Wednesday, August 1, 2012
Pilot Report: The Exciting, Experimental, Exceptional X3
Eurocopter invited Rotor & Wing to fly the X3 on June 18 before the official start of its U.S. tour.
|Overhead panel of the X3 cockpit.|
|X3 landing at Alliance Airport.|
|Up close look at the X3’s rotor hub.|
Picture this: A mad scientist takes a helicopter and a twin-turboprop into his laboratory, wires them together, then sends millions of volts of electricity coursing through their frames. When the smoke clears, a strange combination of the two is left. When released from its iron straps, the creature hovers for a moment, then jets away like an airplane.
That creature is Eurocopter’s X3, a hybrid machine designed to offer the vertical flight capabilities of a helicopter, and the speed of an airplane. To save development time, Eurocopter engineers started with its EC155 medium-twin helicopter. They then replaced the Fenestron with a bobbed tail, tweaked the shape of the fuselage to accommodate new components, and added propellers to stubby wings. Eurocopter says, though, that the X3 testbed flying today will have a different hull design a few years down the road.
Eurocopter invited Rotor & Wing to fly the X3 at Fort Worth Alliance Airport (KAFW), located 22 nm northeast of American Eurocopter’s headquarters in Grand Prairie, Texas, two days prior to its official U.S. public debut. I would only be the second person outside of the company’s own personnel to fly the machine they call the “X cube.” The day began with a technical briefing by Jean-Jacques Ferrier and Paul Eglin, two of the X3’s senior engineers. “It is quite simple,” Ferrier explained. “We slow down the main rotor at high speed to reduce Mach at [the blade tip]. Then we have a wing which develops additional lift to unload the main rotor at high speed and to compensate for retreating blade stall.”
Because the rotor has zero pitch at high speed to reduce drag, Ferrier said that an X3 pilot does not tilt the rotor disk to achieve forward flight. Thrust is provided entirely by the two, five-bladed, constant-speed propellers, which receive power from a pair of Rolls-Royce-Turbomeca RTM322s conventionally mounted atop the fuselage. The turboshaft engines and props are coupled to the same transmission that drives the X3’s five-bladed main rotor system.
The propellers also provide anti-torque services through differential thrust. And to keep it simple for pilots, the pedals control the anti-torque thrust the same as they would in a conventional helicopter.
The wings on the X3 play a key role in its performance. The added lift they produce minimizes the need for lift from the main rotor. Consequently, once 60 kts has been achieved, the collective is lowered. However, this does not cause the main rotor to enter an autorotative state. It merely reduces its RPMs to a point where drag is minimized.
“Fin flaps are used in cruise flight to counteract the main rotor torque,” Eglin said. “Of course, we have less power on the main rotor, so a very small setting is sufficient.” Wing and horizontal stabilizer flaps are used to pitch the aircraft, not the rotor disk. “It is very easy to fly,” boasted Ferrier. Before being escorted out to the X3, I pulled weather for KAFW. The winds were 160 at 12 kts gusting to 17, with a temperature/dew point spread of 34/18.
It’s a tall ship, standing at what I would guess is about 12 feet high. The engine nacelles bulge farther out than the EC155 it was bred with, probably to house the engines and transmission. Inside, the aircraft looks like a standard Dauphin with decorative trim panels missing. (Engineers prefer having easy access to a test bed’s cables, wires and structural members.) The aft cabin was stuffed with flight test gear and an engineer’s station.
The instrument console—as well as the ceiling-mounted starter and fuel management quadrants—is standard AS365 and EC155 fare, with just two major exceptions. First, and most obvious, is the throttle control lever (TCL) on the right side of the center console. It looks like the throttle in a fighter jet, but with three times the travel. The other unusual item is a square, LCD display located on the main instrument panel. It’s called the power management display (PMD), and provides a digital view of how the TCL is set, and how much power it’s delivering to the propellers—amounts that will vary between the left and the right prop during flight. As I got settled into the left front seat, senior X3 test pilot Hervé Jammayrac, clad in a powder blue Eurocopter flight suit, saddled up on the PIC side. Behind us was Dominique Fournier, the flight engineer. Jammayrac and Fournier went through a before-start checklist, followed by an engine start procedure very similar to the one used for the AS365 Dauphin.
