Estimated reading time 23 minutes, 11 seconds.
Well, Ive been to the future… and Ive come back to tell you about it.
Currently, few helicopters exceed 150 knots in top speed (and, owing to the immutable laws of aerodynamics, traditional helicopter designs will always have a top speed that is not much higher than this). Airplanes, meanwhile, are much faster and generally have longer ranges than helicopters, but lack the ability to hover. In the future, though, there will be a type of combined aircraft that can hover, and take off and land vertically, like a helicopter, yet also offer cruise speeds comparable to some fixed-wing airplanes.
Bridging this gap between helicopter and airplane capabilities, of course, will be no small feat, but Eurocopter is one of several manufacturers who are trying their innovative hands at creating this future (see p.62, Vertical, Feb-Mar 2011). Eurocopters entry into this unofficial competition is the X3 (pronounced X cubed) high-speed, long-range, hybrid helicopter, which was designed to investigate configurations that would extend the performance and capabilities of rotorcraft out to speeds where airplanes currently reign.
In June, at Fort Worth Alliance Airport in Fort Worth, Texas, I was lucky enough to get a unique glimpse into this one part of the helicopter worlds future and get the first demonstration flight of the X3! On a warm, gusty day, I flew with Eurocopter experimental test pilot Herv Jammayrac and flight test engineer Dominique Fournier, and was shown that the X3 is, literally, the shape of things to come.
X-Plane on a Budget
Walking out to the X3, my first glance at the vehicle was simply arresting: it appeared to be an innovation unlike any other flying machine on the planet. The pre-flight inspection, however, revealed a constellation of components that were all surprisingly familiar.
The fuselage and main rotor spoke to the helicopter pilot side of my brain, while wings, tail and propellers were familiar from my airplane experience. Eurocopter emphasized that while the X3 is an innovative configuration, budgetary constraints have meant it relies upon only mature technologies. In fact, most of the X3s components were derived from other designs. The fuselage came from the Eurocopter AS365; the rotor is from the 365s successor, the EC155; the engines Rolls-Royce Turbomeca Ltd. RTM322s are the same as those used on the Eurocopter-AgustaWestland-partnered NHIndustries NH90; and the gearbox was derived from the Eurocopter EC175. The propellers, meanwhile, are modified commercial units from German manufacturer MT-Propeller. Only the wings and tail were manufactured specifically for the X3.
Strapping into the cockpit of the X3 felt like getting into any helicopter: cyclic, pedals, collective. The basic layout was reminiscent of the current-production EC155s, because thats basically what it is, with the simple addition of two displays for propulsion system data.
In lieu of airplane-like throttles, control of the big propellers was through a familiar beep trim switch on the collective. The operative sense of the switch was obvious: beep forward to increase thrust. Simple. Of note, on the center console adjacent to the collective was the thrust control lever (TCL), a large, throttle-like lever that served only as a back-up, in case the electric beep trim thrust control failed. (It was never actually used during our flight.) Generally, control of thrust is referred to as setting the TCL, whether it is done electrically through the beep trim or manually through the lever.
Due to his extensive familiarity with the X3, Jammayrac managed the start-up checks from memory. A ground power unit was utilized for the engine start, although the X3 can also get going from battery power. The process was push-button simple, owing to the full-authority digital engine controls (FADECs) on the RTM322 engines. As the first engine approached idle and the starter cut out, things began to whirl. The rotor accelerated overhead while the propellers began to spin at the edges of my peripheral vision. I was surprised by the smoothness of the acceleration, but also by the noise level in the cockpit it was a din reminiscent of a lovesick helicopter courting an amorous turboprop. I had been warned about this, and I was bemused by the attention my French crewmates took to ensure that my active-noise-reduction headset was working properly.
Hovers Like a Helicopter
Taxiing away from our parking space required only a momentary forward beep on the TCL switch. Pedals were used for directional control (as they manage the differential propeller pitch below 80 knots). Direct thrust-axis control via the TCL provided easy regulation of groundspeed, and was overall simpler than taxiing a wheeled helicopter. Jammayrac noted that we could even back up if desired, although I have yet to find an airport that has required me to parallel park!
As I taxied into position on Alliance Airports Runway 16-Left, Jammayrac completed the pre-takeoff checks, including changing the FADEC switches on the two engines from ground to fly mode, which brought the rotor speed up to 312 r.p.m., further increasing the already-impressive noise level. Plus, those two big props were right outside the window!
