Folks,
The initial question was about the difference in drag between a windmilling prop and a feathered prop. Discing, as we commonly use the term, is a different version of the normal windmilling situation.
A feathered propeller presents a stationary profile to the breeze. It generates form drag. It does not generate lift and thus no induced drag. It cannot generate positive thrust.
A windmilling prop is not (normally) being driven by the engine. It rotates because it generates a
net rotational force (lift component) in the direction of rotation (positive component). The rotational force is overcoming the machinery drag of the engine and gearbox only and, importantly, is a force at right angles to the direction of flight.
In helicopter parlance, the propeller is autorotating. That happens because the blade is twisted and the angle of attack (and therefore lift component)on each blade element varies as a function of distance from the hub (rotational velocity), forward velocity and incidence (blade twist). The alpha varies from positive to negative along the blade. The rotational force is a net positive component because there are negative components due to stalled areas of the blade (too much alpha) and non-lifting areas (not enough alpha).
However, the drag we are referring to is in the direction of flight, not the direction of blade rotation. It comes about because the propeller is generating negative thrust. The twisted blades, although rotating, produce an overall drag component in the flight path axis from the stalled sections and the non-lifting (or even negative thrust producing) sections.
The severity of the windmilling drag is related to the windmilling (rotational)speed. Quite often, a feathered prop will continue to rotate slowly, however the drag is not very high. As we unfeather the prop, we are mechanically changing the alpha, thus generating some positive rotational force but substantially increasing the drag. As the prop speeds up, we approach windmilling drag - the steady state drag we get from extracting all of that energy from the airflow to drive our propeller. In most cases, the windmilling RPM is fairly low because the propellor has mechanical stops that prevent the propeller from getting ultra-fine.
In my experience, we tend to speak of discing in terms of turbo props that have either beta ranges or reversing props. These props have stops that permit much higher windmilling speeds because you cannot afford to lose significant propeller RPM when you select idle if you are then about to select beta or reverse thrust. They generally govern the engine to maintain propeller RPM. Hence, the drag increase when you select idle with those propellers is a quantum leap over that which you experience selecting idle in a (typically piston-powered) non-reversing propeller.
Comparisons would best be made by describing discing as entering a pitch range that generates significant negative thrust while the propeller is still autorotating.
The next step is to select even finer pitch which results in lots of negative thrust and a net rotational drag which must be overcome by engine power - "reverse thrust".
Have I clarified or confused? I much prefer diagrams but the techology to do that on PPRuNe escapes me.
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