The noise generated by rotor-blade tip jets is proving a challenge for the designers of a vertical take-off and landing aircraft intended to fly at twice the speed of conventional helicopters.
As a result, the US Defense Advanced Research Projects Agency has given Groen Brothers Aviation an extra six months to design a new blade tip to reduce noise before deciding whether to proceed into the next phase of the Heliplane programme.
Designed to fly at speeds up to 350kt (640km/h), the Heliplane is a gyrodyne: the rotor is powered for take-off, hover and landing, but autorotates in forward flight. Compressed air from a pair of turbofans is ducted to the blade tips where it is mixed with fuel and burned to drive the rotor. In forward flight the engines provide thrust and lift is offloaded from the rotor to the wing.
"With the initial design, we probably could meet the 'noise abatement' requirements of a commercial airport because of its flight profile - tip-jet burning turned off shortly after transition from hover to forward flight and a fairly high climb angle," says chief executive David Groen. "However, we did fall short of the programme noise goal."
A decision on proceeding into Phase 2 of the Heliplane programme - windtunnel testing of a full-scale rotor system - was delayed after issues were uncovered during the preliminary design review in September 2007. Instead, DARPA decided to fund an extension to the first phase.
"We have an alternate design that our initial analysis indicates should have a dramatic effect on tip-jet noise. However, completing the preliminary design of the alternative approach will require additional effort, hence the six-month Phase 1B," says Groen. "Our confidence is high that this design will be effective."