Jammayrac performed the liftoff into the hover, and I noted that, in the absence of a tail rotors asymmetric lateral thrust, the X3 didnt have the tendency of most helicopters to take off into a slightly banked attitude. The hover attitude was dead level. As he pulled on the collective, he also set the TCLs to the hover setting, which provided roughly equal anti-torque thrust from each propeller: forward thrust on the left and reverse thrust on the right. Although, while the propellers thrusts were equal, the torques required were not, owing to the different efficiencies inherent in producing forward thrust on the left and reverse thrust on the right. The propeller torque gauges averaged at about 12 percent and 35 percent, respectively.
I think I made a plaintive noise at this point, because Jammayrac looked over, smiled, and gave me control of the ship.
My time in the hover was limited to only a few minutes under gusty conditions, but the experience brought to mind my now-dim memories of flying the AS365 Dauphin. The control strategy would be immediately familiar to any helicopter pilot. Yaw control using differential propeller thrust was responsive and predictable, with no evidence of the yaw-to-roll coupling that is prevalent in most helicopters. Likewise, in the absence of a tail rotors lateral forces, the pure yawing moment provided by the propellers caused no tendency to drift.
With the automatic flight control system (AFCS) engaged, the AFCS response was an attitude-hold mode with heading hold in the hover. The heading hold function did an excellent job under challenging conditions. About the only nitpicky things I found were that the pedals were very highly damped, necessitating high forces. Also, I found the cyclic centering forces in attitude mode a bit high and elected to hover with the force trim button depressed, suppressing the AFCS attitude-hold function. Doing so, however, exposed a mild tendency for lateral pilot-induced oscillations, a likely outcome of the large propeller/gearbox masses out on the wingtips. Yaw control, though, remained precise and satisfying, if a bit heavy.
Recalling the tendency for pedal inputs to spike the mast torque in the early Arospatiale Dauphin models, I jabbed at the pedals to make some sharp yawing inputs while observing the graphical triplex torque indicator (rotor and propellers). My pedal inputs caused a change in propeller torque, but the propeller torque margins were sufficient that overtorquing with the pedals didnt seem possible.
Meanwhile, I noted with mild astonishment that our hover mast torque indicated only 48 percent, and quickly understood why flight test engineer Fournier had earlier described the X3 as overpowered. Admittedly, the hover power required was reduced by the reported winds gusting from 12 to 20 knots, but I momentarily wondered why the X3 needed so much power I would find out very shortly.
Cruises Like an Airplane
With it now time to go flying, Jammayrac gestured at the TCL switch and pointed down the runway. I held the switch forward and gasped. The X3s acceleration was simply fantastic. Eurocopter claims the machine accelerates longitudinally at 0.3 g: performance comparable to a static thrust brake release in a business jet. The onset of propeller thrust was smooth, without a trace of pitch coupling. Seconds later, as we accelerated through 65 knots indicated airspeed (KIAS), I partially lowered the collective and raised the landing gear. Effectively, we had become an airplane a very powerful airplane.
Dallas-Fort Worth (DFW) air traffic control restricted us to 2,000 feet initially, so I cruise-climbed at 140 KIAS, quickly noting a climb rate around 1,500 feet a minute (f.p.m.). Once cleared to climb higher, I took Jammayracs suggestion and set the aircraft at a 20-degree pitch attitude and beeped the TCL fully forward. This resulted in a climb at the best-rate-of-climb airspeed of 105 KIAS. Passing through 6,000 feet, I noted that the vertical speed was in excess of 3,200 f.p.m. Personally, this climb configuration was the biggest surprise of the flight. I would have thought that a rotor was the most efficient way to generate climbing thrust, but the X3s impressive climb performance is optimal with the collective almost fully lowered.
While Jammayrac and the DFW air traffic controllers sorted out a common dialect of English, we leveled off and the X3 began accelerating… and accelerating. At 6,500 feet and 22 degrees Celsius (72 degrees Fahrenheit) outside air temperature, the X3 finally stabilized at 184 KIAS, which amounted to a whopping 206 knots true airspeed.
The ride quality also immediately made an impression upon me. Often, vibration is the limiting factor in helicopter cruising speed, but the ride was quite satisfactory, especially considering the X3 incorporates no vibration-isolation technology. (Eurocopter engineers had described the transmission as being bolted directly to the airframe, with neither passive dampeners nor active vibration control systems.) I did, however, detect a distinct intermittent bobble, a sort of mini-speed-bump effect, as the X3 penetrated turbulence, which I attributed to flexure of the wings and tip-mounted propellers.
