Following up from my last post, this is figure 4 from NACA report 464 available from NASA. I have added some annotations.



To work out the negative thrust coefficients (i.e. Drag coefficient) for stationary blades of differing pitch find where the blue line intersects the relevant curve. The blue line represent a stationary blade.

To work out the drag coefficient for a free wheeling propeller start by finding where the red line intersects the relevant curve (red line is zero torque). Then 'by projecting down from the point of zero torque to the appropriate thrust curve', move over to left for the drag coefficient.

To work out the correct solution for a windmilling prop with friction, i.e. one with an engine directly attached a friction/torque curve is needed, however the actual result should lie on the respective curves so many quantitative comparisons can still be made.

Don't forget to consider how a CSU effects this. The CSU will move the blades to a finer pitch, and even at the lowest RPM setting may result in a fine pitch.

What this graph tells us:

1. Energy used to turn over the engine, or the resistance/friction of the engine is the not the cause of the drag. It does have an effect, depending on the blade angle it can make it substantially greater, or a minor difference or in the case of a very flat blade angle (> 7degrees) engine friction can actually reduce the drag. Energy used to turn the engine is a red herring.

2. Fine pitch produce very high drag coefficients, higher than a stationary prop, and far high than a feathered prop.

3. For the blade angles 12,17,22 a reduction in negative torque results in reduction in drag, i.e. less friction is better, this is consistent with flight manual check list for closing the throttle in a piston engine aircraft.

4. Blade angles < 7 degrees, an increase in negative torque results in a reduction in drag, positive torque is required for maximum drag.

Take all this with a grain of salt, but none of this contradicts any flight manual or training manual text, with regard to pilot actions. Feathering is imperative to reducing drag when propeller is capable.