# Calculate Propeller RPM from Motor No Load RPM

1. Feb 16, 2016

Hello I am wondering what the relation between motor rpms and propeller rpms. I know that once you put the propeller on you are going to lose some of that speed due to drag and weight of the propeller.

Spec.
Battery: 10 Cell /37V
RPM: 231kv
Max current: 80A
Watts: 2960w
Internal resistance: 0.028 ohm
Pole Count: 24
Weight: 849g
Diameter of shaft: 10mm
Suggested ESC: 120A 10S Compatible
Suggested Prop: 21x13 APC-W

So this shaft maximum rpm would be 231 * 37 = 8500 rpms.

So if I put that prop on that they suggest what would the rpms be of that propeller and how would I go about figuring those numbers out so I can do this for multiple motors. I need to know what the maximum thrust output is to find the most inexpensive motor that has the highest thrust output.

So to sum up is there a mathematical way to figure out the rpms of a propeller with the propeller data and the shaft rpm of the motor so I can calculate the maximum thrust that the motor and prop I choose will put out?

Thanks for the help guys I hope you under stand what I am asking

Peace

Last edited by a moderator: Apr 17, 2017
2. Feb 18, 2016

### Staff: Mentor

The motor and propeller have performance curves: you will need to find where they cross.

3. Feb 19, 2016

### CWatters

+1

Sadly it's hard finding data on model aircraft props. You also need to factor in the flying speed as they unload as the plane accelerates. I've always had to buy several props an experiment.