Conventional helicopters use the rotor for both lift and thrust, but with wings, propellers and rotors at its disposal, it was interesting to note how Eurocopter configured the X3 for cruise. In forward flight, the rotor is used for lift but not propulsion, although not all the lift comes from the rotor. In fact, only about 60 percent of the aircrafts lift comes from the rotor (which requires only about 35 percent of the torque needed to hover), while the remaining 40 or so percent of lift comes from the wings. A trimmable horizontal stabilizer establishes this rotor/wing lift-sharing arrangement by setting the fuselage angle, and thereby the wings angle of attack, in cruise. By cunning design, this also set the fuselage at a level attitude for minimum drag.
As forward-looking as it is, the X3 doesnt actually break any aerodynamic laws; rather, it pushes them out into a higher-speed range. The typical limitations on a helicopters speed are the Mach number of the advancing blade and the angle of attack of the retreating blade, either of which might comprise the limit, depending upon the flight conditions. The X3 pushes back the aerodynamic limits in two ways. First, by reducing the rotor speed in flight it reduces the speed of the advancing blade. In fact, the FADECs regulate the rotor speed to maintain a constant advancing blade speed of 0.91 Mach. The second challenge, retreating blade stall, is handled with one simple and well-proven technology: wings. Using wings to offset rotor lift in cruise allows the X3 to operate with the collective at a far-lower pitch setting than conventional helicopters, which means the retreating blade is likewise operating at a far lower angle of attack.
Friendly Handling Qualities
As we cruised west out of DFW terminal airspace, I took a few minutes to make further acquaintance with the unusual X3; however, my first impressions betrayed nothing unusual. Maintaining altitude was easy and pleasant, with an overall control response that felt as crisp and predictable as an EC155 an understandable similarity since the cyclic was controlling the same rotor. In cruise flight at 180 KIAS, with the AFCS on, turn coordination was excellent.
Eurocopter took pains to point out that the X3s pleasant flying qualities are not the result of complex fly-by-wire trickery. In fact, the flight controls are conventionally mechanical, with a simple electromechanical stability augmentation system (SAS), much like the EC155 from which it was derived. Above 80 knots, the rudders effect yaw control, while each propellers torque is automatically matched and used purely for propulsion. The yaw axis control system which Eurocopter says it has patented was designed to reduce pedal authority with increasing speed. Duplex hydraulic systems, meanwhile, are in place to control the propellers, in light of the fact that using them as primary flight controls make them safety critical.
In preparation for some further handling-qualities tests, I used the TCL beep switch to reduce power to the propellers. The use of a collective-mounted switch to control propeller thrust is simple, however I noticed myself frowning whenever I needed to make small, precise power changes. As currently developed, the propeller power control utilizes a three-way, spring-centered switch with a small, inherent time delay. Its a small matter, but I found it difficult to be accurate with it. For future models, I would encourage use of a rate-proportional controller instead of a discreet switch. Then again, test pilots like myself complain for a living!
The X3 proved so simple to fly with the AFCS on that I was eager to select it off. Jammayrac nodded and poked the AFCS button, disabling it. Virtually nothing happened. Even with the SAS off, the X3 was well-mannered. Although workload was slightly higher, the flying qualities remained pleasant. Pitch and roll damping were naturally sufficient for accurate hand-flying. And, after gently prodding the pedals to assess the lateral-directional dynamic response, I discovered why Eurocopter engineers described the X3s tail as being conservatively over-sized the dynamic response was quickly damped.
Once again, I was not satisfied until I could find something to complain about. So, I performed a turn to 45 degrees bank, and noted no requirement for backpressure on the cyclic with the increasing load factor, which is indicative of neutral maneuver stability. Likewise, conducting a gentle sideslip in forward flight exhibited absolutely no evidence of dihedral effect (the tendency to lift a wing away from the sideslip). In fact, while sideslipping with right pedal applied, the X3 oddly rolled slightly to the left. Both of these flying qualities would raise eyebrows during certification tests, but it is wise to remember that the X3 is a purely experimental design. Quirks such as these described are minor, and can easily be tuned out of a certified design. Overall, within the scope of my limited exposure, I found the X3s flying qualities very enjoyable.
We didnt simulate any emergencies during our flight, but I did ask Jammayrac about the X3s behavior in the event of a power loss. He explained that the ability to autorotate like a helicopter was a critical design objective. To minimize the drag of the propellers on the drivetrain, the AFCS incorporated a helicopter mode (H/C) that is designed to set the propeller pitch to zero thrust. Pressing the AFCS H/C button upon autorotative entry is the only additional step needed compared to any other rotorcraft. Eurocopter claimed that the rate of descent in autorotation was around 2,800 f.p.m., slightly higher than the EC155, due to the drag of the wing. As the prospect of carrying a wing into an autorotation intrigued me, I asked about the possibility of stall. Jammayrac assured me that they had not been able to stall the wing during their flight tests to date.
