What the Propeller Does.

Much of the efficiency of the motor is due to the form and gearing of
the propeller. Here again, as in other vital parts of flying-machine
mechanism, we have a wide divergence of opinion as to the best form. A
fish makes progress through the water by using its fins and tail; a
bird makes its way through the air in a similar manner by the use of its
wings and tail. In both instances the motive power comes from the body
of the fish or bird.

In place of fins or wings the flying machine is equipped with a
propeller, the action of which is furnished by the engine. Fins and
wings have been tried, but they don't work.

While operating on the same general principle, aerial propellers are
much larger than those used on boats. This is because the boat
propeller has a denser, more substantial medium to work in (water), and
consequently can get a better "hold," and produce more propulsive force
than one of the same size revolving in the air. This necessitates the
aerial propellers being much larger than those employed for marine
purposes. Up to this point all aviators agree, but as to the best form
most of them differ.

Kinds of Propellers Used.

One of the most simple is that used by Curtiss. It consists of two
pear-shaped blades of laminated wood, each blade being 5 inches wide
at its extreme point, tapering slightly to the shaft connection. These
blades are joined at the engine shaft, in a direct line. The propeller
has a pitch of 5 feet, and weighs, complete, less than 10 pounds. The
length from end to end of the two blades is 6 1/2 feet.

Wright uses two wooden propellers, in the rear of his biplane, revolving
in opposite directions. Each propeller is two-bladed.

Bleriot also uses a two-blade wooden propeller, but it is placed in
front of his machine. The blades are each about 3 1/2 feet long and have
an acute "twist."

Santos-Dumont uses a two-blade wooden propeller, strikingly similar to
the Bleriot.

On the Antoinette monoplane, with which good records have been made, the
propeller consists of two spoon-shaped pieces of metal, joined at the
engine shaft in front, and with the concave surfaces facing the machine.

The propeller on the Voisin biplane is also of metal, consisting of two
aluminum blades connected by a forged steel arm.

Maximum thrust, or stress--exercise of the greatest air-displacing
force--is the object sought. This, according to experts, is best
obtained with a la