As helicopters near the edge of their performance envelope, radical designs such as tiltrotor and tiltwing are coming into their own

What goes around comes around and that is the case for the rotorcraft industry, which is seeing the re-emergence of radical concepts from the 1950s and 1960s. With the benefit of technological advances in materials, avionics and propulsion, many of these ideas promise to be no less revolutionary in the 21st century.

The conventional helicopter is close to the edge of its performance envelope. The emphasis is on making it a more efficient and environmentally friendly mode of transport. The push to achieve a quantum leap in payload, speed and range is fuelling fresh interest in alternative approaches such as tiltrotor and tiltwing.

There is perhaps no better example of the time it can take a technology to mature than the tiltrotor. The first successful conversion between vertical and forward flight was achieved in 1958 by the Bell XV-3. Only now, more than 40 years later, is the first tiltrotor entering production in the form of the US Marine Corps' Bell Boeing V-22 Osprey.

Acceptance of the tiltrotor has been as much a political battle as it has been a technological challenge, but thanks to the success of the V-22 and earlier Bell XV-15 demonstrator, it has taken hold in the military market and is entering the civil arena. The six/nine-seat Bell Agusta 609 has amassed over 80 orders ahead of its scheduled entry into service in mid-2002. In the aeroplane mode it will have a maximum speed of 275kt (510km/h) and range of 1,400km (750nm) - twice that of a helicopter.

Development of the commercial BA609 has opened the door to cheaper and faster manufacturing processes not used on the V-22. This includes a single-step aluminum frame and carbonfibre skin wing assembly process, cutting manhours in half. "We've been able to streamline processes and design a tiltrotor that is much more competitive with helicopters and aircraft," says Dick Spivey, Bell's director of tiltrotor business development.

Bigger tiltrotors

Bell and others are starting to turn their attention to larger post-V-22 developments for civil and military applications. The Fort Worth-based manufacturer is focusing on a proposed Quad TiltRotor (QTR) capable of lifting 19,000kg (42,000lb) - twice the payload and eight times the internal volume of a V-22. In short take-off and landing (STOL) mode it could deliver around 13,600kg out to a range of 1,800km.

The QTR is intended to use many of the systems developed for the V-22 to keep cost and risk to a minimum. Notionally, it would use the same avionics, drivetrain, nacelles and Rolls-Royce Allison T406 family of engines as the Osprey. Two engines would be mounted on a 15.7m (51.5ft)-span forward wing and two on a 22.9m-span aft wing. The 3.66m fuselage cross-section is comparable to that of a Lockheed Martin C-130 giving the QTR the capacity to accommodate 90 troops or eight cargo pallets.

Bell believes the QTR will have commercial appeal for package carriers such as FedEx. "We've been talking to them for some time about the V-22 and BA609 and know they would like something bigger to carry their standard pallets. From their Memphis hub, the OTR could cover 1,000nm and do drop-offs in each town," says Spivey.

Bell's principal target for the OTR, however, is the US military's future Joint Transport Rotorcraft (JTR) requirement to replace Boeing CH-47F and Sikorsky CH-53E heavylift helicopters. The US Army is looking for transport to carry an 8-12t payload over 1,000km, but this is likely to grow to 20t.

Tiltrotors represent a compromise in efficiency between helicopters with larger rotors optimised for hover and fixed-wing aircraft with smaller propellers sized for forward cruise. Added to the need to fit on ship decks, this has limited the V-22's proprotor diameter to 11.6m. With the QTR likely to measure over 40m tip-to-tip there is renewed interest in more efficient proprotor designs.

With the aid of US Defence Advanced Research Projects Agency (DARPA) funding, Boeing is looking at using "smart" materials, such as piezoelectric actuators and shape-memory alloys (SMA), to optimise blade twist for hover and cruise. "We're primarily looking at improving the twist and twist distribution through high technology actuators that would fit within the same fundamental structures as we use today," explains Dave Harding, Boeing director of future programmes, advanced rotorcraft systems. Using SMAs in the V-22 proprotors would generate a 400kg increase in payload or 130km more range, Boeing says.

