Eurocopter recently took the wraps off its developmental X3 helicopter. What’s unique is it’s configured with two forward-facing propellers on stub wings. At higher airspeeds, the lifting and thrusting duties are progressively shifted to the wings and props. This arrangement is meant to compensate for lost lift on the side of the retreating main rotor blade disk. The X3’s main engines drive the propellers and the main rotor.
The basic physics of the so-called “retreating blade stall” have always capped the cruise speed of conventional helicopters. As the forward speed of the helicopter approaches that of the retreating blades of the spinning main-rotor disk, the blades confront little or no “relative wind” and, therefore, can’t generate lift on that side of their rotational arc. On conventional helicopters, flexible rotor hubs are designed to compensate for this as much as possible by “lagging” the forward moving blades and accelerating the retreating blades to maintain balance. But that design strategy can go only so far.
