Paris Air Show

Paris 2011: Eurocopter’s new X3 compound helo brings X-Factor to Paris show

 - June 19, 2011, 7:00 PM

Eurocopter has delivered what it hopes will prove to be the X-Factor to this year’s Paris Air Show. The daily flying display this week features its new its X3 (“x cubed”) compound helicopter demonstrator, a modified AS 365 Dauphin with a conventional main rotor, two propellers on wingstubs and a conventional empennage. While helicopters usually cruise at 140 to 150 knots, the X3 concept is aimed at proving that 220 knots is a sweet spot where speed is profitable, and last month the demonstrator reached 232 ktas. The X3 was unveiled in September 2010 at Istres’ military flight-test center in France, near the manufacturer’s Marignane factory. According to Jean-Michel Billig, the company’s executive vice president for research and development, the X3 will impress show-goers with its maneuverability, just as the military Tiger wowed them back in the 1990s, when it began performing loops. “In Le Bourget, you will see a flying display no other helicopter can offer–I am talking not only about high speeds,” he told AIN. During the second flight-test campaign, which started in May, the X3  demonstrated a rate of climb greater than 5,000 feet per minute. The crew flew pitch-up attitudes in the 30- to 50- degree range. It performed rolls at 80-degree bank angles. “After the Paris Air Show, we’ll go even faster than 232 knots,” Billig said. That speed was sustained during approximately five minutes at 8,200 feet on May 12. Further flights this year will dig deeper into vibration phenomena and wingstub aerodynamics. Mods Allow Speed Increase During the first test campaign, between September and December 2010, the first goal had been 180 knots. That speed was attained in less than 12 flight hours and it required less power than expected, Billig told AIN. He claimed that the aircraft is so stable it can be flown “hands off” at 180 knots. “It is as stable as a jet,” he said. A vibration problem on the empennage, visible on the video of the first flight, was solved immediately after that flight, according to Billig. Last winter, Eurocopter engineers disassembled the aircraft, thoroughly examined critical parts and analyzed the flight test data. “Our objective was to understand how parts behaved,” Billig said. Modifications were lighter than expected. To allow a speed increase from 180 to 220 knots, Billig and his team had planned to strengthen the main gearbox by installing the unit developed for the new EC 175 medium twin. “However, [the original gearbox] behaved well, a change was no longer needed,” he said. The X3 is powered by a pair of the RTM322 engines developed by Rolls-Royce and Turbomeca for the NH90 military transport. They are so powerful (2,300 shp each) that their full power wasn’t even fully used to reach the higher speeds. Between the first and second test campaigns, the fuel system was modified. The engines now can be fed even at very high pitch-up attitudes. In the cockpit, the information displayed to pilots has been adapted. “At high speeds, some bits of information relating to hover are inappropriate, while the crew needs more explicit speed and slip indication,” Billig said. The second test phase will end after about 12 to 15 flight hours (not counting the hours flown here at the Le Bourget show). This means the demonstration program, at some 20 to 30 hours, will have been shorter than expected; an X3 crewmember had previously talked about a need for up to 100 hours. “We want to go directly where we want to be in terms of understanding high-speed flight,” Billig said, explaining the relatively small number of sorties required. “Our preparation for phase one was very good, which explains our success. I want phase two to be as successful,” he added. The crew had previously tested “99 percent of what they saw in flight” on a simulator. Not a New Concept The idea to add propellers to a helicopter is not new. Several compound rotorcraft have flown since the 1950s but none of them made it to the market. So why try again now? “Today’s computation and simulation capabilities are helping us a lot,” said Billig, explaining why he believes the X3 will have a more positive outcome. Eurocopter’s design engineers can predict the aircraft’s behavior and adjust parameters such as attitude, rotor rotation and speed, and so forth. Moreover, having propellers driven by shafts connected to the main gearbox is a new configuration. Previous compound models had dedicated engines for the side-mounted propellers. These days, the compact design of current turboshafts allows airframers to build a compound that behaves very much like a conventional helicopter. The architecture is relatively simple, with two outputs from the main gearbox (to the propeller’s gearboxes) instead of one (for a tailrotor). At low speed, anti-torque and yaw are controlled through conventional pedals. Instead of acting on the tailrotor, they differentiate the power supplied by each propeller. In cruise, the vertical empennage suffices to control yaw. The pilot can fly the X3 like a conventional helicopter up to 80 knots. Beyond that point an additional control is needed. A trim button, located on the main rotor collective pitch lever, increases the propellers’ pitch and power output. At the same time, the pilot has to reduce the collective pitch. X3 test pilot Hervé Jammayrac told AIN that it would just take no more than 15 minutes for a helicopter test pilot to convert to flying the new compound. In cruise, the wings take over from the main rotor to provide 40 percent of the lift. The main rotor adopts a “flat” profile in which drag is minimal and rotor speed is reduced. Power is then transferred to the propellers. Flying the X3 does not require fly-by-wire controls, the crew said, and the aircraft is stable at high speeds, even without an autopilot. However, Eurocopter is considering adding fly-by-wire controls when the compound concept is turned into a product. Such a decision should be made next year, Billig said. When the X3 gets to market, the price premium is expected to be about 25 percent higher than the cost of a conventional helicopter. Multiple Applications Hoped-for applications for this high-speed, long-range concept include search and rescue (with a rear door for the hoist), border patrol and commercial passenger transport. In six to eight years, Eurocopter’s first compound product may be in the EC 225 category­–20 passenger seats–for oil-and-gas offshore operations. In military service, applications include combat search and rescue, medical evacuation and troop transport, possibly with in-flight refueling. The concept is relevant to those operations where hover is a marginal part of the flight. So how did Eurocopter’s engineering team determine the optimal speed? At 220 knots, the aircraft is about 50 percent faster than its conventional equivalent. The additional operating cost per hour should be about 25 percent more in terms of fuel burn. Balancing this is the assumption that for a given distance, the aircraft will spend fewer hours in the air and thus have lower maintenance costs. According to Eurocopter, the bottom line is a 20-percent cut in costs, measured per passenger-mile. Moreover, by going faster, the aircraft can perform more missions in a given period. The operating cost of the X3at 250 knots (if attained) was calculated to be much too high. Further testing will also help determine the long-range cruise speed–probably close to 200 knots. Jammayrac said the X3’s fuel burn (measured per nautical mile) at that setting will be lower than that of a conventional helicopter at much lower speeds.  Billig did not ignore the conjecture that a purpose-designed compound might deliver more performance than the X3. For example, airframe weight was not an issue with the RTM322 engines. Also, the propellers (a standard set that Eurocopter bought from MT Propellers) might be better profiled if used with a clean-sheet design, resulting in lower noise and more efficient operations. He also was tight-lipped on the X3’s actual fuel consumption.