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 50 pounds per horsepower.

The Voisin machine, with aviator, weighs about 1,370 pounds, 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