Civil tiltrotor designs fly in face of several challenges

 - March 28, 2007, 7:00 AM

Last June Bell/Agusta completed initial testing of its BA609 civil tiltrotor at Bell’s Fort Worth, Texas flight-test center. At the time, project test pilot Roy Hopkins said he was particularly impressed by the tiltrotor’s handling qualities. Over nine flights of the BA609, Hopkins, accompanied by flight test pilot Dwayne Williams, logged 14 hours in helicopter mode. Hopkins and Williams are still in post, currently evaluating reconfigurations to the aircraft in the associated vehicle management system integration laboratory, which replicates all the components and software found on the actual aircraft.

However, technical director John Magee, who had spent much of his life’s work on tiltrotors, retired just as the initial flight-test phase came to an end. Ross Menger took over his post. Since then, the tempo of work on the aircraft has been maintained, although not all of the activities are taking place on U.S. soil. Explained Menger, “We had an extensive reorganization and restructuring of the entire program, renegotiating the work split with Agusta.

“As part of our joint-venture agreement, our Italian partners are now taking on a significantly increased proportion of the work, primarily the flight-test segment. To that end, three out of the four prototypes will find their way to Italy. Number two was actually shipped last summer to Cameri Air Force base, west of Milan, and is currently undergoing safety-of-flight-related proof load tests. In fact, the first one, the ‘empennage pull’–loading the empennage and aft fuselage–has just been completed.

“Over the coming months, that aircraft is also scheduled for proof loads of flight control surfaces, including the elevators, flaperons and rotor controls. These are all certification test requirements to demonstrate that the surfaces will move freely under load throughout their ranges, without any excessive deflections or binding. Aircraft two was sent to Italy as, essentially, two large primary structures–a wing and a fuselage–and so in parallel with the tests it will have the systems integrated and assembly completed.

“Aircraft one will return to flight first toward the end of this year [late October has since been mentioned–Ed.], followed by number two in Italy a few months later, so we are still on schedule. The airplane-mode software development is continuing, as is that of the flight-control system, pressurization system and environmental control system. Aircraft three and four are still here in Texas in various stages of completion, and they will be sent to Italy later this year. The wing and fuselage of aircraft three are taking shape and four is still in early stages of assembly.”

As a result of the initial flight tests, brake-pedal geometry is being reconfigured to ease operation, the main landing gear shock strut is being modified to allow softer landings, the glareshield size is being reduced to improve visibility and, to add protection in the event of a birdstrike, two small overhead windows in the cockpit have been eliminated.

Additionally, the Pratt & Whitney Canada PT6-67As will have different mechanical fuel-control units, designed both to improve fuel scheduling during start and to help it achieve faster acceleration during OEI conditions.

“Once the aircraft starts flying again,” noted Menger, “we will be concentrating on expanding its flight envelope and then looking at certification issues. We are still on target for FAA certification in 2007 and first deliveries a year later.” The BA609 team cannot find out for sure about any weight savings until the full flight envelope has been defined, but Hopkins said they are keeping their fingers crossed.

Bell/Agusta claims it has orders for 60 civil tiltrotors and that the price will be confirmed two years before scheduled delivery.

While progress on the BA609 remains tied–even constrained–to that of the Bell Boeing V-22 military tiltrotor, Europe’s Erica (enhanced rotorcraft innovative concept achievement) tiltrotor project continues for now as a purely paper exercise led by AgustaWestland. In a departure from the design of existing aircraft, a much heavier 22,000-pound Erica will have smaller-diameter prop-rotors. Its outer-wing sections will move independently of the nacelles, and a rigid continuous shaft will run between them, both to support the nacelles and drive their rotation.

Significantly, to allow heavier payloads and a longer range, the 24-foot-diameter rotors will allow sufficient ground clearance for takeoffs and landings in airplane mode. A highly twisted rotor blade could raise Erica’s maximum cruise speed to 350 knots, a full 75 knots faster than that of the BA609 and V-22.

Since a smaller rotor inevitably affects hover performance, the engineering team’s solution is to improve the angle-of-attack by designing the outboard wing sections to move independently of the nacelles. This feature should also deliver a wider airplane-to-helicopter (and reverse) conversion corridor and improved characteristics in autorotation.

To reduce weight, the support shaft between the nacelles will have pipes and wires routed inside. It will also dispense with the need for swiveling joints and synchronization mechanisms on the nacelles.