Originally Posted by
Ascend Charlie
For 100kt, the fuselage angle is noticeably nose-down - what will it look like at 280kt?
It would be near level body to minimize drag at speeds greater than ~180-200 knots. Looks like at least several of the shots had the aircraft flying in "helicopter mode" with the main rotor producing horizontal force and the prop freewheeling. The earlier XH-59A, with it's comparatively poor rotor aerodynamics, and limited power/weight ratio compared to some modern X-2 aircraft, could do 156 knots as a pure helicopter. Given the advancements and installed power in the more recent aircraft, they should have an even larger "helicopter mode" flight envelope to work in before prop thrust "airplane mode" flight is required. So if the prop driveline of SB>1 isn't ready yet, they still have plenty of flight envelope to explore.