Bell has been pursuing stop fold tilt rotor technology for almost as long as the tilt rotor itself. The link above is an unpowered XV-15 sized stop fold rotor in the 40 x 80 wind tunnel tested in 1972, several years before the XV-15 flew. This current testing goes beyond what could be done in a wind tunnel (where the airspeed is imposed on the rotor by the tunnel) and demonstrates the transfer of thrust from one propulsion type to another while managing the fold in a less controlled environment.