Here we have three dramatically different approaches to speedy rotorcraft design, and yet each has the same goal: greatly increased cruise speed while maintaining VTOL (vertical takeoff and landing) characteristics without taking the impractical leap of, say, adding a 43,000-pound-thrust, fully afterburning Pratt & Whitney F135 turbofan engine to the equation. (In case you’re wondering, that describes the propulsion system of a $240 million, VTOL-capable Lockheed Martin F-35B Lightning.)
An Aerodynamic Puzzle Solved
AgustaWestland has been focused for several years on perhaps the most conservative of fast-cruise-speed VTOL designs (as if that adjective even fits here) with the AW609 civil tiltrotor. The approach is conservative if only because it’s proven. Based on engineering work pioneered by Boeing and Bell Helicopter, the AW609 is a commercial tiltrotor aimed at government and corporate VIP markets. Sikorsky, meanwhile, has created a wildly advanced machine called the X2 that relies on rigid, twin-coaxial rotors and a rear facing five-blade propeller that are designed to propel the craft beyond the 250-knot barrier. Not to be outdone, Eurocopter has created the X3 (pronounced “X Cubed”) technology demonstrator, which combines the fuselage, engines and main rotor of an EC 155 with two high-efficiency propellers mounted on short wings astride the fuselage. The strange look of the design has given rise to the nickname “Flying Cuisinart.” A funny moniker, yes, but nobody was laughing when the machine last year attained a sustained forward speed in level flight of 232 knots.
In Eurocopter’s case, the X3 concept is about much more than merely building a faster helicopter. It’s about unraveling the engineering mysteries that will enable the commercialization of a helicopter that can fly 50 percent faster than a traditional helicopter at a cost of just 20 percent more.
The biggest challenge will be in the transformation needed to turn the X3 idea from a technology demonstrator to an honest-to-goodness product. Obviously, a hybrid helicopter based on X3 technology will need bigger engines to drive the main rotor and the propellers, which are linked through combining gear boxes. It’s very likely that a helicopter based on the X3 demonstrator will be tailored to military needs given the obvious benefits of an aircraft that is capable of flying at high speed and inserting troops at precise locations on the battlefield. The Bell/Boeing V-22 Osprey tiltrotor is slowly proving its usefulness as a troop transport — and now Eurocopter and also Sikorsky believe the time is ripe for a more agile VTOL craft capable of serving in both transport and light attack roles. It should come as no surprise, then, that a U.S. tour of the X3 this summer included visits to several military bases, including the Redstone Army Arsenal Airfield in Alabama, Fort Bragg in North Carolina and multiple demonstration stops for Pentagon brass at sites around Washington, D.C.
So what makes the X3 different from other compound rotorcraft under development? The most obvious disparity is the use of two five-blade propellers mounted on short-span fixed wings and driven by the same turboshaft engines that turn the craft’s five-blade main rotor. An obvious difference from conventional helicopters is the absence of a tail rotor, the need for which is obviated by the propellers, which turn at slightly different speeds to offset the torque created by the main rotor. What the X3 concept doesn’t completely solve, of course, is the issue of retreating rotor blade stall (to partially overcome it, the X3’s main rotor turns more slowly than an EC 155’s), meaning that 232 knots is likely about as fast as this helicopter will be capable of flying, even in its final configuration.
Now on to the question almost everybody asks the first time they see a photo or a video of the X3: Won’t those large propellers mounted on each side of the passenger compartment pose a clear and present danger during entry and exit? Even some Eurocopter execs admit they felt the configuration bordered on ludicrous when it was first presented — but the designers are thinking of ways to prevent passengers from being torn to ribbons the moment they step out the X3’s cabin door. Two main ideas are currently under consideration. The first would require that the propellers be stopped during passenger loading. The other would move the cabin door from the side of the helicopter to the rear, which would allow for quick entry and egress, an enticing compromise that would be especially useful for carrying troops into or out of a hot landing zone.