Pilot lifts veil from BA609 flight testing
The first two prototypes of the Bell/Agusta BA609 civil tiltrotor had covered around 60 percent of the certification flight-test program in more than 350 flight hours and 225 hours of ground running by the middle of last month, in the process reaching the type’s maximum operating altitude of 25,000 feet, its certification speed of 310 ktas and G loadings of +3.1 and -1.0.
The latest in a line of Bell powered- lift designs that stretches back to the XV-3 of 1955, the BA609 has the configuration of a typical general aviation turboprop, with its high wing, T tail, conventional landing gear, 17,000-pound mtow and pair of 1,940-hp PT6C-67A engines, though with a rudderless tail inherited from the Bell Eagle Eye UAV. At 43.6 feet long and 59 feet in span its footprint resembles that of a King Air or S-76, so it should be able to use the same infrastructure. But its most important feature, according to AgustaWestland BA609 project pilot Pietro Venanzi, is the triplex digital fly-by-wire control system.
“It is the control system that makes the payload available,” Venanzi told the Royal Aeronautical Society in London last month. The XV-15 had a control system like a Swiss watch, he said, “a piece of art, but extremely complicated, extremely heavy. The only payload available was the two pilots flying it.” The pressurized BA609, on the other hand, will be able to carry up to nine passengers and be equipped with an ice-protection system.
The cockpit controls are like those on a helicopter, with a cyclic, two pedals and a power lever that acts as a collective pitch lever in helicopter mode and a thrust control in airplane mode. Pitch is controlled by symmetrical cyclic of the two rotors in helicopter mode, with the inputs diverted to the elevator in airplane mode. There is provision for lateral cyclic control, but “we don’t intend to use it unless there is a particular need,” Venanzi said.
Roll is controlled by differential thrust in helicopter mode and by the flaperons in airplane mode, yaw by differential cyclic when the aircraft is rotor-borne and differential thrust when it is wing-borne. In all three cases, Venanzi said, the flight-control system handles the transfer of authority during transition between the two modes. The digital engine controls are also interfaced with the control laws, “so the fly-by-wire speaks to the engine and makes it behave the way it wants.”
There is no need for a rudder because there is no critical engine failure case. “If we have an engine failure, the two gearboxes are connected by a shaft so the motion of the rotors is always assured. This way we can get rid of weight, complexity and hydraulic lines going to the tail, and we manage well with differential thrust.”
The Rockwell Collins Pro Line 21 cockpit has been adapted for the tiltrotor, with a nacelle angle indicator in the top left corner of the primary flight display and helicopter-style engine information display with torque indication. It is a “no-memory cockpit” Venanzi said, with everything color-coded so the pilot does not have to memorize parameters. “In the case of an engine failure it will immediately reconfigure to show the new limits.”
The prototypes have a flap lever fitted for protection, but the feature probably will not be included on production aircraft because the flight control system takes care of extension and retraction. To alleviate the load imposed on the wings by the rotors’ downthrust in helicopter mode, the flaps are automatically drooped near vertical at 67 degrees.
Taxiing is “pretty easy” using differential cyclic to steer, supplemented by differential braking for sharp turns. The nacelles can be moved up to 15 degrees forward and five degrees aft while the BA609 is on the ground, making it easy to modulate taxi speed. And tilting the nacelles forward permits a short takeoff roll and alleviates the download on the wings.
The hover is straightforward, Venanzi said, with the aircraft behaving “like a CH-47 Chinook sideways.” The best hover height is around 15 feet, “and at that height it’s a hands-off task; once you are in the hover you can release the controls and it stays there pretty much by itself.”
The transition to airplane mode is controlled by a thumbwheel in the grip of the power lever. Between 95 and 75 degrees from the horizontal–the range allowed on the ground–the nacelles move continuously at eight degrees per second, but at 75 degrees a single click initiates the transition from 75 to 50 degrees at three degrees per second, and at 50 degrees, a second click moves them to zero degrees at the same rate. The hydraulic actuators, one on each nacelle, are connected to ensure that if one fails the conversion will still be possible, and synchronize the two nacelles to 0.01 degrees. A third click reduces engine rpm from the 100 percent used in conversion mode to the 84 percent used in cruise mode.
Seamless Conversion from Rotorcraft to Fixed Wing
Everybody wants to know what happens when the conversion starts. Venanzi said, “Nothing happens. It’s incredible how from helicopter mode just tilting the nacelle forward a few degrees brings a dramatic acceleration, so immediately we start feeling wing-borne lift. At the same time, getting rid of the downwash on the wing helps maintain level flight. During conversion we don’t have a height excursion of more than 50 feet, and the control movements required in the cockpit are minor.”
Reconversion to helicopter mode for landing is a similarly low-workload reverse of the process, and the different nacelle angles can be used to advantage, he said. On the ILS approach to Milan Malpensa, for example, with the nacelles at 15 degrees and 140 knots airspeed, it fits into the airport’s normal traffic flow. “We are also examining the possibility of slowing down during the approach and bringing the aircraft to a stop right on the numbers on the runway,” Venanzi said. The aircraft’s ideal application will be to take off from a rooftop, follow an IFR route to an airport and return to the rooftop, “so we’re optimizing the controls to do that,” he added.
Eighty percent of training for pilots at Bell’s Alliance, Texas training academy will be accomplished on full-motion simulators. Candidates will need fixed- and rotary-wing licenses, with a commercial helicopter license and instrument ratings on either.
The BA609 is fun to fly, Venanzi concluded, especially in turbulence, because the wing loading is double that of a conventional aircraft, while helicopter pilots will appreciate the low noise and vibration at high speeds.