Gyroplanes, so long forgotten, may be about to reclaim centre stage
Guy Norris/LOS ANGELES
If David Groen, president of Groen Brothers Aviation (GBA) is right, the skies may soon be populated by fleets of gyroplanes. Combining the vertical short take-off and landing capabilities of the helicopter with the lower costs of fixed-wing aircraft, the gyroplane is gearing up for a comeback.
Groen is developing the four-seat, turbine-powered GBA Hawk 4T for US Federal Aviation Administration (FAA) certification and believes the time is right for a concept that essentially missed the boat. "Gyroplanes should have taken their own evolutionary course - just like aircraft and helicopters," he says. Instead, gyroplane development was sidetracked into helicopters in the 1940s, and subsequent gyroplane growth became severely hobbled by what he describes as "politics and a misunderstanding of the technology and the concept".
The basic principles of the gyroplane - or autogyro as it is sometimes still known - remain unchanged from the first flight of the Juan de la Cierva-designed aircraft in Spain in 1923. Unlike helicopters, the gyroplane's main rotor system is unpowered, and lift is instead provided by "steady state autorotation". To achieve this, the aircraft is pushed (or pulled) forward by a propeller which provides the required air velocity to keep the blades turning. Steady state autorotation exists when there is zero torque on the rotor, resulting in constant revolutions per minute (RPM). Since the unpowered main rotor develops no torque, an anti-torque tail rotor is not needed.
Two key design innovations since Cierva's time include a direct control rotor head, and a pre-rotator system for the main blades. Early machines flew with the rotor head fixed, and roll, pitch and yaw authority was provided by conventional ailerons, elevators and the rudder. The development of a direct control rotor in 1932 allowed full control in pitch and roll, independent of forward airspeed.
The mechanical pre-rotator enabled the rotor to be spun up to higher speed than that needed for flight. In previous machines, the blades were started by hand, and the aircraft taxied at greater and greater speeds until rotor RPM was achieved. In light winds this often resulted in very long take-off runs.
David Groen's interest in gyroplanes was stimulated at an early age by a picture of a Benson kit-built machine on the front page of Popular Mechanics in 1966, a year before James Bond took to the skies in the fabled "Little Nelly" gyroplane in the film You Only Live Twice, popularising the concept. Together with brother Jay, Groen established the Salt Lake City, Utah-based company which has been developing the Hawk series for 16 years. The latest Rolls-Royce 250-C20S-powered 4T is described by Groen as "the world's first turbine-powered gyroplane", and it is on track for certification by the third quarter of 2002. Test flying is undertaken at Buckeye, Arizona, close to the Glendale Airport site west of Phoenix earmarked for the Hawk production line and the company's future headquarters.
Keys to success
Groen's turbine-powered 4T, which succeeds a Teledyne Continental TSIO-550 piston-powered Hawk 4 model, has attracted cash deposits for 142 aircraft. The first are due for delivery in late 2002 based on a production plan calling for the assembly of 12 in the first year, 50 in 2003 and up to 300 in 2003.
The company believes it can succeed where others have failed for several reasons. One is that the GBA-proprietary blade design and rotor system is optimised for sustained autorotation, rather than having been adapted from standard helicopter systems. Virtually every previous gyroplane designer used the NACA 0012, a helicopter-derived blade-shape. In its place, the Groen machines have a new asymmetrical, cambered aerofoil that exhibits a small pitching moment change as the angle-of-attack varies. The shape means advancing blade tip Mach numbers are low (around Mach 0.6), which allows faster forward flight before encountering retreating blade stall.
Unlike earlier machines, with the notable exception of the Fairey Rotordyne - developed in the UK in the late 1950s - all gyroplanes have been underpowered, the company says. Virtually all used a 134kW (180hp) piston engine and had weight-to-power ratios of more than 6kg/1kW. GBA's design, using a 310kW turbine engine, changes that. Further developments will all have higher powered engines, adds Groen. Additionally, all earlier designs had two seats and small useful loads of around 230kg (500lb). The Hawk 4T, on the other hand, is designed with a gross weight of around 1,500kg, a useful load of around 545kg, a range of approximately 925km (500nm), a take-off roll of between 0 and 45m (150ft) and a landing roll between 0 and 15m. Maximum level flight speed is 135kt, and rate of climb is 1,000ft/min (5m/s).
Besides having a large market in its own right, Groen views the Hawk 4T as a bridge to larger gyroplanes. GBA is also close to taking the first steps towards this second goal with the maiden flight of a Hawk 6 - a converted Cessna C337 Skymaster - due to take place as Flight International closed for press. Intended as a proof-of-concept test vehicle for carrying either larger cargo or eight to 10 passengers, the Cessna gyroplane has been greatly modified.
Inverted twin booms
The rear pusher engine has been removed to provide access to what, on a production version, would be clamshell cargo doors. The forward 'tractor' engine is replaced by a Rolls-Royce 250-B17F2 turboprop, while the twin booms have been inverted so the tails point down, which keeps them out of the way of the two-bladed rotor taken straight from the 4T.
GBA expects the wing stubs - required as structural supports for the booms - to provide lift which it will cancel with new spoilers. Ultimately, it plans to add wing extensions to help offload the rotor, though GBA adds that this will be done cautiously to avoid upsetting the weight and airspeed balance of the autorotative rotor system.
"We needed to learn about tractor gyroplanes, though since we started it we have had interest from government agencies about using it as an uncertificated aircraft for things like border patrols," says Groen. The FAA turned down GBA's request for supplemental type certification for the conversion. The main aim, therefore, remains the development of a larger version for companies such as Fedex, interested in an aircraft that could carry at least 1t of payload. GBA's original designs for such a vehicle "looked very much like a Cessna Skymaster, so that's where we got the idea to use one as a technical demonstrator".
Ultimately, Groen foresees a large family of gyroplanes, the largest of which could be a range of regional transports carrying 18, 35 and 50 passengers. Inspired by the Fairey Rotordyne, which was technically a compound aircraft, GBA is "working with a major aerospace firm" on this. Like the Rotordyne, which was scrapped in 1962, the proposed transport versions will have tipjet-powered rotors to allow vertical take-offs and landings. Groen sees this as the key to the family's promised flexibility, freeing it from the need for runways and slot congestion at airports. "The Fairey Rotordyne was killed by politics and not technology, so with what we have available today in terms of modern plastics and composites even if we did no better than the Rotordyne we'd be doing well," he adds.
Thanks to GBA and CC, the gyroplane may be set for a renaissance in the 21st century.
Advanced technology also lies at the heart of another gyroplane venture, the CarterCopter (CC). Founded in 1994 by Jay Carter and Paul Redding, the Texas-based company has completed the last of a three-phase NASA small business initiative-sponsored research contract and is pursuing technology for a final design that will cruise as high as 50,000ft (15,000m) and at speeds up to 435kt (500mph).
Despite several flight test setbacks, including three crashes, CC is setting its sights on breaking the "Mu-1 barrier" - the theoretical limit on the top speed of any rotorcraft. Flight above Mu-1 would involve generating forward speed greater than the tip-speed of the rotor, which Carter believes is possible by slowing the rotor and yet still maintaining rotor stability. The company has proved the concept's viability on a 1/6 scale model, but needs to repeat the feat using the full-scale gyroplane. Carter says: "We want to break world records, and demonstrate the capability of this aircraft which we believe could be as good as a second-generation [V-22] Osprey without having to tilt the rotors."Source: Flight International
