Despite recent technological advances, obstacles still remain in the way of building a safe, reliable aircraft which is as comfortable with fixed-wing flight as it is with rotary-wing flight

The idea of an air vehicle capable of converting from a conventional aircraft to a helicopter has challenged aeronautical engineers for almost 50 years. There has been a succession of experimental tiltwing craft, such as the Canadair CL-84, Hiller X-18 and Vertol VZ-2, or tiltrotors - for example, the Bell XV-3, Curtiss-Wright X-100 and, more recently, the Bell XV-15.

While the idea has made huge technological strides in the last few years, major hurdles remain before there can be the long-awaited revolution in rotorcraft. When Bell and Boeing first agreed jointly to develop a tiltrotor for commercial use in late 1996, the V-22 Osprey was in full-scale engineering and manufacturing development (EMD), and it was thought, at the time, that by now the sky would be populated with civil and military examples.

But much has happened since, with the US Marine Corps MV-22B still grounded in the wake of two disastrous accidents in 2000, and the civil BA609 programme has been steadily delayed, with the first prototype yet to make a maiden flight.

Delays have been blamed on a series of re-organisations, starting with Boeing's exit from the civil rotorcraft business. Bell then had to find a new partner, Agusta, for the BA609. There followed the decision in 2000 to replace fuselage supplier Aerostructures with Fuji Heavy Industries, and, finally, there has been the financial and political fallout from the V-22 programme. The net result is more than a two-year delay in flight testing and certification. "We expect to fly in the second quarter of this year, but we are event-driven," says Don Barbour, Bell executive director civil tiltrotor programmes.

BA609 funding was predicated on expected revenues from the V-22, with full-rate production initially targeted to begin in fiscal year 2001. Bell Boeing, instead of building 36 Ospreys a year, is having to eke out a living based on a low-rate output only one-third that number. This is unlikely to change for several years until the Pentagon is satisfied that the tiltrotor is ready for operations. This has not only limited cash, but the resumption of V-22 EMD is occupying engineering resources.

Bell stresses that the V-22 and BA609are fundamentally different machines. The former is a 27,500kg (60,600lb) transport, incorporating highly complex electro-hydraulic redundancy, folding wing and proprotors, while the latter is more akin to the successful XV-15, weighing a mere 7,630kg, incorporating its own distinct turbo-machinery, avionics architecture and a six-to-nine-seat pressurised composite cabin. What the two machines do have in common, however, is their mode of operations and, until doubts raised about the safety and reliability of the V-22 are addressed once and for all, the BA609 is effectively in a holding pattern.

Perception problems

"People's perception of tiltrotors was affected by the two major crashes in 2000," says Rhett Flater, American Helicopter Society executive director. "The tiltrotor, to win acceptance in the civil market, must also win the acceptance of the military. They need to get the V-22 back on track, and address the deficiencies that have been pointed out by the various investigations and assessment panels - and they need to get it into full production to establish credibility," he adds.

The V-22 experience offers some important lessons for the BA609, not least the adoption of a much more conservative approach to flight testing and certification. Accordingly, Bell/Agusta has slowed work on the second pair of prototypes in order to concentrate efforts on the first two aircraft, which will be used for airworthiness testing and envelope expansion.

Preparatory ground testing includes elevating the lead prototype onto a bridge to ensure the necessary ground clearance for its two Pratt & Whitney Canada PT6C-67A-driven proprotors to rotate through the full conversion range.

Ground runs also include electrical continuity testing of navigation systems and hydraulic testing of flight controls, landing gear, pumps and nacelle rotation, along with checks for leaks from the BA609's 3,000lb/in2 (207bar) hydraulic lines. Only then will the aircraft be cleared for first flight. The next pair of tiltrotors will be used to test handling qualities, icing and instrument flight rule (IFR) operations. "The certification schedule has not been announced. We have a large flight envelope to demonstrate - we are going from gate to gate," says Barbour.

Particular attention is having to be paid to the danger posed by vortex ring state, or power settling, in the wake of the MV-22B crash at Marana, Arizona, in April 2000 that killed 19 marines.

The BA609's Rockwell Collins Pro Line 21 avionics suite, which has had to be reconfigured around the helicopter right-hand command seat, offers unique features, including primary flight-display data such as airspeed and altitude, for keeping the pilot flying within the conversion/ reconversion corridor. There will be three nacelle positions represented graphically on the display, and controlled from the collective. Conversion between aircraft and helicopter modes would take around 40s, compared to under 20s for the XV-15.