Variable diameter

DARPA is also funding demonstration of a Variable-Diameter Tilt Rotor (VDTR), as part an advanced rotorcraft technology effort in support of JTR. Sikorsky is heavily involved, having already windtunnel tested full-scale telescopic blade components that allow proprotor diameter to be increased in the hover and reduced for forward flight. "Physically it makes sense. The $64 billion question is whether it is safe and reliable," says Doug Halley, Sikorsky director of advanced technology.

Studies show that a 50% extension to each of the V-22's two three-blade proprotors would deliver a 35% improvement in payload, which if translated into fuel would extend the tiltrotor's range by 70-100%. Aside from the V-22 and larger 34,000-45,000kg size JTR applications, Sikorsky is looking at VDTR for smaller 11,500kg-class tiltrotors that could be employed as armed escorts.

Sikorsky is looking at two different types of actuators to reel the blades in and out, using either jackscrews or composite straps. The challenge in the view of some critics will be in regulating engine and tip speed as proprotors are either extended for hover or retracted for forward acceleration.

Bell is looking at an alternative "stop-and-fold" approach to improving tiltrotor efficiency. This would entail decoupling, feathering and folding the proprotor back along the nacelle once the tiltrotor had passed 100kt and there was sufficient wing lift. The tiltrotor's turbine powerplant would switch from turboshaft to turbofan mode, permitting speeds of 400-500kt.

European research into tiltrotors centred on the early-1990s Eurofar programme, which yielded a concept being proposed for a far more ambitious multinational effort, called Eurotilt. The aim is to produce a design that could be applied to a range of convertible helicopters - and although a full-scale programme is still a long way off, the Eurotilt consortium is convinced that its novel approach will give it a strong contender for the utility market once a programme is launched.

In the Eurofar design, conversion from horizontal to vertical flight was achieved by rotating only the forward part of the nacelles, leaving the engine in a horizontal position. This is the scheme adopted for Eurotilt, and it is fundamentally different from the fully tilting engine/rotor concept used in the V-22 and BA609. "We believe it is a better approach because it means the engine needs no modification for vertical operation," says Eurocopter research director Philippe Galland. He points out that the rotating forward nacelle also allows for a simpler wing design, because the rotating mass does not include the engine.

A fully tilting wing was ruled out early on, says Galland, "because translating the entire wing causes a lot of drag during translation to horizontal flight, and adds extra weight for the vertical take-off. This reduces payload by at least 3t".

Another major difference between the Eurotilt and the BA609 is its size, which Eurocopter believes firmly should be in the 10t, 19-seat category, compared with the 7t, six/nine-seat Bell Agusta machine. Range would be around 1,500km, at a 300kt cruising speed. "We're looking at the offshore and utility markets," says Galland. "These are areas that we think are likely to provide real economic benefits for this kind of machine."

Eurotilt has been proposed for European Commission funding under its Fifth Framework technology research programme, and Eurocopter has set up a team comprising 33 partners from nine European countries. They would carry out initial research into the most technology-critical areas, such as engines, rotor dynamics and fly-by-wire flight controls, with ground tests of the dynamic system taking place in the final phase. While the Eurotilt programme does not lead directly to a flight test demonstrator, Galland says: "It would certainly be our aim once the technologies are in place".

Eurocopter joins race

Eurocopter admits it is starting "from a lagging position", but says that the programme would enable it to "catch up within four years, using distinctive but competitive technology for key components". It adds that it would be ready to "meet anticipated market demands around 2008 which are complementary to those chosen by Bell for the BA609".

Agusta, meanwhile, has assembled a 16-strong team to pursue European funding for its convertible helicopter programme, called Erica. This and the Eurocopter project are the subject of negotiations between the two teams to decide on a common programme - the European Commission having made this a condition for funding under its Fifth Framework research programme.

The Italian proposal is based on a 20-seater. The design features rotating outer wings - in some ways similar to the 1960s Canadair CL-84 and LTV Hiller Ryan XC-142 tiltwings - the idea being that the proprotor downwash does not impinge on the fixed horizontal wing, improving lifting efficiency by around 12%, says Agusta. This allows for a reduction in the size of the proprotors, in turn making them more efficient and raising cruising speed to around 350kt.

