The US and Europe have been working on actively controlled rotors for decades, sometimes jointly, but the benefits have never been worth the complexity. Now it looks like operational experience in the extremes of Iraq and Afghanistan is giving some momentum to efforts to close the wide gap in state of the art between rotor blades and aeroplane wings.

While wings today have actively controlled leading-edge and trailing-edge devices providing variable camber, load alleviation and a host of other fly-by-wire advantages, rotor blades look and behave pretty much as they did when Igor Sikorsky flew the first practical helicopter.

Sikorsky is whirl-testing a helicopter rotor system with active blade flaps as part of a research effort jointly funded with the US Army's Aviation Applied Research Directorate. The active rotor will be tested in the 40 x 80ft windtunnel at NASA Ames next year.

Boeing tested its SMART rotor in the same tunnel last year. Based on the MD 900's five-blade rotor, this uses piezoelectric-actuated flaps on the blades. Sikorsky is using the four-blade Schweizer S-434 rotor, modified with electromechanically actuated blade flaps.

Earlier this year, in the same Air Force-operated tunnel, Sikorsky and the Army completed tests of an H-60 rotor modified for individual blade control (IBC), the rigid pitch links between the swashplates and blades replaced with hydraulic actuators developed by ZF.

As part of the 30-month active rotor program with AATD, Sikorsky will also study active blade slats, adaptive flight controls and a dual-frequency rotor head-mounted vibration suppression system. The study will produce the conceptual design for an H-60 size active rotor.

Sikorsky says the study's objective is to determine how fine-tuning of the blades using active flaps, slats and controls can increase rotor performance while reducing noise and vibration. The goals are a 90% reduction in vibration, significantly lower noise and 25-45% higher blade loading.

The next step is expected to be DARPA's new Mission Adaptive Rotor (MAR) program, which aims to flight-demonstrate a rotor system that can reconfigure in flight to maximize performance for each phase of a mission. Goals include increasing rotorcraft payload by 30% and range by 40% while reducing acoustic detection range by 50% and vibration by 90% compared with a fixed-geometry rotor.

DARPA's concept is that rotor characteristics would be automatically or manually varied between and during missions to optimise performance. As a minimum, the agency wants speed and reliability to be comparable with today's rotors, and parts count to be no more than 10% higher, but its objective is a 50kt speed increase and 25% fewer rotor and rotor control parts.

The agency has a long list of potential adaptive technologies, which include varying the blade planform - diameter, sweep, chord and tip shape; varying the airfoil with flaps, slats and active flow control; and varying blade twist, anherdal/dihedral, tip speed, stiffness and damping.

DARPA plans to award multiple contracts for conceptual design studies lasting about 12 months. Phase 2 would involve whirl-stand and windtunnel tests of a large-scale rotor, and Phase 3 would be flight tests of a full-size adaptive rotor. If successful, MAR could shape the next iteration of the AH-64 and UH-60 - and their ultimate replacements.