Eurocopter unveiled a compound helicopter demonstrator called the X3 (“x cubed”) designed to cruise at 220 knots–about 50 percent faster than today’s medium twins. The helicopter manufacturer believes it has found a sweet spot at a target speed that should make time savings profitable, and the technology could be incorporated into helicopters in less than a decade.
The purpose of the 30-minute first flight on September 6 was to calibrate the rotor and propellers. Eurocopter plans an 18-month, 100-flight-hour test program. The manufacturer is testing the airplane-helicopter hybrid at the French defense procurement agency’s flight- test center, a 40-minute drive from Eurocopter’s headquarters in Marignane.
F-ZXXX looks like a Dauphin, with two short wings and two propellers in tractor configuration. A conventional empennage replaces the tail rotor. Under the cowlings, the main gearbox has been adapted from that of the in-development EC175 medium twin; two drives have been added for the propellers. The two turboshafts are RTM322s, which usually power the NH90 military transport.
The first phase, which will last through next month, will open the flight envelope to 180 knots. Then Eurocopter will modify the main gearbox to transmit more power to the propellers for higher airspeeds. Testing will resume in March to expand the envelope to the target speed of 220 knots.
Hoped-for applications for this high-speed, long-range concept include search-and-rescue, border patrol and commercial passenger transport. In the military, the roles include combat search-and-rescue, medical evacuation and troop transport, possibly with in-flight refueling.
The engineering team has determined an optimal speed close to 220 knots, making the aircraft approximately 1.5 times faster than its conventional equivalent. Eurocopter expects hourly operating cost to be about 25 percent higher than for an equivalent conventional helicopter, but the rotary/fixed-wing hybrid would spend fewer hours in the air for each mission. According to Philippe Roesch, who heads the X3’s engineering team, the bottom line is a 20-percent cut in costs, measured per passenger-mile.
Moreover, its greater speed would allow the aircraft to perform more missions in a given period. Fuel consumption per mile should be similar to that of a conventional helicopter, despite the higher speed, Eurocopter predicts.
In cruise flight, the wings take over from the main rotor to provide most of the lift, a concept explored but abandoned by the British Fairey Rotodyne in the 1950s. The main rotor adopts a “flat” profile for minimal drag, and power is then transferred to the propellers.
The pilot can fly F-ZXXX like a conventional helicopter up to 80 knots. From there, it requires an additional control. A trim button, located on the main rotor collective pitch lever, increases the propellers’ pitch and thus the power they supply. The pilot simultaneously has to reduce the collective pitch. In the future, fly-by-wire controls may simplify this process.
At low speed, antitorque (and thus yaw) is controlled through conventional pedals. Instead of acting on the tail rotor, they differentiate the power supplied by each propeller. In cruise flight, the vertical empennage has sufficient authority to control yaw.
Flight-test engineer Daniel Semioli explained to AIN how modern technology makes this innovation possible. Engines are now powerful enough without excessive fuel burn; manufacturers have a better command of materials, both metal and composites; and finally, test equipment, especially telemetry, is more sophisticated. Engineers can thus conduct a test campaign without too much trial and error. CEO Lutz Bertling declined to reveal the budget for the X3 demonstration program.
The first application of the compound concept could appear in about eight years, in the category of the 20-passenger EC225. Eurocopter hopes such a compound helicopter will not command a premium of more than 25 percent above the price of an equivalent conventional helicopter. An equipped EC225 currently sells for approximately $18 million. As for weight, a compound would be close to 13 metric tons, rather than the EC225’s 11 tons.