US researchers are reviving the gyrodyne concept and designing the Heliplane, a rotorcraft that promises faster speeds than conventional helicopters
Since the perfection of the helicopter, the autogyro has lain on the shelf alongside other almost forgotten aeronautical ideas. Now US researchers are dusting off the concept and looking back almost half a century for their inspiration, to the Fairey Rotodyne vertical take-off and landing airliner of the 1950s that perished more for political than technical reasons.
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The Heliplane is a gyrodyne – a rotorcraft in which the rotor is powered for take-off, hover and landing, but autorotates through most of its forward speed range and has a propulsion system independent of the rotor. In concept, the aircraft is similar to the UK-designed Rotodyne, which had a tipjet-driven rotor and turboprops. The Rotodyne first flew in November 1957 and set a record speed of 166kt in 1959, but was cancelled in 1962.
“We have taken a 40-year-old concept and revived it,” says DARPA Heliplane programme manager Don Woodbury. “It is hard to see why the Rotodyne did not progress. It was revolutionary back then and, if it was flying today, it would be at the top end of rotorcraft performance.”
Woodbury sees this as a lack of progress in rotary-wing technology over the last 40 years. “We will see if we can advance VTOL with the Heliplane,” he says “If we can get performance out of a rotary wing that is comparable with a fixed wing in speed and efficiency we will change the nature of VTOL. It could be the birth of a new age of rotary-wing aviation.”
This is a lofty goal for a small, Salt Lake City, Utah-based company. GBA was founded by former military and commercial helicopter pilot David Groen and brother Jay to develop autogyros, or gyroplanes, that incorporated rotor collective-pitch control for improved performance. Previous autogyros had been fixed pitch, and the Groens believed this contributed to their decline.
After building the first aircraft in their office, the Groens began investigating autorotative flight. “We found the Rotodyne, and the history of the gyrodyne, and set out to make it happen,” says president and chief executive David Groen. “There was nothing in the Rotodyne that does not make real good sense,” he says. “It was not technology that killed the programme, it was politicians.”
GBA went on to develop the Hawk 4, a turbine-powered four-seat gyroplane but, when private investment dried up after 9/11, the certification programme was put on hold and most of the employees laid off. “We stopped certification, laid off 100 employees, but the company survived and the technology survived,” says Groen.
GBA instead began developing a single-seat kit gyroplane, the Sparrowhawk, and is now working on a two-seat design for the emerging US light-sport aircraft market. At the same time, the company began pressing DARPA to fund development of the gyrodyne, “using the Fairey Rotodyne as proof it was a good idea”, says Groen.
The initial concept was for a heavylift aircraft and a GBA design was offered for the US Army’s Joint Heavy Lift (JHL) requirement. The proposal called for a demonstrator based on a modified Lockheed Martin C-130 airlifter with full-scale tip-drive rotor. But the Georgia Institute of Technology team, of which GBA was part, did not win one of the five concept design analysis contracts awarded in November last year.
The GyroLifter proposed for JHL was a 9,000kg (20,000lb) empty-weight aircraft capable of cruising at 215kt. By comparison the Heliplane is much smaller – and much faster. “The GyroLifter was designed for up to 250mph, which is fairly easy to accomplish,” says Groen. “DARPA wants significantly higher speed, which changes the blades and the drive.”
DARPA’s goals for a Heliplane demonstrator are a 350kt cruise speed, 1,850km (1,000nm) unrefuelled range and 455kg payload. The design mission of combat search and rescue (CSAR) demands speed and range. “The initial mission is CSAR, but there are many other potential uses for an aircraft with speed, efficiency, VTOL, hover and the ability to operate from unimproved surfaces,” says Woodbury.
Best of both worlds
GBA and DARPA believe the Heliplane combines the best characteristics of a helicopter with those of a fixed-wing aircraft, giving it the ability to take off and land vertically, hover with low disc loading and low downwash, yet fly as fast as a turboprop and more efficiently than a helicopter. Efficiency goals include a cruise lift/drag ratio greater than 10 – compared with 5-6 for a helicopter and more than 15 for a fixed-wing aircraft – and a disc loading in hover that is lower than a tiltrotor’s.
