DARPA*, the Pentagon warboffinry outfit for whom the only field worthwhile is the left one, is having some problems with its "Heliplane" programme.
The Heliplane is essentially a modified autogyro (aka gyrocopter or gyroplane), an aircraft that gets its lift from a rotor disc outwardly resembling that of a helicopter. However, an autogyro rotor is normally unpowered and spins due to the craft's forward (or downward) motion. The craft is driven forward through the air by normal jets or propellors, getting lift from the spinning rotor as a plane would from its wings.
Heliplane - a tip-jet blast from the past.
A regular autogyro can't lift off vertically or hover, as it needs forward speed to make its rotors spin. It can fly much slower than a plane, though, and descend to land with almost no forward speed. It can also be faster and cheaper and carry more load than a comparable helicopter.
Under DARPA's Heliplane project, begun in 2005, the basic gyrocopter idea gets some additions to suit it for a military "combat search and rescue" (CSAR) or special-ops role. The heliplane's rotors are powered, but they use "tip jets", a grand old whirlybird notion from long ago, rather than a normal turboshaft and gearbox. Fuel and compressed air from the craft's engines are piped along the rotors and burned in jets at the tips, generating thrust to spin the blades.
This can deliver a lot of vertical lift, allowing a substantial load to get airborne off a pad. Then forward thrust from the turbofans kicks in and the plane accelerates. Soon the tip jets can be cut out as the rotors begin to spin in autogyro mode - then as the speed increases further still, lift begins to develop from ordinary stub wings. At 400mph this is just as well, as the rotor lift is suffering from problems with blade airspeeds (the blade going forward is tending to go supersonic, and the one going back is temporarily almost stationary).
All in all, the concept bears more than a passing resemblance to the UK's Fairey Rotodyne, which perished following a loss of government backing in 1962. The Rotodyne could take off and land vertically and fly 700km at 300kph. The V-22 Osprey tiltrotor can beat that handily, but on the other hand it's only just gone into service - 50 years after the Rotodyne's debut flight. DARPA are hoping their Heliplane would be even better.
Enthusiastic gyrocraft manufacturers Groen Brothers have been more than happy to take an initial $6.4m from DARPA and have a go, but the trials have hit a snag. One of the great problems of tip jets is the terrific racket they make, and it seems that the Heliplane is no exception.
Flight International reports today that DARPA have been reluctant to take Heliplane to phase 2.
"We probably could meet the 'noise abatement' requirements of a commercial airport," CEO David Groen told Flight.
All it would take, he said, would be "tip-jet burning turned off shortly after transition from hover... However, we did fall short of the programme noise goal."
DARPA has refused to move forward with the $40m programme until noise is sorted out. Still, they have given Groen Bros a six-month extension on phase 1.
"We have an alternate design that... should have a dramatic effect on tip-jet noise," said Groen. "Our confidence is high."
Meanwhile here's a YouTube vid of the Fairey Rotodyne:
Deja vu strikes. ®
*The Defence Advanced Research Projects Agency.