Navy tries to counter tiltrotor criticism with shape memory alloy actuators to boost hover and forward flight modes

Boeing has been given a follow-on contract by the US Navy's Office of Naval Research (ONR) to mature and demonstrate the concept of a reconfigurable rotor blade (RRB), which, through the use of shape memory alloys, offers the potential for optimising proprotor twist on the Bell Boeing V-22 Osprey tiltrotor to improve performance in hover and forward flight modes.

The technology could address some of the recent criticism levelled at the V-22 by Pete Aldridge, the US undersecretary of defence for acquisition, logistics and technology, and a self-admitted tiltrotor sceptic. A characteristic of the tiltrotor, due to its 11.6m (38ft)-diameter proprotors, is heavily loaded blades and a relatively restricted hover performance.

"It's a compromise between a helicopter which wants very big blades and an aircraft which wants relatively small blades. Because the blades are smaller than would be required for a helicopter, you have to load the blades up. In terms of the pounds per square inch that you have to have on the blades to hover, you don't have much control authority. You don't have much margin left," says Aldridge.

Shape memory alloy actuators integrated into the proprotor spar would enable the twist of the blade to be decreased from the 47.5¡ root-to-tip twist to improve hover efficiency, or increased for higher forward speed. The US Marine Corps estimates this could produce an extra 1,360kg (3,000lb) of payload capability or up to 30kt (55km/h) of additional forward speed.

"No decision has been made to include shape memory alloy actuators inside a full scale V-22 proprotor. Such a decision cannot be made until the critical enabling technology can be adequately developed, matured and evaluated," says the US Navy. Initial work undertaken by Boeing has assessed the feasibility of an RRB and produced a conceptual design.

The follow-on $10.25 million contract from ONR is intended to mature the technology and demonstrate its performance benefit in a sub-scale windtunnel test. This will be followed by the bench test of a full-scale system by December 2006.

Source: Flight International