Europe is evaluating a rather different approach to convertible rotorcraft that would involve tilting part of the wing and nacelle together. Tilting the wing minimises downwash masking in helicopter mode, and allows for more efficiently sized proprotor for aircraft mode. The drawback of traditional full tiltwing designs, such as the 1960s LTV Hiller Ryan XC-142, is poor hover control in strong winds. "The idea is to improve hover efficiency, while reducing the size of the rotor, and allowing it to also land and take-off in fixed-wing mode," says Marc Allongue, of the Eurocopter technology directorate.

The hybrid tiltwing/rotor idea was first tabled by an Agusta-led team in 1999 as the basis for a 20-seat transport named Erica, for which it was seeking European Union (EU) funding under its Fifth Framework technology research programme. The Eurocopter-led Eurotilt consortium proposed a similar-size vehicle, but in which only the forward part of the nacelle would translate.

Both submissions were twice rejected by the EU, before the two groups could agree on a single effort involving component manufacturers, research centres and universities from seven European nations.

Critical technology

The result is a €43 million ($37 million) five-year programme to research six component level Critical Technology Projects (CTP), with industry contributing around half the cost. The three Eurocopter-led CTPs are focused on rotorcraft handling, interactions and load predictions or flight controls and mechanics; development of an advanced rotor for tiltrotor, which will include laboratory testing of a full-scale rotor head; and active control technology for tiltrotor, centred on pilot-in-the-loop simulator testing of new flight-control laws.

AgustaWestland will co-ordinate work in the area of tilt aerodynamics examining the interaction between the rotor and wing, and fuselage, which will include a powered wind tunnel model.

The advanced European tiltrotor dynamics and noise programme will concentrate on high-speed forward flight and external noise reduction, while a third project, tilt rotor integrated drive system development, will involve the design, manufacture and testing of a full-scale main transmission and tilting mechanism.

"Each of these projects is being done in co-ordination with the other," says Allongue. "The idea is to acquire technology, integrate it and produce a good test article and flying demonstrator in order to be ready with a product by the end of the decade," he adds.

The European thrust is to harness new emerging technology and simplify system design and piloting, compared with the complex V-22 or BA609. A European industry demonstrator could form the basis of an EU Sixth Framework-funded programme starting as early as 2003/4.

Infrastructure issues

The question being asked by many potential operators is not whether six- or 20-seats is the optimum-sized vehicle, but whether the necessary infrastructure exists, in the air and on the ground, to make tiltrotors a worthwhile proposition.

Bell/Agusta sees the BA609 as being able to fly IFR between downtown business centres, without having to route via an airport to enter and exit controlled airspace.

Only then will the tiltrotor have the operational flexibility to exploit the machine's 275kt (509km/h) speed, 25,000ft (7,600m) ceiling and runway- independent capabilities to give it the edge over fixed-wing aircraft on sectors up to 490nm.

"If the question is whether the infrastructure is in place to allow vertical flight aircraft to flourish as they exist today, then the answer is no it's not," says Roy Resavage, Helicopter Association International president. "We're trying to develop infrastructure, and make the system more efficient. Global positioning system (GPS) technology will allow us to break the yoke of some of the constraints of terrestrial navigation, and if we can get to free flight, it would give us a competitive economic and time advantage."

The other major infrastructural question hanging over the tiltrotor is access to a suitably equipped network of city heliports. The BA609 will be capable of executing much longer and steeper 9-12º approaches than conventional helicopters, lowering the noise footprint on approach and making it an attractive proposition for operations in and out of confined areas.

The BA609's Pro Line 21 suite includes an FMS-3000 flight management system option, making GPS non-precision approaches possible - but the number of urban heliports appears to be decreasing.

"Without infrastructure, you have nothing," says Eric Walden, American Eurocopter vice-president sales and marketing. "Tiltrotors will be left operating between airports - so you might as well go by aircraft. If we don't turn this around and there is a place to land, I don't see us selling large numbers of tiltrotors."

Bell/Agusta claims to have advance orders for around 80 BA609s from 42 customers in 18 countries. Operators hope to use the tiltrotor in a variety of roles, such as long-range offshore oil and gas rig support, corporate and private transportation, and island hopping. The firm also plans to promote potential versions of the machine, including US Coast Guard search-and rescue, medevac, and as a lead-in tiltrotor trainer for USMC MV-22B and US Air Force CV-22s.

An early customer for the BA609 is Air Center Helicopters (ACH), which plans to use the tiltrotor in the Caribbean as a high-speed VIP passenger transport between islands without runways. The Fort Worth, Texas and Virgin Island-based operator had been scheduled to take delivery of its two machines last year.

But despite the delays ACH owner Rod Tinney remains a strong supporter of the programme, saying: "Bell/Agusta is very well aware that the BA609 has got to be better than perfect and be the safest thing in the sky. "For a small company like mine to put an investment down, I must be feeling pretty good about it."

Source: Flight International