Conventional wisdom is being challenged on both sides of the Atlantic, where 's X2 and now 's X3 are showing that helicopters need not be slow and that a 50% increase in speed is achievable at reasonable cost. The prospect of cruising at 220 kt., not today's 150 kt., has both commercial and military helicopter operators seriously interested, particularly for long-range missions.
The X3, or “X-cubed,” is a technology demonstrator designed to validate Eurocopter's High-speed, long-range Hybrid Helicopter, or H3, concept, says Jean-Jacques Ferrier, head of innovation. The H3 would be a family of helicopters ranging from 5-14 tons for markets such as long-range search and rescue, deep offshore oil and gas exploitation, corporate transport and military missions.
Hybrid helicopter technology will allow Eurocopter “to offer our customers about 50% more cruise speed and range at very affordable costs,” says President and CEO Lutz Bertling. The company's goal is an increase in life-cycle cost of only 20-25%. The H3 will “increase productivity as long as the benefit of those higher speeds outweighs the cost of purchasing, operating and maintaining the machine. The overall mission price will thus be significantly decreased whenever mission time is a determining factor,” he says.
Aviation Week was given the opportunity to fly the X3 shortly after it arrived at American Eurocopter's headquarters in Grand Prairie, Texas, in mid-June to begin a U.S. marketing tour. This was to include demonstrations to the U.S. military at Redstone Arsenal in Huntsville, Ala., home of the Army's Aviation & Missile Command; Fort Bragg, N.C.; and Fort Belvoir, Va., just a few miles south of the Pentagon.
The basic principle of the X3 is that two propellers provide forward thrust while wings, albeit short, produce lift that reduces the load on the rotor. The lift provided by the wings increases with speed, reducing the load on the rotor by 40-50%. It is this reduction in blade loading that enables the X3 to fly faster.
In a conventional helicopter, the velocity of the advancing rotor blade increases and that of retreating blade decreases as the speed of the helicopter accelerates. The advancing blade is traveling at the rotor rotational velocity plus the forward speed of the helicopter; the retreating blade is traveling at the rotor rpm minus helicopter speed. The tip of the advancing blade can go supersonic while the trailing blade can stall. As this dissymmetry in blade speed widens, drag and vibration increase. The key to increasing helicopter speed is to fix these two problems.
Sikorsky's X2 tackles this through a coaxial contra-rotating rotor system, the advancing blades on opposite sides producing the lift and allowing the retreating blades to be offloaded. Eurocopter resolves it by removing from the main rotor the need to provide thrust and all of the lift. Propulsion is shifted to the propellers and lift partially offloaded to the wing, allowing rotor rpm to be reduced. Rotor rpm is automatically slowed by up to 15% as forward speed increases, Ferrier says. This reduction in rotor rpm allows a significantly higher forward velocity before the aircraft reaches its never-exceed speed.
Primary objectives of the X3 program are to develop and validate the H3 concept in terms of aircraft control and trim strategies, anti-torque control, propulsion systems, variable rotor speed and power-system management, Ferrier says.
The aircraft first flew September 6, 2010, with testing performed at the Istres flight test center in southern France. Initial flights took the aircraft to 180 kt., the speed limit set by the initial transmission. After upgrading the gearboxes, flights resumed in March 2011, achieving a sustained maximum speed of 232 kt. in May that year, “while using less than 80% of available power,” Ferrier says. Speed will be increased gradually throughout the test program. Current cruise speed is 180 kt. Operational specifications for the future H3 aircraft include a high cruise speed in excess of 220 kt.
To produce an experimental concept demonstrator as rapidly as possible, but at minimum cost, Eurocopter borrowed heavily from its existing helicopters. The fuselage is from the AS365N3 Dauphin and the entire rotor system, from swashplate to blades, is from the EC155. The main gearbox is from the new, but instead of a driveshaft to an anti-torque tail-rotor it has two shafts going to left and right reduction gearboxes to drive the two wingtip propellers, supplied by Germany's MT-Propeller. The twin RTM322 engines, rated at 2,300 shp each, are from the .
