Flying Machines: Construction and Operation / A Practical Book Which Shows, in Illustrations, Working Plans and Text, How to Build and Navigate the Modern Airship

by William J. Jackman; Octave Chanute; Thomas Herbert Russell

What the Designers Seek.

Every designer is seeking for the same result—the securing of the greatest possible thrust, or air displacement, with the least possible energy.

The angles of any screw propeller blade having a uniform or true pitch change gradually for every increased diameter. In order to give a reasonably clear explanation, it will be well to review in a primary way some of the definitions or terms used in connection with and applied to screw propellers.

Terms in General Use.

Pitch.—The term "pitch," as applied to a screw propeller, is the theoretical distance through which it would travel without slip in one revolution, and as applied to a propeller blade it is the angle at which the blades are set so as to enable them to travel in a spiral path through a fixed distance theoretically without slip in one revolution.

Pitch speed.—The term "pitch speed" of a screw propeller is the speed in feet multiplied by the number of revolutions it is caused to make in one minute of time. If a screw propeller is revolved 600 times per minute, and if its pitch is 7 ft., then the pitch speed of such a propeller would be 7x600 revolutions, or 4200 ft. per minute.

Uniform pitch.—A true pitch screw propeller is one having its blades formed in such a manner as to enable all of its useful portions, from the portion nearest the hub to its outer portion, to travel at a uniform pitch speed. Or, in other words, the pitch is uniform when the projected area of the blade is parallel along its full length and at the same time representing a true sector of a circle.

All screw propellers having a pitch equal to their diameters have the same angle for their blades at their largest diameter.

When Pitch Is Not Uniform.

A screw propeller not having a uniform pitch, but having the same angle for all portions of its blades, or some arbitrary angle not a true pitch, is distinguished from one having a true pitch in the variation of the pitch speeds that the various portions of its blades are forced to travel through while traveling at its maximum pitch speed.