The vertical takeoff hybrid splits its engine power between the main rotor and two variable pitch propellors on wings. The propellors provide counter-torque during low-speed operations and forward thrust for cruise. The wings are used to unload the main rotor at higher speeds, reducing vibration and avoiding the retreating blade stall problem that limits traditional helicopters to about 155kt cruise speed. By also slowing the rotor, more power can be diverted to the propellers to boost forward speed. Control surfaces on the tail will allow for aircraft-type control during cruise flight.
Sikorsky has proposed overcoming the traditional helicopter "speed limit" using counter-rotating main rotors (the retreating blade can be offloaded by its forward moving counterpart) and a pusher propeller on its X2 technology offering. The company is now testing a two-place demonstrator using the technology. The animation below shows some of the potential applications of the X2 techs.
Piasecki Aircraft has a different solution with its X-49A SpeedHawk technology demonstrator, which uses wings to offload the rotor and a ring-tail propulsor/clamshell arrangement for speed and anti-torque control.
Photo credit: Piasecki
