Test pilots of Eurocopter's hybrid helicopter demonstrator X3 say the aircraft has more than proved its architectural concept of side-mounted propellers during the flight-test programme over the past two years.
Hervé Jammayrac, experimental test pilot, says the rotorcraft handled as predicted - and trained - in the simulator prior to the first sortie in September 2010.
However, he adds that he was fascinated by the rotorcraft's much faster acceleration than conventional helicopters, saying the X3 was the most exciting aircraft he has flown to date.
The Dauphin-based helicopter has logged about 120 flight hours so far, and took part in a demonstration tour in the USA in July and August.
While designers targeted a speed of 220kt (400km/h), the aircraft reached 232kt using only 80% of its power. The distribution of power between the main rotor and propellers changes across the speed range. As the aircraft flies faster, the short wings - on which the propellers are mounted - generate up to 40% of lift and consequently reduce the lift requirement from the main rotor. But at the same time the propellers need more energy to maintain airspeed.
In cruise flight, the main rotor can receive as little as 25% of the available power. As a result of the reduced need for lift, the pitch of the five main rotor blades can be reduced in cruise flight, which in turn significantly cuts vibration.
The size of the wings was a key focus area of the test programme, as fixed airfoils can reduce the helicopter's autorotation capability. But Dominique Fournier, experimental flight-test engineer, says the X3 showed the same autorotation characteristics as the standard Dauphin.