§ 1. The design and construction of propellers, more especially the former, is without doubt one of the most difficult parts of model aeroplaning.
With elastic or spring driven models the problem is more complicated than for models driven by petrol or some vaporized form of liquid fuel; and less reliable information is to hand. The problem of weight, unfortunately, is of primary importance.
We will deal with these points in due course; to begin with let us take:—
The Position of the Propeller.
In model aeroplanes the propeller is usually situated either in front or in the rear of the model; in the former case it is called a Tractor Screw, i.e., it pulls instead of pushes.
As to the merits of the two systems with respect to the tractor, there is, we know, in the case of models moving through water a distinct advantage in placing the propeller behind, and using a pushing or propulsive action, on account of the frictional "wake" created behind the boat, and which causes the water to flow after the vessel, but at a lesser velocity.
In placing the propeller behind, we place it in such a position as to act upon and make use of this phenomenon, the effect of the propeller being to bring this following wake to rest. Theoretically a boat, model or otherwise, can be propelled with less horse-power than it can be towed. But with respect to aeroplanes, apart altogether from the difference of medium, there is at present a very considerable difference of form, an aeroplane, model or otherwise, bearing at present but little resemblance to the hull of a boat.
Undoubtedly there is a frictional wake in the case of aeroplanes, possibly quite as much in proportion as in the case of a boat, allowing for difference of medium. Admitting, then, that this wake does exist, it follows that a propulsive screw is better than a tractor. In a matter of this kind constructional considerations, or "ease of launching," and "ability to land without damage," must be given due weight.
In the case of model aeroplanes constructional details incline the balance neither one way nor the other; but "ease in launching" and "ability to land without damage" weigh the balance down most decidedly in favour of a driving or propulsive screw.
In the case of full-sized monoplanes constructional details had most to do with the use of tractors; but monoplanes are now being built with propulsive screws.[24]