The CarterCopter development is going to get very interesting when it attempts to break the mu = 1 barrier. [Tip Speed Ratio (mu) = forward speed / rotor's tip speed]. At tip speed ratios greater than 1 the complete span of the retreating blade will be in reverse air flow. In Carter's words "..the air flow over the entire retreating blade passes first over the trailing edge to the leading edge rather than the from the leading edge to the trailing edge."
To my knowledge, no helicopter has even achieved mu = 0.8. This includes the Sikorsky XH-59A ABC, which partially unloaded the retreating blade to prevent tip stall. Nick Lappos would be the best person to comment on this subject.
CarterCopter's patent 6,024,325 is for an airfoil profile that is halfway between a NACA 0015 profile and a shallow ellipse. This has probably been done to improve blade stability during reverse airflow. Probably as an attempt to keep the chord-wise center of lift fairly close to the center of mass and the center of feathering rotation.
A number of interesting concerns jump to mind.
~ Is it going to be possible to put enough kinetic energy into the rotor to give a jump takeoff at gross weight; considering that the semi-elliptical blade has a much higher drag/lift ratio then a conventional airfoil?
~ As Lu has mentioned, will the high inertia rotor turn fast enough to retain stability, while not rotating so fast that the gyroscopic rigidity will be detrimental to maneuvering?
~ Most interesting, will be the rotor's ability to maintain rotation by aerodynamic means at high forward speeds. They state "... the lift on the retreating blade increases as the blade drops, and works whether the air flows from leading edge to trailing edge or from the trailing edge to the leading edge, allows the rotor to operate at a mu greater than 1. The flapping automatically increases until the vertical velocity component changes the angle of attack on both the advancing and retreating blades until they both have the same lift."
At mu greater than 1, both blades must be aerodynamically acting as advancing blades.
[ 04 November 2001: Message edited by: Dave Jackson ]