After only about 20 minutes total flight time, we commenced our descent to Alliance Airports Runway 16-Left. As disappointing as it was to end the test, the approach and landing revealed another of the X3s interesting secrets. Jammayrac had me fly onto final approach at 120 KIAS, maintaining our airspeed until long after a helicopter would have been too high and fast to negotiate the landing. Then, upon his cue, I reduced the TCLs to the zero thrust position and watched the aircraft do its imitation of a parachute with the collective still in its cruise setting, presenting the flat pitch of the propeller blades to the airflow allowed us to perform a remarkably steep decelerating approach. Plus, in contrast to a helicopters normal nose-high descent attitude, the X3 was able to decelerate with its nose pointed down, right at our landing area: a capability that provided for a terrific field of view over the instrument panel.
The transition back to the hover was anti-climactic. As the airspeed bled below about 40 KIAS, my helicopter instincts told me to level the fuselage and to apply collective. Voila, we were hovering again… and my time aloft in the unique X3 was over.
Bringing the Future Forward
So, what are we to make of a rotorcraft with wings and propellers?
In the X3, Eurocopter isnt offering some incremental upgrade to a product line, but an entirely new category of flying machine. During our post-flight debriefing, Eurocopter showed conceptual designs for future high-speed, hybrid-helicopter variants scaled from five to 14 tonnes (5.5 to 15.4 tons). Taking a thinly veiled poke at tilt-rotor technology, Eurocopters engineers emphasized that their plans would proceed only if those designs were cost-effective to produce and operate. Its not hard to imagine the civil, military and parapublic applications of a machine that can hover like a helicopter and cruise like a turboprop, and Eurocopters plans include designs with aft cargo ramps and declutchable propellers to facilitate ground operations.
Plans or not, Eurocopters investigation of the hybrid rotorcraft configuration is hardly complete. Being built from existing components, the X3 is far from a performance-optimized design. The companys engineers admitted that the AS365 fuselage is too draggy for this application, that the rotor turns too fast, and that the design probably has too much tail surface area. The company also wants to investigate the merits of deploying wing flaps in the hover to reduce rotor downwash impingement upon the wing, and are keen to establish the minimum practical rotor speed for cruise, in the interest of reducing noise. Future designs will also likely take advantage of fly-by-wire flight-control technology.
I would venture that one of Eurocopters main challenges in turning the X3 concept into a viable product will be packaging its breathtaking performance into an airframe that retains sufficient payload. Adding wings, gearboxes and propellers to a helicopter all add weight. For our evaluation, the X3 was at its maximum gross weight of 5.2 tonnes, with a crew of three and a full fuel load of 800 kilograms (1,760 pounds), giving it a payload of around 1,060 kilograms. Admittedly, it carried a significant mass of flight test instrumentation, and as Eurocopter engineers took pains to point out, as an experimental prototype it was by no means representative of a production design. Nevertheless, comparing its payload to the EC155 is interesting. The EC155 can carry a payload of roughly 2,300 kilograms. Therein lies the compromise inherent in the X3s rotor-plus-propeller configuration: such a vehicle faces a steep rise in drag as speed increases during cruise flight, necessitating powerful engines and a relatively heavy gearbox.
The stated objective of any future products Eurocopter develops from the X3 would be to increase both speed and range over existing helicopter designs. However, given the high power required in cruise, it might be more correct to say that a future hybrid helicopter would have increased speed or increased range, but not necessarily both, because the fuel flow required to push a rotor up to turboprop speeds is considerable. This would explain why Eurocopter engineers emphasized that although it can potentially fly faster, the planned high-speed cruise for the X3 is only 220 knots. Nevertheless, such a blend of capabilities adds up to impressive operational flexibility.
In my short 20 minutes aloft, the X3 flew far enough to afford me an intriguing glimpse into what tomorrow may look like. And, it showed me that Eurocopter today has produced an innovative combination of mature technologies that allows for impressive performance with familiar handling qualities. All in all, it was an appealing vision of the future of rotorcraft technology.
A graduate of the U.S. Naval Test Pilot School, Rob Erdos is an experimental test pilot licenced for fixed- and rotary-wing aircraft. In addition to being an engineering graduate from the Royal Military College, and holding a masters degree in aviation systems research, Rob is a former Canadian Air Force SAR pilot. An avid airplane builder, and a passionate flyer of historical aircraft for Vintage Wings of Canada, Rob flies such iconic planes as the Spitfire and Hurricane.