AW609 rotors tilting. Photo by Ernie Stephens
The idea of a flying machine that will take off and land vertically, then achieve airplane speeds, has been around since the development of engine-driven aircraft. Several designs have been toyed with, but arguably the most successful have fallen into one of two categories: a hybrid using a main rotor and propellers, or vectored thrust (i.e., a tilting rotor). In 2012, I conducted an evaluation flight of the Eurocopter (now Airbus) X3 experimental aircraft, which uses the hybrid system. So, it was only fitting that I accept AgustaWestland’s offer to fly the AW609 tiltrotor, which employs vectored thrust. The 609 actually began life in 1998 as a Bell-Boeing project, and later became the Bell/Agusta Aerospace BA609. But in 2011, when Bell Helicopter officially pulled out of the partnership, the rebranded “AW609 tiltrotor” was placed under the control of the newly formed AgustaWestland Tilt-Rotor Company (AWTRC) of Arlington, Texas, a wholly-owned subsidiary of AgustaWestland.
Upon my arrival at AWTRC’s combination headquarters, development facility and hangar at Arlington Municipal Airport (KGKY), I was greeted by experimental test pilot Dan Wells, a former Army helicopter pilot who had also worked for Boeing on the V-22 Osprey program, the only other tiltrotor flying today. Fellow test pilots Pietro Venanzi and Paul Edwards, also former military pilots, joined Wells. The AW609 is simple in concept, but complicated in execution. Its pair of three-bladed, 26-foot in diameter “prop-rotors,” coupled to their 1,930 shp Pratt & Whitney PT6C-67A turboshaft engines, must lift its 16,800-lb bulk into the pure vertical. They must then be able rotate forward and propel the 40-foot-long, 33-foot-wide, 15-foot-tall airframe at forward speeds of 275 KTAS and altitudes of 25,000 feet MSL.
To accomplish this, the engines must simultaneously rotate back and forth between 95 degrees (straight up, plus another 5 degrees aft), and zero degrees (straight forward). The engines must also be able to turn both rotor systems at any tilt angle as a team, or independently.
To make the units tilt as a matched set, the engineers employ one tilt-axis gearbox on each engine nacelle. Lubricating systems and chip detectors are carefully positioned to function throughout their range of motion, and a shaft-driven fan helps cool the engine when the normal ram air intake is rotated up for helicopter mode.
The 609’s rotor mechanism can best be described as a main rotor head that can rotate. So, if you were to look behind the cowling, you would see a set of swashplates with some clever modifications to control blade pitch. Their inputs are tied one engine to the other so that pilot commands will act upon each equally. My AW609 classroom training covered substantially more details, such as hydraulic, electrical, and structural systems, but space prohibits me from sharing it here. I’ll say this, though: The designers of the aircraft did their best to make as much of the aircraft safe, reliable, and simple to maintain by using tried-and-true aeronautical engineering practices found in today’s most sophisticated airplanes and helicopters.
Flight day arrived bringing with it clear skies, winds out of the south at 12 knots, a temperature of 24 degrees C, dew point of 12 degrees C, density altitude of 1,900 feet, and an altimeter setting of 29.99. Our takeoff weight would be 16,200 lbs.
The aircraft, N609TR (serial #0001) had returned earlier that day from its date with the paint shop outside of Dallas. The “BA” had been replaced with “AW,” and the company’s logo now dominated the fuselage.
The interior, like all aircraft under development, was still free of trim panels, so that its wire runs, fasteners and other critical components could be easily reached. A circular escape trunk dominated the center of the floor inside of the single cabin door... just in case.
|AW609 in flight. Photo by Ernie Stephens
Today, I would be flying with Venanzi, who took the right seat. I settled into the left one. I immediately noticed that both he and I were able to achieve comfortable flying positions, in spite of a substantial difference in our heights and weights, thanks to seats that could travel forward and back, as well as up and down. They even had the nerve to be rather comfortable.
The windscreen and side windows bear a mild family resemblance to those on the AW139, in that they droop at the corners to give an impressive field of view when looking toward the ground.
Facing me was a well-thought-out instrument panel containing a three-display Rockwell Collins Pro Line 21 glass cockpit system. Several switch panels were in customary places, such as center and overhead consoles. Something foreign to me as a helicopter, though, was a cabin pressurization and oxygen control panel. Venanzi was quick to remind me, however, that any experimental aircraft’s cockpit layout is subject to change as the design team makes tweaks.
Like the V-22, the 609 has a cyclic – called a “center stick” – that is held in the pilot’s right hand. But unlike the Osprey’s fighter jet-like thrust lever, the 609 has a conventional-looking collective they call a “power lever.”
The pedals and toe brakes look normal, and control pivoting about the yaw axis. But they are not connected to a tail rotor, and there is no rudder. In a hover, pedal inputs change the pitch on the rotors cyclically, causing one or the other set to drive the aircraft’s nose around its yaw axis. In forward flight, pressing on the left pedal adds thrust to the right rotors, and vice versa. Like the other flight controls, it too is fly-by-wire.
Venanzi handled the start-up procedure, because every operation is recorded and monitored live in the flight test control center on the ground level of the hangar.
