Man's physical universe

xanabras

366 FORMS OF ENERGY

An airplane propeller acts like a boat propeller, but it has to be

much larger relative to the size of the ship because the density of the

air is much less than the density of water. Propellers act like wood

screws, bits, or augers, in the way in which they force their way through

matter.

The denser the air is, the greater will be the forward motion produced

by a revolution of the propeller, because a greater mass of material has

to be moved by the propeller and therefore a greater force is exerted.

Take-ofTs require longer runs, and the rate of climbing is lower on

damp days than on dry days, because the density of damp air is

than that of dry air.

Metal propellers are more efficient than wood propellers because of

their thinner sections.

less

A change from a wood to a metal propeller may

result in an increase of air speed (speed of the airplane relative to air)

as high as 5 per cent.

The pitch of a propeller determines the distance which the propeller

advances per revolution. It is important that all portions of the propeller

have equal pitch, and for that reason the propeller is so designed

that it has increasingly greater angles toward the hub (center), so

that each portion of the blade will perform an equal share of the

propelling action. Inasmuch as the blade travels faster at the tip

than at the hub the blade is tapered.

When taking off and climbing, the pitch of the propeller should be

least, thus obtaining maximum power by allowing the number of

revolutions per minute of the engine to increase; but when flying at a

cruising-altitude, the propeller pitch should be increased in order to

permit it to take the maximum "bites" out of the air. The maximum

amount of power is required when taking off and climbing. When

flying at a level, the highest efficiency is obtained by maintaining the

manufacturer's recommended number of revolutions per minute by

increasing the pitch of the propeller. Large, high-speed airplanes have

devices by which the propeller pitch may be varied automatically so

as to keep the number of revolutions per second constant.

This is of

special importance in multi-engined aircraft, as it is a means of keeping

the engines accurately synchronized.

The Force of Gravity Must Be Considered in Designing an Airplane.

The performance of an airplane depends upon the power and the

weight of the airplane. The speed of an airplane is not greatly increased

by increasing the power of the engine because the horsepower varies

as the cube of the speed — doubling the speed requires eight times as

many horsepower.

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