Ernest Christley wrote:
Jim, you're always looking for simple DIY projects for your kitplane
articles, aren't you?

Some guys on the flyrotary list have be batting around ideas on how to
determine if the prop on a pusher is windmilling. Would a piece of
reflective tape and an optical pickup suffice?

What I'd do is to use two magnets securely affixed to the shaft, and a
hall sensor, to read them.
Optics get dirty IME.
I assume the prop is connected to the crankshaft in some manner that
means if one turns the other does.
You may already have suitable crank sensors.