As the FADEC brought the first engine to life, both rotors began turning, proving that they are inter-connected. As the second came online I noticed the interior was not as noisy as I would have expected. Venanzi explained that placing the engines and rotors out to the sides creates a relatively quiet cabin.
Once the pre-taxi checklist was complete, Venanzi took the controls and hopped us from the AW ramp across the runway to the taxiway near the tower. Once there, he set it on the ground and gave me the controls.
For takeoff, the nacelles are set for 87 degrees – as indicated on the nacelle angle indicator integrated on the PFD – which is accomplished by pushing one click forward on the nacelle tilt control (NTC), a thumbwheel switch mounted on top of the power lever. But failing to hold the brakes until airborne will cause the machine to slowly drift forward. Shortly after the nacelles hit 87 degrees, and I gently pulled up on the power lever and got the only true scare of my flight. Because even though I had been warned that the 609 did not like to hover below 15 feet, I did not realize how unsettling it would actually be.
The 609 imparts downwash on its own wings, causing the aircraft to be very unstable about its roll axis. Called “darting” by the pilots, the way to avoid it is to pull straight through to a 20-foot hover. Once there, it settles down nicely. And because the counter-rotating disks cancel any yawing induced by power changes, no real pedal work is required.
After a few pedal turns, which felt like a pedal turn in a conventional helicopter, I conducted a standard 609 departure by tilting the rotors to 75 degrees, which vectors thrust forward and up into a helicopter-like climb. But you must remember to use the center stick to keep the deck level.
|AW609 serial #0001 is equipped with a Collins Proline 21 avionics suite, as well as a myriad of test equipment. The pilot-in-command sits on the right.
During climb out, I had to monitor the limiting airspeed indicator (LAI). This display reads the angle the rotors have assumed, and adjusts its green arc to show how much power you can pull using the power lever at that setting. As the rotors tilt farther forward, the green arc shifts to authorize higher speeds. And in case you aren’t minding the LAI as closely as you should, the aircraft will impart a gentle vibration through the power lever as a tactile indication that you are about to exceed a speed range. (You can still pull past it, however, if an emergency dictates it.)
As the green arc on the LAI climbs, the nacelles can be rotated farther forward. When they reach an angle of 45 degrees or less, the aircraft is wing-borne and should be controlled like an airplane. Unlike a standard airplane, the flapperons on the wings automatically extend and retract as speeds change, but can be manually overridden. With the nacelles at zero degrees, which is full forward, you may as well be in a turboprop plane. The center stick commands climbs and rolls just like a yoke, and the power lever performs like a throttle in a jet. Flying the 609 like an airplane was... well... like flying an airplane, which is great, considering it recently lifted off the ground at zero airspeed.
At an altitude of about 2,500 feet MSL, I started the nacelles on a rearward rotation to 90 degrees to put us in an out-of-ground-effect hover. It was interesting enough to feel the ship dynamically breaking from 220 KIAS down to zero so quickly, but downright bazaar to look to my left and see a huge portion of the aircraft silently rotating aft. Once in a hover it felt and performed like any other helicopter.
After about an hour executing a variety of maneuvers, we returned to the airport where I practiced normal, steep and run-on landings. Each was followed by either a normal, maximum performance, or running takeoff. We did a normal takeoff when we first departed, so our first landing back at GKY was also normal. We entered it from a right downwind to runway one-six, giving us a 30-degree quartering right headwind.
The secret to landing – or taking off, for that matter – in the tiltrotor is to select the nacelle angle that will deliver the velocity and vertical speed you want for the various phases of your maneuver.
For a normal landing from a 1,000-ft., I reduced and the nacelles to 75 degrees on downwind, which slowed us from 200 to about 120 KIAS. On final, I incrementally stepped the nacelle angles up and brought the power back to put me as about 50 KIAS on the lower half of my final. As I neared the point where I would have flared a normal helicopter, I tilted the rotors back to 91 degrees to really bleed off the speed. I forgot, however, to return the nacelles to 87 degree prior to reaching zero airspeed, which caused the ship to inch backwards instead of holding its place over my touchdown point.
Autorotations, however, are a little different from what one might assume. When power is lost, the pilot must rotate the nacelles back to 95 degrees, which is done by pulling the thumb switch on the power lever all the way back past the detent. As the aircraft descends, the nose will naturally hang at a 5-degree nose-down angle, giving the pilot a better chance of spotting a suitable landing zone. Altitude permitting, the pilot can maintain an airplane configuration long enough to glide to a suitable location before rolling the nacelles back to 95 degrees for autorotation.
When the flying was over, Venanzi let me ground-taxi us back to the hangar. That was accomplished by tilting the rotors forward to about 88 degrees, easing the power lever up an inch or two, and using the rudder pedals and differential braking for steering. Ninety minutes after takeoff, I stepped out of the 609 feeling like I had just sampled a portion of civil aviation’s future. Part helicopter, part airplane, the AW609 Tilt-Rotor is an unusual but fun machine to fly. And if it attains its FAA certification in 2017, as the company hopes it will, you may see a lot of them.
Related: Flight Testing News