Is there some way to calculate thr rpm of a bldc motor under load if you know the motors max rpm?

No, i’d say not really. ”Load” is too undefined. On ebikes i’ve used 85% with an ok correlation. Propellers can vary a lot in load.

So basically, for max mechanical (output) power → aim for 70% or so from free spining rpm

For max efficiency-> aim for 85-90% of free spin rpm, but you’ll only be getting half the max mechanical power your motor can deliver

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I didn’t find those numbers there?

So max mech power at; 5500/9000= actually 60%

Max eff at 8000/9000 = 90%

Efficiency depends on a specific motors’ losses so data can’t be used generally like from that diagram.

An example can be a motor with thick laminations but a hand wound rotor with max copper fill, efficient at lower speed, higher load/current.

Or the opposite, a motor with thin laminations but poor winding, efficient at higher speed, lower load/ current.

assuming you have got the correct sized components for the application, under load you got an average of 60-80% of the max rpm to make it simple.

that was my question 4 years ago…

Thanks for the knowledge!

Yea, thanks. Very helpful to have a ballpark figure. The thread Alexandre linked to was a good read too. Now I have a starting point