When designing a rotary aircraft such as a helicopter or tilt-wing, engineers must strike a balance between the blade angle that works best for forward flight and the angle that works best for vertical flight. Through a project called the reconfigurable rotor blade (RRB), the Office of Naval Research is supporting development of a device that twists the blades in-air to optimize the angle for each mode of flight and thereby increase aircraft fuel efficiency, range, and payload.
ONR and the Naval Air Systems Command are jointly developing the RRB, with Boeing acting as the prime contractor. “At the heart of the RRB is a solid-state, electronic torsional actuator that changes the ´spanwise´ twist of a blade, allowing the rotors to operate at maximum efficiency during takeoffs and landings, and while cruising,” said ONR program officer Lawrence Ash.
The small package—about 18 pounds—is compatible with the existing blade and hub structures on many Department of Defense helicopters and the tiltrotor V-22 Osprey, for easy retrofit. The torsional actuator is made from an alloy called NiTinol (for nickel titanium Naval Ordnance Lab—where it was first developed in the early 1960s). NiTinol is a shape memory alloy, that is, even after being deformed or reshaped, it “remembers” its original shape and will return to it when heated.
The actuator sits inside the blade spar near the blade´s hub. When its NiTinol tube is heated by an electric current, it twists a predetermined amount. An “energy shuttle” developed at Boeing amplifies the force of the twist and applies it to the blade, twisting it to the correct angle for a particular mode of flight. The twist variability can be incorporated within the aircraft´s flight controls, but a pilot would have the option to engage or disengage it. When engaged, the system would automatically provide the optimal blade angle for each mode of flight.
The RRB project began in 2001, and will culminate in 2008 with a ¼-scale wind tunnel demonstration and full-scale bench component test. Since the project´s start, the weight of the full-scale actuation system has been reduced from 52 to 18 pounds, the power requirement has been reduced from 3500 watts to 700 watts, and deployment time has been reduced from three minutes to 25 seconds. In March 2004, the program successfully demonstrated a ¼-scale actuator.
A number of educational institutions and companies support the project through various Small Business Innovation Research and Small Business Technology Transfer awards, as well as the Navy´s Manufacturing Technology program. The Shape Memory Alloy Consortium sponsored by ONR and the Defense Advanced Research Projects Agency also supports the effort.