The Theory and Practice of Model Aeroplaning

by V. E. Johnson

In the case of models, not models of full-sized machines, but actual model flyers, the writer considers propulsive screws much the best.[25]

In no case should the propeller be placed in the centre of the model, or in such a position as to shorten the strands of the elastic motor, if good flights are desired.

In the case of petrol or similar driven models the position of the propeller can be safely copied from actual well-recognised and successful full-sized machines.

§ 2. The Number of Blades.—Theoretically the number of blades does not enter into consideration. The mass of air dealt with by the propeller is represented by a cylinder of indefinite length, whose diameter is the same as that of the screw, and the rate at which this cylinder is projected to the rear depends theoretically upon the pitch and revolutions (per minute, say) of the propeller and not the number of blades. Theoretically one blade (helix incomplete) would be sufficient, but such a screw would not "balance," and balance is of primary importance; the minimum number of blades which can be used is therefore two.

In marine models three blades are considered best, as giving a better balance.

In the case of their aerial prototypes the question of weight has again to be considered, and two blades is practically the invariable custom.[26] Here, again, constructional considerations again come to the fore, and in the case of wooden propellers one of two blades is of far more easy construction than one of three.

By increasing the number of blades the "thrust" is, of course, more evenly distributed over a larger area, but the weight is considerably increased, and in models a greater advantage is gained by keeping down the weight than might follow from the use of more blades.

§ 3. Fan versus Propeller.—It must always be most carefully borne in mind that a fan (ventilating) and a propeller are not the same thing. Because many blades are found in practice to be efficient in the case of the former, it is quite wrong to assume that the same conclusion holds in the case of the latter.

By increasing the number of blades the skin friction due to the resistance that has to be overcome in rotating the propeller through the air is added to.

Moreover a fan is stationary, whilst a propeller is constantly advancing as well as rotating through the air.