Initial testing of BA609 civil tiltrotor flawless
Successfully completing phase one of what will be the world’s first civil certification of a tiltrotor aircraft, veteran convertiplane pilot Roy Hopkins recently found himself in possession of something he hadn’t had much of in the last few months: spare time. Plans announced before the first flight of the Bell/Agusta BA609 called for a near-total teardown of the first flight-test prototype following what was expected to be that aircraft’s first 20 flight hours of function-and-reliability tests. After that, several months would go by before the test program was resumed using the modified prototype-one BA609 and the first of an expected three additional prototypes.
“We managed to accomplish all of the flight-test goals we had budgeted for the first 20 hours of flight in just 14 hours,” said Hopkins, the 56-year-old Bell test pilot who, through his experience in flight test of the XV-15 testbed and V-22 Osprey, has logged slightly more than 1,000 flight hours in tiltrotors (both actual and simulated), the highest total of its type on record. “Those 14 hours of flight were logged during a total of nine flights between March 7 and April 14,” Hopkins reported, “and those flight tests followed 41 hours of ground runup tests.” Like all initial forays aloft in newborn rotorcraft, Hopkins and copilot kept their first flights conservative, but soon branched out into tilted nacelle flight while still in helicopter mode. So far a speed of 120 knots has been achieved in forward flight.
“The BA609 has been flown with the nacelles tilted as far forward as 75 degrees, and as far aft as 95,” Hopkins said, 90 degrees indicating that the prop-rotors are in the horizontal position, like a helicopter’s main rotor. “That’s pretty much the helicopter envelope as far as the BA609 is concerned. Any more than those settings and you start to transition out of helicopter mode.”
Like its much larger, military cousin, the BA609 is a digital, fly-by-wire rotorcraft. Together they represent Bell’s first foray into production fly-by-wire rotorcraft and, indeed, are among a small handful of fly-by-wire rotary-wing designs to make it past the experimental stage worldwide. (Additionally, the BA609, with a top service ceiling of 25,000 feet, will be the first civil rotorcraft with a fully pressurized cabin.)
“Control software for the BA609 was developed specifically for that aircraft and not derived from the code written for the V-22’s flight control systems,” said Hopkins. “Both aerodynamically and from a systems point of view, there really is no comparison between the two aircraft. The 609 is much smaller, more like the XV-15, which was always a treat to fly. In fact, the 609’s prop-rotors are only one foot larger in diameter than those of the XV-15, totaling 26 feet, compared with the Bell Boeing V-22’s far larger 38.1-foot-diameter prop-rotors. The overall feel is that of an aircraft much lighter and smaller than the V-22, which of course it is.”
For this first phase of what promises to be at least another four years of extensive flight testing (Bell declined to speculate at this point on total flight test hours), the prototype BA609 was flown at an average weight of 14,400 pounds, well under its target max gross of 16,800 pounds.
‘Just Like an Airplane’
It’s a well established cliché of the aviation business that the seasoned test pilot returns to base after the first time aloft in the new aircraft and, no matter how appalling that aircraft’s performance, the grizzled captain describes the flight as “fine,” and the aircraft as “honest,” summarizing the machine as “flying just like an airplane.”
Hopkins’ compliments for his first time aloft in the BA609 last March 7 were probably the best that can be bestowed on a new general aviation aircraft, especially one that represents a whole new level of technology. “Flawless” and “stable” were his two most noteworthy compliments. When asked to elaborate, he did so. “In all my years of flight test, I have never been given a new aircraft that was as clean and as mechanically ready as this aircraft. And as we came into the hover, I was just overwhelmed by the stability that the fly-by-wire technology brings to the aircraft. It beat the simulator, which surprised me.”
The triply redundant FBW system is backed up a triply redundant hydraulic backup control system. Each operates independently, providing flight commands from, as well as feedback to, a dedicated microprocessor, which evaluates the existing flight regime while comparing it with actions called for by the pilot under pre-set parameters of the BA609’s performance-derived software.
As part of a overall philosophy to keep the BA609 as user-friendly as possible, Bell and Agusta engineers have abandoned the so-called “blottle” (after Marine Lt. General Harry Blot, an early V-22 program advocate and manager), the sidearm throttle-style control on the V-22 that combines power and nacelle tilt in a way that some have found confusing.
