Study and optimization of a contra-rotating propeller hub for convertiplanes part 1: VTO and hovering

Contra-rotating propeller seems to be a convenient solution for the tilt-rotor convertiplanes of the V22/BA609 type. With this propeller arrangement the rotor diameter is significantly reduced. In the case of an aerial vehicle similar to the V22 the rotor diameter can be reduced from 11.6m down to 7m. In this case the emergency 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. The V-22/BA609 configuration has a high roll polar moment of inertia with roll control implemented through differential rotor thrust. VRS is then particularly critical. The contrarotating propellers are less subject to the roughness zone of the VRS as demonstrated in wind tunnel tests. Furthermore, the airfoil chosen (NACA 0006) is particularly suited to have a smooth transition from the propeller working state to the windmill brake state. The stability of the contrarotating propellers and the possibility of the two hubs to rotate at different speed in windmill brake state, make it easier to enter into a stable autorotation state. The autogyro (autorotation) and the airplane mode landing are fundamental requirements for the certification of V22/BA609 as civilian transport. To make the certification easier it is possible to identify three flying modes for the aerial vehicle: VTOL with the hubs tilted vertically, STOL with the hubs at an intermediate angle and aircraft (horizontal hubs) for cruise. The transition can be restricted to most favorable conditions. The turboshafts may have two working conditions: in the cruise one the maximum pressure (and maximum efficiency) of the reference Brayton cycle is necessary. This pressure is achieved with the contribution of the air intake. In this mode the propeller tip speed can be near 0.5M. The lower disk diameter of the contra-rotating propeller guarantees better propulsion efficiency. The VSTOL mode is with maximum power, with lower maximum pressure in the turboshaft and with higher propeller tip speed (0.91M). The hub can be simplified with the Advancing Blade Concept (ABC) that requires only the blade feathering DOF. The upper and lower rotors require about 1 DEG of difference in AOA (Angle Of Attack) in most conditions. The gyroscopic effects are neutralized and the force necessary to tilt the rotors is much lower than the single rotor solution. This assures also better handling in most flying conditions. Finally, the two-contra-rotating-rotors-tilting mechanism is not more complicated than the single rotor one.