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Heliplane design gathers speed

Rotor system requirements review completed as Groen and DARPA tackle 350kt challenge for gyroplane

Design of the Heliplane high-speed gyroplane is progressing, with developer Groen Brothers Aviation and the US Defense Advanced Research Projects Agency (DARPA) completing a system requirements review on the rotor last week.

Under the DARPA contract, Groen is designing an X-plane demonstrator that can take off and land vertically, reach 350kt (400mph or 645km/h), and carry a 455kg (1,000lb) payload over an 1,850km (1,000nm) radius without refuelling. "We did not realise how much of a challenge 400mph was, but we have a design that converges, and we have got some margin," says DARPA Heliplane programme manager Don Woodbury.

Recent design changes include relocation of the twin Williams FJ44 turbofans to either side of the rotor pylon to improve integration of the engines, from which cold bypass air is diverted and ducted to the tips of the blades to be burned with fuel to drive the rotor. The Heliplane is a gyrodyne - the rotor is powered for take-off, hover and landing, but auto-rotates through most of its forward speed range.

Under Phase 1 of the Heliplane programme, Salt Lake City, Utah-based Groen will take the complete rotor and drive system to a preliminary design review next April, aiming to conduct a full-scale windtunnel test in the next phase. "We plan to go into the large-scale tunnel at NASA Ames to demonstrate we can control the rotor at 400mph," says Woodbury.

The 15-month first phase includes several model and component tests to reduce risk. These include a 10%-scale windtunnel model of the airframe - based on the Adam A700 very light jet - minus the rotor a 40%-scale rotor hub and pylon model to characterise drag, a major challenge a scale model of the overall drive system to measure duct losses static and rotating rig tests of full-scale tip burners to determine thrust, efficiency and noise and component tests for the "very rigid" rotor blades.

"The challenge is stabilising the rotor at high speed," says Woodbury. As forward speed increases and lift transfers to the wing, blade angle-of-attack will be controlled to minimise drag while keeping the rotor spinning for stability. "As we go faster, [control] tolerances decrease, but are well within what we can do," he says. Plans call for the X-plane demonstrator to be windtunnel tested in the programme's third phase, and flown in mid-2008.