Convertiplane cruise performance optimization with contra-rotating propellers

A major problem of convertiplanes is that they need high disk-loading for efficiency in cruise and very low diskloading for lift at vertical take-off. Contra-Rotating Propeller (CRP) seems to be a convenient solution for the tilt-rotor convertiplanes derived from the tilt-motor V22/BA609. With this propeller arrangement the rotor diameter can be significantly reduced. In this way the disk-loading for vertical take-off is the same of the traditional propeller. In the case of an aerial vehicle similar to the V22 the rotor diameter can be reduced from 11.6m down to 7m. The lower diameter and disk area significantly reduces the drag at cruise speed. In this way propulsion efficiency is increased. In the V22 case a secondary reason for using contra-rotating propellers is to increase the propeller disk-area, and achieve higher vertical lift efficiency, within a propeller diameter limited by the height of the aircraft's undercarriage. In this way the emergency "conventional" horizontal landing is possible by giving a small amount of dihedral to the wings (8 DEG). This is easy to implement due to the absence, in the contra-propeller version, of the interconnecting transmissions between the two rotors at the wingtips. Moreover, the huge gyroscopic moments of the V-22/BA609 propulsion system that induces slow response on these aerial vehicles is annulled by the contra-rotating solution. Finally, the contra rotating propellers are less subject to the roughness zone of the VRS (Vortex Ring State) as demonstrated in wind tunnel tests. The higher complication of the two contra-rotating rotors is compensated by the fact that only the blade feathering DOF is implemented in the hub. In helicopter mode, the advancing blades of each rotor operate at higher pitch angles to produce more lift without prejudice to roll trim, since the difference in lift between the advancing and retreating blades of the upper propeller are balanced by the equal and opposite lift distribution of the lower one. This concept is called the Advancing Blade Concept (ABC) and has been successfully implemented in the Sikorsky's X2 high-speed technology demonstrator. Flutter due to lack of stiffness of the contra-rotating solution have been successfully solved in the fifties (see the nearly sonic Tu95 Bear). In this paper the cruise performance of the contra rotating solution is fully analysed with Solid Woks Flow Simulation. As expected the propulsion cruise efficiency is significantly higher than the traditional propeller solution.