The aircraft would also be able to take off and land conventionally - albeit with the outer wing tilted up 7°. Propeller tip clearance would be increased by mounting the wing high on the fuselage. The wing spar would be a carbonfibre tube housing a shaft linking the engines, mounted inboard of the tilting section, to the outboard proprotors, which Agusta says simplifies the power transmission system. The aircraft would be certificated for single-engined take-offs and landings at maximum take-off weight.

A further adaptation on the 1960s tiltwing concept is the Boeing No-Tail Advanced Theatre Transport (NOTAIL ATT), formerly known much more simply as Super Frog. The design is not a true 90° vertical take-off and landing tiltwing, as was the Canadair CL-84, but rather an innovative effort at developing a "super-STOL" (SSTOL) transport capable of lifting 30t loads into 180m-long unprepared strips at the battlefront.

Tiltwing technology has benefited from important advances over the past 10 years, including a breakthrough in tailless design. Earlier tiltwing efforts such as the LTV Hiller Ryan XC-142 suffered from tail stalls at low speed, requiring the installation of a tailrotor for better lateral control. The ATT instead uses counter-rotating port and starboard propellers and split flaps/ailerons for lateral control.

"The real push," says Gerry Janicai, Boeing Phantom Works director of business development, "has come through the use of computational fluid dynamics to help to design an aircraft to fly an extremely slow, 35-40kt approach speed with wings tilted and to understand its low speed characteristics in terms of stability and control".

For SSTOL, the ATT's wing is tilted up to 42° to increase lift, using two jackscrew actuators housed within the wing/fuselage fairing. Low-speed lift is augmented by propeller flow over the wing, which simulates an 130kt approach speed, and by engine thrust. The 56.2m-span wing is swept to cover 80% of the compact 37m-long fuselage.

The ATT would be powered by four 9,000kW (12,000shp)-class turboprops cross-shafted for redundancy. With one engine inoperative it will still be required to take off from a 340m strip and clear a 23m obstacle. Fully loaded, the aircraft is projected to have a range of 3,300km.

Boeing believes there is a US Air Force requirement for ATTs around 2011, and earlier for the US Army. The aircraft's size and lift capability would put it firmly in the C-130J camp, rather than the JTR, and accordingly it will face strong competition for funding. Boeing is flight testing a 7% scale model and if it was to secure money in the next 18 months for a demonstration, "2011 would be completely feasible", according to Janicai.

Compound comeback

Another rotorcraft configuration vying for a comeback is the compound helicopter, a promising 1960s concept that is being looked at again to the maturation of technology. One of the first applications was the ambitious Lockheed AH-56A Cheyenne attack helicopter. In spite of achieving an impressive speed of 220kt, the programme fell victim to development problems and cost escalation and was cancelled in 1969.

The concept in its simplest form entails fitting a helicopter with a lifting wing and some form of auxiliary thrust. The net result is a reduced load on the main rotor, which delays retreating blade stall and provides the opportunity to reduce rotor speed to mitigate compressibility effects on the advancing blade. This permits an expansion in flight envelope out to speeds of 240kt, higher operating ceilings approaching 20,000ft and extended range.

"More important to performance benefits is the compound's ability to unload the rotor in forward flight, offering the opportunity to lower rotor vibration and fatigue loads, which can significantly reduce helicopter operating cost," says John Piasecki, vice-president of Piasecki Aircraft, which has been pursuing the compound concepts for decades.

Lightweight composites, more efficient vectored ducts and modern flight controls are making compounding viable. Piasecki, which flew its first shaft-driven Ring Tail compound demonstrator in the 1960s, is working on a $30 million US Navy advanced technology demonstration. This entails modifying a Sikorsky CH-60 with a vectored-thrust ducted propeller (VTDP) and lifting wing and will culminate in flight testing in 2003.

The VTDP is a second generation Ring Tail, and provides anti-torque/yaw control and thrust vectoring. "In windtunnel tests, the VTDP demonstrated a 46% improvement in hover efficiency on the original Ring Tail," claims Piasecki.

