Windmilling and electrical power

Viperdoc writes:

Do you know the difference between a windmilling prop and one that is
feathered?

Yes. I also know that not all props have adjustable pitch, and that
the fully feathered position on a prop will not necessarily be exactly
parallel to the wind.

I suppose the question can apply to jet engines as well, but someone I
doubt that they would windmill with speed sufficient to generate
useful power, and I'm not sure which stage of the engine drives
generators, anyway.

Obviously windmilling itself is practical, since some aircraft have a
tiny deployable windmill specifically for this purpose.

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