Propellers and weight vs RPM

In summary, the propeller will spin at max rpm when the drag from the air interacting with it equals the torque generated by the engine.
  • #1
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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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  • #2
Ryder S said:
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.
 
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  • #3
tnich said:
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.
 
  • #4
Ryder S said:
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.
 
  • #5
Reduction in mass or increase in mass will affect the angular acceleration or retardation.
 
  • #6
anorlunda said:
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!
 
  • #7
Thank you, tnich!

That's perfect.
 

1. How does the weight of a propeller affect its RPM?

The weight of a propeller directly affects its RPM, as a heavier propeller will require more energy to rotate and therefore will have a lower RPM than a lighter propeller.

2. What is the optimal weight for a propeller to achieve maximum RPM?

The optimal weight for a propeller to achieve maximum RPM depends on various factors such as the size and type of the propeller, the engine power, and the intended use. Generally, a lighter propeller will result in a higher RPM, but it is important to consult with a professional to determine the best weight for your specific needs.

3. Can a propeller's weight be adjusted to increase its RPM?

Yes, a propeller's weight can be adjusted to increase its RPM. This can be done by either decreasing the weight of the propeller or by increasing the engine power. However, it is important to note that altering the weight of a propeller can impact its performance and should be done carefully and with the guidance of an expert.

4. How does the shape of a propeller affect its RPM?

The shape of a propeller can also affect its RPM. A more streamlined and aerodynamic shape can result in a higher RPM as it reduces drag and requires less energy to rotate. Additionally, the pitch of the propeller plays a role in its RPM, as a higher pitch can result in a higher RPM.

5. What is the relationship between propeller weight and fuel efficiency?

The weight of a propeller can impact fuel efficiency, as a heavier propeller will require more energy to rotate, resulting in higher fuel consumption. Therefore, choosing a propeller that is the appropriate weight for your needs can help improve fuel efficiency.

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