It is difficult to speak positively on the question of the best speed at which a screw should rotate, as the loading per square foot of surface enters into the proposition. If a model has 1 sq. ft. of surface for every ounce of its weight, there is a speed at which the main surface will give a maximum of lift for a minimum of power, and a screw must be fitted whose pitch, multiplied by its revolutions per minute, equals the distance per minute the model should fly. If a screw that is too fast is fitted the model will show a tendency to “stall,” or ascend nose first, and if too slow a one is used the model will appear to be under-powered.
The writer has outlined these points to emphasise the fact that no definite rules, but only approximations, can be laid down, owing to the large number of unknown quantities which would have to be taken into consideration. As the aero-modellist, however, becomes accustomed to puzzling out the many little problems connected with model aeroplaning, he speedily diagnoses the complaint of a refractory machine, and applies a remedy accordingly.
Fig. 54.—Bentwood Shaft
Attachment
Fig. 56.—Safety Hook
Fig. 55.—Carved Screw Shaft
Fig. 57.—Proportions of Camm Air-Screw