|The X3’s instrument panel varies little from its cousin the EC155. The primary differences are the test gear mounted atop of the glare shield, a special power management display (PMD) for the propellers (circled), and the thrust control lever (indicated by the arrow).|
As the No. 1 engine spooled up, so did the main rotor and both propellers—proof that only one transmission drives all three spinning members. Once the second engine was online and everything was ready, Jammayrac gave me a quick lesson on how to read the special instrument aboard the X3, and we were ready to roll. Taxiing the X3 requires the same technique used in an airplane: Just give it some gas to start moving, and steer with the pedals. The only difference is in how the X3’s throttle is manipulated. In the X3, the TCL on the center console isn’t touched. Forward thrust is applied by thumbing a small, momentary-contact, hat switch mounted on the collective. Push it forward to increase the power, tap it backwards to decrease it. The TCL moves forward and back as the hat switch is actuated, but is only manipulated by hand as an emergency procedure if the hat switch fails. (The logic behind that design would become apparent to me soon.)
Once we were cleared for takeoff, the procedure was simple: Just lift the X3 off the ground and hover it like any other helicopter. In a 10-foot hover, the hydraulically boosted controls felt a little heavy to me, but I was still able to hold it over a spot, and do some pedal turns with little effort. In fact, it spun through a 15-kt quartering tailwind without protest. “Are you ready to go flying?” asked Jammayrac. “Yes, sir,” I replied. “All you have to do is push the throttle switch forward,” he instructed. “Don’t do anything else.” And that was when the X3 stopped being a helicopter and became a King Air!
Regardless of whether I “beeped” the throttle hat switch on the collective or held it forward, I could feel the power come in. At 60 KIAS, Jammayrac told me to lower the collective all the way down. It didn’t feel natural at that speed while still 10 feet off the deck, but I did as instructed. By the way, it was at that moment I understood the logic behind controlling the throttle with the hat switch instead of the jet-like throttle lever: You need to simultaneously have your hand on the collective and thumb on the throttle when transitioning between flight modes that close to the ground! Bumping the hat switch forward changed everything. Almost instantly, the thrust developed by the props pushed me back into my seat. “It just doesn’t feel right,” I said without bothering to key my microphone. “I’m moving at corporate-plane speed with the collective down!”
Climb out was both thrilling and abnormal. As covered in the briefing, the rotor disk remains at a zero-degree pitch, so aft cyclic isn’t used. Instead, there’s a hat switch on the cyclic that changes pitch during forward flight above 60 KIAS. When I beeped it back, the horizontal surface on the tail of the X3 pointed the nose up, and the ship climbed into the air with great ease.
At 2,000 feet MSL at an airspeed of 167 KIAS, the X3 demonstrator handled like a twin turboprop airplane. Turns were accomplished by deflecting the cyclic to the left and right, speed changes by bumping the hat switch on the collective, and altitude adjustments were the responsibility of the switch on the cyclic. The onboard computers pretty much handled coordinating the turns, as well as keeping the aircraft trimmed.
Exploring the X3’s envelope at altitude was surreal. It offered the surroundings of a helicopter, but the performance of a fast plane. I wish I could have taken a video of the return to KAFW, but had my hands full. Jammayrac had me shoot my approach fast, as if in an airplane. As a guy with precious little fixed-wing time, it required my undivided attention. The X3 handled just as smoothly on the descent to the runway as it did when I was twisting and turning it over the Texas countryside. The odd part for me was to be in a “helicopter” on a two-mile final, pitched down, doing 100 KIAS, collective on the floor, and tweaking my rates of closure and descent with two thumb switches. Somewhere around 2,500 feet out from the touchdown zone, every bone in my body wanted to start slowing down. But Jammayrac insisted that I keep the speed up until he gave me the word to back off on the throttle. It seemed to take forever—a pretty funny illusion, considering our speed—but at about 100 feet off the deck and 1,000 feet from our touchdown point, he finally gave me permission to slow down.
The same way advancing the throttle smartly pushed me back in the seat during takeoff, holding the switch back pulled me into the harness as if I had just hit the brakes. Of course, the entire approach could have been made at a crawl—or straight down—but the purpose of that technique was to demonstrate the responsiveness of the propellers to power demands, and to show that the X3 could have also landed like a corporate plane had I kept the speed up. As my intended touchdown point began to disappear under the X3’s nose, it occurred to me that in order to land in helicopter mode, I would have to transition soon. There’s a blue range on the propeller and power torque display that tells the pilot when to switch, but it’s easy to figure out without it: When your speed drops below 60 KIAS and the ship begins to settle, add collective. The main rotor wakes up, the propellers counteract the torque, and it’s a helicopter again! Whether it’s in rotorcraft or hybrid mode, the X3 concept looks like a winner.