View Single Post
Old Feb 21, 2011, 02:07 AM
Bruce Abbott is offline
Find More Posts by Bruce Abbott
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
5,217 Posts
Originally Posted by A Useless Geek View Post
If they want us to buy their product the least they can do is provide some numbers that they must have generated themselves even before building a prop.
I think you will find that most prop manufacturers have not comprehensively tested all of their props, and I doubt that they have much (if any) test data to hand.

Another problem with this idea is that manufacturers could have different testing methods and/or calibration issues, so their results may not be comparable. Far better would be to collate data from independent testers (who have verifiable testing methods) and compensate for calibration issues by comparing their results for identical props.

I am hoping the prop OEMs can give us standardized numbers like thrust per RPM
'thrust per rpm' is not a constant. Thrust is (theoretically) proportional to rpm^2. Plastic props bend under load, so their exponents may not be exactly 2. To properly describe a prop's performance at different rpm you need a 'thrust-vs-rpm' constant and a 'thrust-vs-rpm exponent' constant.

Originally Posted by rgoble
Heres an example graph of an APC SF 9x4.7 showing thrust and g/W vs RPM.
Could you fit a curve of thrust vs. rpm to the averages of all those points, then show the derived constants? The curve's equation would be of the form 'thrust = a*rpm^b' (where constant a = thrust.rpm.factor, and b = thrust.rpm.exponent).

As your graph shows, 'g/W' is almost worthless. The reasons for this are firstly that 'W' is the input power to the motor, not output power - so the result is dependent on the efficiency of the motor, and secondly that 'g/W' is also not a constant!

To get the true picture of a prop's thrust vs power you need to measure motor output power with a dyno (which few people have). This is a pity, because thrust vs power is a better measure of prop performance than thrust vs rpm. However, thrust vs input power is useful for comparing the performance of different motors with the same prop.
Bruce Abbott is offline Find More Posts by Bruce Abbott
Reply With Quote