when the diameter is made larger, the machine has a tendency to capsize laterally in the opposite direction to which the screw revolves. This force is known as torque.
It can, however, be fairly claimed that, for a given torque or turning power, better results are usually obtained with carved screws, whether short or large ones are used. The writer personally prefers a large diameter and short-pitched screw, because, as the screw thrust is equal to the weight of air displaced, the larger the screw the greater is the proportion of air driven back in proportion to diameter. That is to say, double the diameter and four times the volume of air is displaced for only a double expenditure of power.
Fig. 51.—Bentwood Screws
Fig. 52
Fig. 53
Figs. 52 and 53.—Standard Types of Bentwood Screws