The Eurocopter X3 hybrid made its record-breaking speed run just 10 days before the opening of the Paris Air Show, so it was not surprising that the EADS company brought both the unique aircraft and its crew to the Le Bourget biennial event. On June 7, the X3 flew at 255 knots in level flight and 263 knots in a dive, besting the previous record set by the Sikorsky X2 in September 2010 (250 knots level and 260 knots in descent). Among a crush of onlookers and well-wishers, AIN had a few minutes to talk with the pilots about how one actually flies this aircraft.
The X3 (pronounced “X-cube”) is a twin-engine, medium-size helicopter based on a heavily modified AS365N3 Dauphin. Two Fadec-controlled Rolls-Royce-Turbomeca RTM322 turboshafts drive the main gearbox. This EC175 gearbox was modified with two outputs, one to drive the X3’s five-blade, EC155 main rotor and the other to drive two five-blade, constant-speed propellers that are fixed on short anhedral wings attached to the fuselage above the cabin. On startup, as soon as one engine begins turning the gearbox, the rotor and both propellers start turning at the same time.
The X3 does not have a tail rotor, but instead a horizontal stabilizer supporting vertical stabilizers, or fins, on each end. A wide trim tab spans the aft end of the horizontal stabilizer for trimming pitch when above 60 knots by use of a coolie hat switch on top of the cyclic. Flaps on the fins are used to counteract the torque of the main rotor in cruise. The wings also have flaps, which help optimize wing-lift performance depending on weight, altitude and airspeed, according to Dominique Fournier, Eurocopter flight-test engineer. Fournier and Eurocopter test pilot Hervé Jammayrac were on board the X3 when it made its record flight.
Achieving the Speed Sweet Spot
The basic operating premise of the X3 is to use the main rotor for lift when hovering and the propellers to provide thrust in high-speed forward flight, when the fixed wings give additional lift. The pitch angle of the (main) rotor blades is reduced in forward flight compared with a “standard” helicopter, although not to zero, and the angle depends on a large number of parameters. The pilot reduces the pitch angle by lowering the collective. Rotor rpm is set at a compromise value that is the best combination between Mach effect on the advancing blade and retreating-blade stall. The variable-pitch propellers do double duty by providing anti-torque control when the X3 hovers.
“Hovering turns are controlled by differential pitch on the two propellers,” Fournier explained to AIN. “If you want to turn nose right, you increase the pitch on the left propeller and decrease the pitch on the right one,” Fournier said. The pilot does this with the pedals, as with a conventional helicopter. “The pedals are controlling the pitch of the propellers,” he explained, via the mechanical flight control system, making it feel entirely natural for an experienced helicopter pilot.
A coolie hat-shaped control mounted on the collective provides precise control of the throttle control lever (TCL) located on the right side of the center console. The TCL beep increases and decreases the pitch on the two propellers by the same value. Beeping the switch forward adds pitch; beeping it back reduces pitch. A gage on the instrument panel shows a TCL scale. “This scale,” Fournier explained, “has a recommended area–shaded in blue–corresponding to the optimum, in terms of power, hover-flight regime.” This also corresponds to the optimum range of anti-torque sharing between the two propellers, although the pilot does not have to think about anti-torque sharing, because it is adjusted automatically.
“One thing is very particular to this machine,” Fournier continued. “When you are in hover, you can choose the [aircraft] pitch [attitude] you want in a hover, from minus 10 [degrees] to plus 15. If, from the optimum hover position [the blue area], the pilot beeps the TCL forward, he will create a forward movement. If he then counters this by pulling back on the cyclic and putting the nose up, this will result in a hover with the nose up.”
Conversely, if the pilot “beeps the TCL backward, he will create a backward movement. If he counters this with forward cyclic, putting the nose down, the result will be a nose-down hover. This is totally intuitive. You can just hover over a spot with the nose up or down. You can also do this when making a nose-up slope landing.”
For a takeoff from a hover, the pilot increases power by beeping the TCL switch on the collective forward, increasing pitch on both propellers. The X3 accelerates forward in level flight and at about 40 knots the pilot lowers the collective, bringing the main rotor to low pitch. The helicopter is now an airplane. To change the pitch attitude of the aircraft when above 40 knots, the pilot beeps another coolie hat on the cyclic or uses fore and aft movement of the cyclic itself. To change airspeed, the pilot uses the beep switch on the collective: forward to go faster; aftward to go slower. The cyclic is used to make turns, which are coordinated by the flight control system.
The test crew has practiced autorotations in the X3, although not all the way to the ground, because, said Fournier, there is, after all, only one X3. “You enter autorotation by reducing the pitch of both propellers to zero thrust,” he explained. The collective is already all the way down in forward flight. “In autorotation, it’s a helicopter. You do a flare and then pull collective and land. We do it all the way to the ground only in the simulator.”
Fournier estimated that an experienced helicopter pilot would require about four hours in the simulator and one flight hour to transition to the X3. “Flying the X3 is intuitive in all flight regimes,” he said.
The X3 made its first flight in September 2010 and to date it has flown 148 hours. Fournier said Eurocopter planned to retire the aircraft last month, but the technology lessons the company has learned from the X3 will be carried forward in future aircraft.