The Salt Lake Tribune has just run a profile of local gyroplane developer Groen Brothers Aviation. It contains a brief reference to “design issues” uncovered in September during the preliminary design review for the Heliplane high-speed rotorcraft demonstrator GBA is designing for DARPA.
So I contacted DARPA, which said: “We underestimated the difficulty in achieving 400mph cruise performance with an existing engine and airframe. Nobody has ever flown a rotorcraft at 400mph.” Understatement – the fastest a rotor has flown sideways is 249.1mph, attached to a Westland Lynx in 1986.
GBA calls the Heliplane a “gyrodyne”: it takes off and lands like a helicopter using a tipjet-driven rotor, but cruises like an autogyro, with the rotor unpowered and thrust provided by a pair of turbofans. Gyrodynes are not new, but pushing one to 400mph is – that’s twice the speed McDonnell’s XV-1 Convertiplane tipjet compound autogyro achieved in 1956.
Designing a reaction-drive rotor system that can produce sufficient lift, generate minimum drag, carry the loads and be stable at high speed has proved to be a challenge. DARPA says GBA is working to resolve “a few remaining design issues”, then it will decide whether to proceed to full-scale windtunnel testing of the rotor system.