Where the Heliplane follows the Rotodyne is in its use of a reaction-drive, instead of transmission-driven, rotor. “To keep weight down, reduce development time and cost, and enable efficiency, we need to get rid of the transmission, torque and tailrotor,” says Woodbury. The Heliplane is “a simpler concept than a helicopter, a little more complex than an aeroplane,” he says. Compared with a helicopter, the rotor is simpler because it does not have to power the aircraft in forward flight, while the wing is shorter and has a less-complex high-lift system than in a fixed-wing aircraft.
Driven by tipjets, the rotor is powered in vertical flight and through the transition to forward flight. “As soon as it passes through the transitional lift, power to the rotor is turned off,” says Groen. Conventional propulsion takes over and, as speed increases, lift transfers to the wing and the now-autorotating rotor is slowed to reduce drag. “At the top end, the rotor carries as little lift as possible,” he says.
The Rotodyne used “cold-cycle” reaction drive. Compressed air from the turboprops was ducted to the rotorblade tipjets and burned with kerosene to drive the rotor. Other pressure-jet concepts used either a hot cycle, in which efflux from a gas generator was ducted to the blade tips, or a warm cycle, which used cooler turbofan bypass air to drive the rotor.
The choice of reaction drive is one of several trade studies underway during the initial 15-month, $6.4 million phase of the Heliplane programme. Groen says GBA is looking at cold- and warm-cycle tip drive. Another trade study will decide whether compressed air to drive the rotor is drawn from the the Heliplane’s twin Williams FJ44 turbofans or a separate engine in the aircraft belly. “We are likely to use just two engines,” Groen says.
Noise was a major issue for the Rotodyne, but Fairey had made progress before the aircraft was cancelled, Woodbury says. “Part of the noise was from tip burning,” he says. “Heliplane noise levels will be similar to a helicopter’s and it will be quieter in forward flight.” Citing research by team member Georgia Tech, Groen says: “We believe we have a solution to tipjet noise and the Heliplane in autorotative flight makes less noise than an helicopter because the tip speed and lift is lower.”
Both DARPA and GBA agree the biggest technical challenge in the Heliplane programme is stability and control of the rotor at high forward speed. “Control is a risk area. We have got to get rotor stability at very high advance ratios,” says Woodbury.
The Heliplane will fly at ratios of rotor advancing-blade speed to aircraft forward speed far higher than conventional helicopters. Advance ratio is typically 0.3-0.4, but a Westland Lynx reached almost 0.5 – and its blade tips almost Mach 1 – when it set the world helicopter speed record of 217kt. Slowed-rotor autogiros have been flown to advance ratios above 0.8, but the Heliplane could exceed 1.6.
Autogiros have significant high-speed advantages over a helicopter, says Groen, because of differences in rotor disc angle-of-attack and blade collective pitch. Where a helicopter requires a negative disc angle to produce thrust, an autogyro has a positive angle, which allows higher advance ratios before the retreating blade stalls. And where a helicopter must increase collective pitch to fly faster, an autogyro reduces it, delaying retreating-blade stall.
Hub drag challenge
Minimising the drag of the autorotating rotor by controlling its speed and angle-of-attack is key to achieving the ambitious performance goals. “If we do not substantially reduce drag, we will not get to 400mph,” says Woodbury. The simple rotor will help. “The blades are not twisted and we do not need an extremely rigid rotor. Hub drag is still a challenge, but a gyroplane hub is less complex than a helicopter’s,” he says.
Phase 1 of the $40 million programme will take the rotor system to a preliminary design review and address primary risk areas: windtunnel tests of a full-scale blade section and subscale fuselage to measure drag and downwash; static and rotating tipjet tests to assess aerodynamic and propulsive performance; and acoustic and structural tests. Phase 2 would include windtunnel testing of a full-scale rotor and detail design of a demonstrator based on the Adam A700 very-light jet. The demonstrator would be built and windtunnel-tested in Phase 3 and flight-tested in Phase 4. DARPA wants the first flight in about 42 months, says Woodbury.
Leading a team including Georgia Tech, Adam Aircraft, Williams International and the NASA Ames/US Army rotorcraft research division is a challenge for GBA. “Groen has experience of taking a concept to flight in a short time,” says Woodbury, adding: “We could not deliver this programme for the dollars or in the time available if we used one of the big guys.”
Performance goals are high to ensure the Heliplane is not ignored. “We have set the standard high enough that, if we succeed, it will not be at the margin - we will have an overwhelming advantage over existing configurations,” says Woodbury. “We will change the nature of VTOL.”
GRAHAM WARWICK / WASHINGTON DC
Source: Flight International