The X3 has no tail-rotor. The aft section consists of a wide horizontal stabilizer with twin vertical stabilizers. At low speed, rotor anti-torque compensation is provided by differential pitch angle between the left and right propellers, directly controlled by the pilot putting in the appropriate foot pedal movement. At high speed, anti-torque is provided by computer-controlled fin flaps.
Since the rotors on French-designed helicopters turn clockwise, as viewed from the cockpit, the advancing blade comes from the left, causing the nose to yaw to the left as power is applied. The pilot instinctively applies right pedal to counter this. As right pedal input is applied, propeller pitch increases on the left side while decreasing on the right, bringing the nose back around to the front.
Our initial “walk around” at Grand Prairie revealed two additional features designed to help with torque control. The twin vertical stabilizers each have a narrow aileron at the trailing edge. These move automatically above 80 kt. to balance propeller torque at high speed. A series of metal tabs just forward of the vertical ailerons acts as vortex generators to improve their efficiency.
The aircraft also has a strake along the starboard side of the tailboom. As the retreating blade passes over the tail, the downwash accelerates around and under the boom, creating a low-pressure area. Just as airflow over a wing provides lift, the low-pressure area on the boom tries to pull the tail to the right. The strake breaks up the airflow, increasing anti-torque control as well as reducing the power required for hovering.
As with any experimental aircraft, the X3 is set up to record every aspect of flight. Cameras are located both on the fuselage and in the cabin, including a 3-D camera on the front panel focused on the pilots and flight-test engineers.
The instrument panel was based on the EC155, but greatly expanded to provide a wide array of readings for every system in the aircraft. For the purposes of our flight, the primary instruments were two multi-function displays on each side of the panel showing standard flight parameters such as airspeed and altitude, as well as a single display that showed only the true airspeed, propeller pitch and vertical climb speed in ft./min. Another highlights propeller pitch and both rotor and propeller torque.
Since the rotor and propellers are driven by the single gearbox, starting the No. 1 engine begins all three turning simultaneously. The No. 2 engine is then brought on line as in any twin-engined aircraft. Conversely, on shutdown, stopping the rotor with the rotor brake simultaneously stops the two propellers.
I was flying with Eurocopter test pilot Herve Jammayrac, who I had flown with eight years earlier for a pilot report on the EC225. Since this is not a production aircraft, we spent virtually no time on specifications such as payload weight limitations or power settings in various flight modes. Of key interest were propeller pitch settings and speeds at various stages of flight, as well as rotor rpm readings. I was also interested in torque readings, since the transmission can be a limiting factor.
Jammayrac says that, in a production H3, standard rotor speed will be about 340 rpm, dropping to about 310 rpm at higher airspeeds as lift shifts to the wing. For ease of manufacture, the X3's rotor speed is fixed at 310 rpm.
Pitch of the propeller blades is set by the throttle control lever (TCL), a “coolie hat” toggle switch on the collective-grip control box. A much larger back-up throttle, similar to fixed-wing engine power levers, is located on the center console, but not generally used, Jammayrac says. Only a slight thumb movement on the TCL is needed to either increase or decrease prop pitch.
When we were cleared to taxi from Eurocopter's helipad out to the Grand Prairie Municipal Airport, Jammayrac simply released the brakes and applied minimum pressure to the “coolie hat” to get us rolling along the taxiway, “driving” the X3 like a prop-powered fixed-wing aircraft.
Once outside the company area, Jammayrac brought the X3 to about a 5-ft. hover, with both engines producing a combined 48% torque on a 93F day. Hovering the X3 is the same as in any helicopter, just a matter of being steady on the controls. Low-speed flight seemed perfectly normal, both forward and sideward.
Winds were gusting around 18 kt., but seemed to have little impact on the hover. One difference from a helicopter is that without a tail rotor, there is no loss of tail-rotor authority in winds. In helicopter mode, using rotor cyclic instead of prop pitch to accelerate, the X3 would stay below 80 kt. as the propellers act as aero brakes.
