It is the object of all men who design propellers to obtain the maximum of thrust with the minimum expenditure of engine energy. With this purpose in view many peculiar forms of propeller blades have been evolved. In theory it would seem that the best effects could be secured with blades so shaped as to present a thin (or cutting) edge when they come out of the wind, and then at the climax of displacement afford a maximum of surface so as to displace as much air as possible.
While this is the form most generally favored there are others in successful operation.
There is also wide difference in opinion as to the equipment of the propeller shaft with two or more blades. Some aviators use two and some four. All have more or less success. As a mathematical proposition it would seem that four blades should give more propulsive force than two, but here again comes in one of the puzzles of aviation, as this result is not always obtained.
Difference in Propeller Efficiency.
That there is a great difference in propeller efficiency is made readily apparent by the comparison of effects produced in two leading makes of machines--the Wright and the Voisin.
In the former a weight of from 1,100 to 1,200 pounds is sustained and advance progress made at the rate of 40 miles an hour and more, with half the engine speed of a 25 horse-power motor. This would be a sustaining capacity of 48 pounds per horsepower. But the actual capacity of the Wright machine, as already stated, is 50pounds per horsepower.
The Voisin machine, with aviator, weighs about 1,370pounds, and is operated with a so-horsepower motor.
Allowing it the same speed as the Wright we find that, with double the engine energy, the lifting capacity is only 27 1/2 pounds per horsepower. To what shall we charge this remarkable difference? The surface of the planes is exactly the same in both machines so there is no advantage in the matter of supporting area.
Comparison of Two Designs.
On the Wright machine two wooden propellers of two blades each (each blade having a decided "twist")are used. As one 25 horsepower motor drives both propellers the engine energy amounts to just one-half of this for each, or 12 1/2 horsepower. And this energy is utilized at one-half the normal engine speed.
On the Voisin a radically different system is employed.
Here we have one metal two-bladed propeller with a very slight "twist" to the blade surfaces. The full energy of a 50-horsepower motor is utilized.
Experts Fail to Agree.
Why should there be such a marked difference in the results obtained? Who knows? Some experts maintain that it is because there are two propellers on the Wright machine and only one on the Voisin, and consequently double the propulsive power is exerted.
But this is not a fair deduction, unless both propellers are of the same size. Propulsive power depends upon the amount of air displaced, and the energy put into the thrust which displaces the air.
Other experts argue that the difference in results may be traced to the difference in blade design, especially in the matter of "twist."The fact is that propeller results depend largely upon the nature of the aeroplanes on which they are used.
A propeller, for instance, which gives excellent results on one type of aeroplane, will not work satisfactorily on another.
There are some features, however, which may be safely adopted in propeller selection. These are: As extensive a diameter as possible; blade area 10 to 15 per cent of the area swept; pitch four-fifths of the diameter;rotation slow. The maximum of thrust effort will be thus obtained.