The benefits of compounding are mission dependent and not universal. While the VTDP compound helicopter is more efficient in hover than tiltrotor/wing concepts, it usually requires more power than conventional helicopters in the low-speed regime. The advantages of compounding are realised in missions where speed and range are a priority.

The US Navy's interest is in augmented power for its planned airborne mine-countermeasures version of the CH-60, which will be required to tow loads with a hydrodynamic pressure of up 2,700kg. For the SH-60 maritime patrol and HH-60 search and rescue versions, compounding would provide faster transit times and, thanks to a wet wing, longer periods on station, says Piasecki.

The company is also hoping to attract US Marine Corps interest in a further enhancement to its modernised Bell AH-1Z attack helicopter and has already conducted concept studies. Besides speed and improved nap-of-the-earth flight control, the thrust vectoring tail provides a unique pitch pointing capability, Piasecki says, which decouples pitch attitude and airspeed. This would permit a sustained nose-up pitch for air-to-air or over-the-hill engagements.

EH101 experiment

Agusta and GKN Westland have also been studying a compound development of the EH101 helicopter for applications such as the UK Royal Navy's Future Organic Airborne Early Warning aircraft, where a higher ceiling is needed to increase radar range. The two companies are hoping to secure funding for a Rolls-Royce Turboméca RTM322-powered Lynx compound demonstrator.

Sikorsky experimented throughout the 1970s and 1980s with concepts to bolster helicopter performance. These efforts included the S-69 Advancing Blade Concept demonstrator equipped with coaxial counter-rotating rigid rotors and two 3,000lb-thrust (13kN) Pratt & Whitney J60 turbojets. This was re-equipped with a ducted pusher propeller.

This was followed by the S-72 Rotor Systems Research Aircraft, intended as a testbed for a variety of different concepts. Features included a removable 13.7m-span wing and two 9,300lb-thrust General Electric TF34 turbofans. The most ambitious concept was to install in place of the main rotor a compressed-air blown X-wing which would be stopped at high speeds. Pneumatically it proved too complex for its time and was cancelled.

A much refined variation on this theme is the joint Boeing/DARPA-funded Canard Rotor Wing (CRW). The reaction-drive rotor is propelled by a warm-cycle mix of engine fan and exhaust air ducted to the blade tips, a concept first tested in 1964 on the Hughes XV-9A. As speed reaches a 100-120kt conversion corridor, lift is offloaded from the rotor on to the canard and horizontal stabiliser, allowing the rotor to slow and gas to be diverted to the aft thrusting nozzle. The main blade is then locked, allowing speeds of 400kt-plus.

"The X-wing was a concept in which, as you slowed the rotor, you lost the ability to generate lift. The breakthrough for CRW is the fact that we're offloading the rotor. This allows us to balance the aircraft's transition. We also don't have the X-wing's complex drive, which was getting all its flight control from circulation blowing along the edges of the blades.

"We've basically got a reaction-drive tip, a gimballed rotor and traditional helicopter controls," says Andy Logan, Boeing Phantom Works' general manager of advanced rotorcraft systems.

Under the current $24 million demonstration programme, Boeing is planning in 2001 to flight test two 80%-scale versions of a proposed unmanned air vehicle (UAV). "We're setting out to prove the whole CRW concept is viable. It will hover like a helicopter, transition to a controlled rotary-wing flight and we will then stop the rotor and fly it as fixed wing vehicle," he says.

CRW benefits

As with compound helicopters, the benefits of CRW are mission-specific and combine the flexibility of VTOL with high-speed performance. Two applications have been initially envisaged for the CRW: a 9,000kg-class manned escort for the USMC's V-22 tiltrotor and a smaller 5,900kg-size UAV for the US Navy.

"What intrigued us and started us on the CRW was that it promised reductions in weight and a dramatic increase in efficiency because you eliminated, rather than added, complexity. There is no transmission, direct controls, tail rotor or cross shaft. It actually goes the way you want to go, which is to take things away," says Logan. Given the complexity of today's helicopters, that seems like a good idea.

Source: Flight International