Thanks Lu.
Not enough lateral thinking on my part, as I had ignored such vehicles that have their wing surface travelling faster than the fuselage
Thanks Pigboat.
My source, Allied Aircraft Piston Engines of WWll, ISBN 1 85310 734 4, does not specify any dash F-50/60 DD, as the references make no mention of a "DD", and neither do they reference any rotary winged aircraft.
Many thanks for the 'heads up'