MjH,
The thrust of a propeller is roughly 5 lbs per horsepower for relatively small aircraft.
The following are approximate values for the X2. The gross weight is 6,095 lbs. The power of the engine is 1,450 SHP. The X2's rotors maybe in a low-rpm powered state or in an autorotative state at maximum forward speed. It has been mentioned that the craft was using somewhere around 80% power at this speed.
The high parasitic drag of the X2 is one of it's primary limitations, since drag increases at the square of the forward velocity. Hypothetically, the X2 could just about takeoff and hover on its prop as a
Tailsitter. 6,095 lbs / (1450 x 0.8) = 5.25 lbs per horsepower.
The following may provide you with a more exact answer.
I think that
Aero Composites, Inc. provided the propeller.
The propeller specifications plus this article
Theoretical Max Propeller Efficiency may be of value. The article was written by an engineer at CarterCopters.
If you get some information from Sikorsky, please post it. There is nothing better then the facts.
Dave