Active control of a helicopter rotor blade is attractive because it moves part of the aerodynamic optimisation problem out of a fixed geometry and into a structure that can be commanded. A blade able to change its twist during each revolution could, in principle, suppress selected vibratory loads, alter the interaction between a blade and the wake shed by the blade ahead of it, and improve behaviour in flight conditions that impose contradictory demands on a conventional rotor. The underlying aerodynamics has been investigated in Europe, the United States, Japan and Korea for two decades, by an international research consortium assembled in 2005. What has changed recently is the experimental evidence: a four-metre model rotor with piezoceramic actuators embedded in the blade structure has been run through a three-week campaign in a large European low-speed wind tunnel, and the organisations involved have published reductions in noise and vibration for the conditions they tested. The distance between that result and an aircraft is a change in the kind of evidence required, in the conditions under which it has to be produced, and in who has to produce it. Those are different questions from the aerodynamic one, and the public record answers them to very different degrees.