Propeller RPM

[Duchess_Driver]

Fixed pitch blade.

Constant RPM.

Increase in forward speed = increase in RPM.

Why...???

(And don't just say "It windmills...")

Notes:

I understand that the inflow to the disc increases with an increase in TAS and that therefore the relative airflow changes with a 'constant' rotational velocity decreasing the angle of attack for the blade. I'm thinking that with a reduction in alpha, you get a consequent reduction in thrust (lift) and as a consequence of that a reduction in torque (drag) which in turn provides less resistance to the engine which is working at the same throttle setting.

I am ignoring implicit changes in alpha due to the change in the aircraft attitude.

My problem with understanding this is that an increase in RAF produced by this increase in rotational velocity (Vsquared) would create a greater amount of 'lift' and a consequent increase in torque (drag) and the status quo is returned. Neither does this consider an increase in alpha as a result of the increase in RV....

Get the feeling that I'm over thinking it somewhere....


Duchess

[Runaway Gun]

You are over thinking it.

Essentially you are 'unloading' the propellor, and it accelerates, due to less work being required of it.

[hugh flung_dung]

Using non-technical terms ... initially the engine power is absorbed by the drag of the propellor; as the inflow increases the prop average AoA decreases so the prop drag decreases, so the RPM is able to increase. As the RPM increases the prop AoA increases and therefore the prop drag also increases so the RPM stabilises.
Which is basically what you said, I think!

HFD

[Oktas8]

My problem with understanding this is that an increase in RAF produced by this increase in rotational velocity (Vsquared) would create a greater amount of 'lift' and a consequent increase in torque (drag) and the status quo is returned. Neither does this consider an increase in alpha as a result of the increase in RV....

For a typical propeller aircraft, the propeller's rotational speed at the most effective part of the blade (around 200m/s for a light aircraft) is much much bigger than the forward speed (around 50 to 100m/s for a light aircraft).

Therefore, an increased forward speed produces a proportionally large reduction in propeller blade alpha with only a proportionally small increased airspeed experienced by the blade.

A large reduction in angle of attack with a proportionally much smaller increase in speed will have a very large effect on lift & drag, as you've already said. So, the blade will be "unloaded" and will increase RPM. Because rotational speed is (usually) much bigger than forward speed, a small change in RPM will restore the balance that existed before the aircraft accelerated.

I really wish I could post a diagram!

Cheers,
O8