Once established in a stable hover, takeoff is accomplished by simply pushing forward on the TCL toggle switch. As propeller pitch and aircraft speed increase, rotor pitch decreases and more power is directed to the propellers, to the point where the X3 is basically a fixed-wing aircraft taking off 5 ft. above the runway.
The aircraft maintains a level attitude, with no dropping of the nose. But Jammayrac pointed out that by using rotor cyclic control, combined with prop pitch, you can take off with a nose-low, -level or -high attitude, depending on your preference.
If there was any translational lift as we moved from hover to forward flight, I did not notice it. The aircraft just started climbing. Our takeoff was done with roughly 29-31% torque, then we climbed up to about 2,000 ft. at 110 kt. at 27% torque.
Once the takeoff is established and the aircraft reaches 60 kt., rotor collective pitch was reduced to a fixed setting of 5 deg. Acceleration and deceleration is then performed with the TCL beep button while flight direction is controlled through the cyclic stick. Feet come completely off the pedals since the aircraft is computer trimmed through propeller pitch. Essentially, the aircraft is flying in airplane mode, with the cyclic being the control yoke and TCL toggle switch being the throttle.
When we were clear of the controlled area, Jammayrac pushed the TLC forward and put us in a 3,000 ft./min. climb at 118 kt. at 20% torque, climbing to 7,000 ft. The X3 literally pushes you back in the seat as though it is a corporate jet on climb-out. Performance limits for climbs are up to 5,500 ft./min. with a climb slope of 40 deg.
As mentioned, the dissymmetry in blade speeds causes increasing vibration. Traditionally these are controlled through use of either passive dampeners or active devices that sense and counter the vibration frequencies. No anti-vibration systems are installed on the X3 and test pilots who have flown the aircraft at speeds in excess of 232 kt. say that neither vibration nor stability appear to be a problem, and that the aircraft can be flown hands-off without either anti-vibration or stability-augmentation systems installed.
It is still too early to determine whether such systems will be needed in a production aircraft, but Eurocopter says the X3 “has validated the H3 concept beyond expectations, [and] even at 232 kt. is behaving like a flying carpet without autopilot or stabilization systems, and can be flown hands-off.” We took the X3 up to 220 kt., where I found the aircraft can indeed be flown hands-off with good stability, but with a noticeable amount of vibration.
As for stability, I was able to put the aircraft into a series of turns increasing to 60 deg. of bank, with feet off the pedals and collective lever down, maintaining both altitude and airspeed—more or less. In unfamiliar helicopters I tend to lose a couple hundred feet of altitude while losing or gaining 10-20 kt. in sharp turns, but in the X3 the loss or gain was about half that, or less. Turn-speed limitations are 45 deg. at 220 kt. and 60 deg. at 210 kt.
The aircraft does have a four-axis autopilot, taken from the EC155. At one point while I had the aircraft in straight and level flight, Jammayrac turned off the autopilot. The cyclic got just “squirrelly” enough to notice it, but not so much that it would present a problem.
Returning to the airfield, Jammayrac set us up for a standard arrival, maintaining level flight during the entire approach—which was just a bit disconcerting since on conventional helicopters the view changes as the nose is pulled up to reduce speed. Speed was adjusted with the TCL beep button until about 50 kt. At that point, the aircraft transitions back to helicopter mode, where speed is controlled with the cyclic and altitude with the collective.
Eurocopter is not predicting when a next-generation hybrid helicopter will roll off the production line, although executives acknowledge that “usually a period of 10 years is necessary before an aircraft of this type can be introduced on the market.” But the company also says X3 technology may be integrated into “certain products” in the Eurocopter family by the end of the decade.
|Bell 533 HPH||275 kt.||1969|
|Westland Lynx||174 kt.||1972|
|MBB BO105 HGH||218 kt.||1975|
|Sikorsky XH-59A||263 kt.||1977|
|Aerospatiale Dauphin Grande Vitesse||201 kt.||1991|
|Sikorsky X2||262 kt.||2010|
|Eurocopter X3||232 kt.||2011|