University of Virginia Library

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.

On this subject Mr. R. W. Jamieson says in Aeronautics:

"Take for example an 8-foot screw propeller having an 8-foot pitch at its largest diameter. If the angle is the same throughout its entire blade length, then all the porions of its blades approaching the hub from its outer portion

would have a gradually decreasing pitch. The 2-foot portion would have a 2-foot pitch; the 3-foot portion a 3-foot pitch, and so on to the 8-foot portion which would have an 8-foot pitch. When this form of propeller is caused to revolve, say 500 r.p.m., the 8-foot portion would have a calculated pitch speed of 8 feet by 500 revolutions, or 4,000 feet per min.; while the 2-foot portion would have a calculated pitch speed of 500 revolutions by 2 feet, or 1,000 feet per minute.