# Propellers and weight vs RPM

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## Main Question or Discussion Point

Hello all...
I have a question about an engine-driven propeller. While a propeller is being spun, the air resists it and the maximum power of the engine will turn at "max rpm" when the drag from the air interacting with the propeller becomes equal to the torque generated by that engine.

Now imagine that you replace the prop with an IDENTICAL one, except that it half the weight (less dense material).

What happens to max RPM and why?

Conversely, imagine adding a propeller that is IDENTICAL except it's twice as heavy.... What happens to max RPM in this case?

Thank you!

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tnich
Homework Helper
Hello all...
I have a question about an engine-driven propeller. While a propeller is being spun, the air resists it and the maximum power of the engine will turn at "max rpm" when the drag from the air interacting with the propeller becomes equal to the torque generated by that engine.

Now imagine that you replace the prop with an IDENTICAL one, except that it half the weight (less dense material).

What happens to max RPM and why?

Conversely, imagine adding a propeller that is IDENTICAL except it's twice as heavy.... What happens to max RPM in this case?

Thank you!
Torque is the rate of change of angular momentum. So if a propeller is spinning at a constant rate, the net torque on the propeller is zero. In your equation for net torque, the drag of air on the propellor balances the torque generated by the engine.
$T_{engine} = \int_0^R D(r)~r~dr$, where D(r) drag per unit length at distance r from the center of the propeller.

The weight of the propeller does not appear in that equation, so it has no effect on the max RPM. It would have an effect on how long the propeller takes to spin up to max RPM.

A.T.
It would have an effect on how long the propeller takes to spin up to max RPM.
And spin down to zero when the power is cut.

anorlunda
Mentor
the engine will turn at "max rpm" when...
No, that's not true. When you fly a propeller powered airplane, changing throttle position changes RPM. Max RPM happens only at max throttle.

Reduction in mass or increase in mass will affect the angular acceleration or retardation.

No, that's not true. When you fly a propeller powered airplane, changing throttle position changes RPM. Max RPM happens only at max throttle.
As I had mentioned: "the maximum power of the engine". This is not a discussion about how engines produce max power. It is: *given* an engine at max power (by whatever means that is achieved... magic dust, planetary alignment, act of God, throttle position etc.)

Thank you!

Thank you, tnich!

That's perfect.