View Full Version : Relation between Diameter, Pitch, RPM, and HP

B Hosticka
02-04-2006, 02:37 AM
Is there a standard formula for relating the Diameter, Pitch, RPM, and HP for a propeller? The best case would be to know RPM, HP, assume a Diameter and calculate a pitch. But any formula with these four variable will do since a little algebra can isolve for the unknown (1) parameter.

I guess that an efficiency would have to be assumed for this. Of course the number of blades is specified also. Lets say two.

Also what is done to make a test club instead of a propeller. HP and RPM would have to be the same as a real propeller but diameter would be smaller and thrust is a disadvantage. Low efficiency?

SO many questions.

Brian Mahoney
05-25-2006, 06:20 PM
The clubs for test installations I think tried to replicate the angular momentum of the installed prop without the bothersome thrust and wind issues.

In terms of power/speed/pitch questions, one needs to also think in terms of 'swept area per unit time.' Scale modelers are aware of this... tiny scale flying propeller models need to oprerate at hideous RPMs—not the same RPMs as their namesakes—to meet the swept area requirement. (It is in proportion to the SQUARE of the radius... )

There is a long technical and theoretical passage in Graham White's "R-2800—Pratt and Whitney's Dependable Masterpiece" (©2001 Airlife Publisher, England) about the tendency of freely floating blades in a propellor to precess toward the fully feathered orientation, which has design implications for pitch actuators and hubs.

At some level, while an oversimplification, it is conceptually useful to think of the effective pitch of the blade as analogous to a car's gear- finer attach angles being like lower gears in a ground vehicle. One uses many rotationst of them to go a short distance, but that gives an engine 'good advantage' over the loads of accelerating. An aggressive pitch is like a high gear, and one can design too-high a gear fro any car or too-high a pitch for any plane, if not mindful of torque limits at the intended speed.

Hope this imagery is helpful.