Aboard the BA609, a far more familiar version of the traditional helicopter-style cyclic and collection-control configuration is used, with some important variations. Between the pilot’s legs is a control stick performing both the functions of a helicopter’s cyclic and a fixed-wing aircraft’s stick. To the left is a “power lever,” closely resembling a conventional helicopter collective. Pulling up on the lever increases power to both prop-rotors. On the “blottle” arrangement, additional power past a certain point also began automatically rotating the nacelles forward out of helicopter mode.
“What the new software and controls have done is dramatically lower the pilots’ workloads, especially as compared with that encountered on the V-22,” said Hopkins. “The BA609’s flaps are automatic, deployed to be the optimal setting for the desired nacelle angle. Moreover, unlike the V-22, there are pre-set detents on the nacelle controls that automatically tilt the nacelles forward or aft to produce desired forward flight speeds. Nacelle angle is controlled via a thumb- operated detent switch located at the top of the power lever.
“The system is set very much along the lines of an automatic automobile transmission. For instance, 90 degrees nacelles, or the straight-up helicopter mode, is good for zero to 60 knots in forward speed or zero to 35 knots in rearward flight. The transitional 75-degree detent allows forward flight up to 100 knots while in the 60-degree setting flight as slow as 80 knots and as fast as 120 knots are within the envelope. We’ve also subjected it to 1.5 gs.”
The BA609 is designed to take as much three gs positive/one-half g negative during helo operation and throughout the conversion envelope, and three gs positive and one g negative in airplane mode. “The next detent is for the 45-degree full airplane auto setting,” Hopkins said. “Once you’re there, you’re in airplane mode.”
Helo Jock or Fixed-wing Pilot?
Ever since the idea of an aircraft that could take off like a helicopter but cruise like an airplane was first bandied around the hangar coffee urn, the debate has continued: Who would be better-suited to fly the beast–fixed- or rotary-wing pilots?
Hopkins adds his vote in favor of, not surprisingly, rotorcraft pilots being the best suited to transition to tiltrotors. “It’s really kind of obvious,” he said. “The two most critical operations of an aircraft’s flight are landing and takeoff. The tiltrotor does both of these as a helicopter. Therefore, pilots should have more helicopter experience.”
Even when cruising in airplane mode, the BA609 is a unusual kind of airplane. Unlike its predecessors, it has no rudder. In helicopter mode, thrust is used for vertical control while pitch attitude is used for velocity control. In airplane mode, these roles are exchanged, with thrust controlling airspeed and pitch attitude controlling vertical flight path angle. Lateral maneuvering in forward fixed-wing fight is accomplished chiefly by banking, the result of some wing mounted flaperons controlling roll. Quite simply, if a right turn is desired, the right turn input is applied to the stick. Flight control computers then deflect the right-side flaperon up and the left-side flaperon to bank the aircraft.
Yaw control is accomplished via some conventional-looking floor-mounted pedals, which in “helicopter and conversion mode control differential longitudinal cyclic with a tad of DCP, or differential collective pitch, thrown in,” Hopkins explained. “In airplane mode, pedal inputs result in DCP inputs to the prop-rotors, meaning that the one on the outside of the turn produces slightly more thrust to ‘center the ball.’ The BA609’s fly-by-wire system provides full-time turn coordination, meaning that essentially no pedal inputs are required by the cockpit crew above 45 knots. “All this may sound very intimidating,” said Hopkins, “but really flying the previous tiltrotors and, so far, the BA609 is very intuitive. One you get the hang of it, it’s like riding a bicycle; you don’t think about it as much as you just do it.”
The BA609’s control systems marks a major departure from the V-22, which uses both differential thrust and rudders for airplane-mode yaw control.
Back in the Air
Following a successful teardown evaluation of the first BA609 prototype, flight test is scheduled to resume in July next year and will be split nearly equally between Bell Helicopter’s Arlington, Texas facility and program partner Agusta’s Cascina Costa test site outside Milan, Italy. Each program partner will have two prototypes for flight and ground test. Following that, present plans, highly subjective at this time, call for a joint FAA/JAR certification of the six-to nine-passenger tiltrotor transport in mid-2007.