> how much drag off the propeller disk can be modulated by
> using the prop rpm control ?

A lot as noted by Scott.

> but then again . . . if the engine is ceased . . . the spring in the
> prop hub makes the blades go flat pitch . . . maximum drag !

As long as the engine is turning, the oil pump is typically making normal
oil pressure, so most engine failures give you the option of pulling the
prop back to course pitch (greatly reducing drag).

If oil pressure is lost, you are correct, the prop will go to max rpm, but
insufficient oil pressure may subsequently cause the engine to seize,
which is fairly low drag:(

Craig