The aircraft, which has been dubbed a heliplane, also must be able to carry a 1,000-pound payload for 1,000 miles before refueling.
Aeronautical engineers have for decades grappled with the challenge of designing rotor craft capable of flying at the speeds achieved by fixed-wing aircraft, Groen said. "And one thing is certain: A helicopter simply cannot be designed to fly that fast."
Try to push a helicopter much above 200 mph and its rotor blades will begin to buckle and warp. Eventually, the blades will disintegrate under the dual pressure of providing lift, while also trying to move it forward through the air at such speeds.
Groen Bros., which has been working for the past two years under a $10.4 million Department of Defense contract to design a rotor system for a heliplane that can hit the 400 mph barrier, believes the solution can be found in its gyroplane technology.
Since its founding in 1986, Groen Bros. has spent nearly $100 million trying to fulfill its founders' vision of filling the world's skies with gyroplanes, which differ from helicopters in that their rotors are not powered by an engine. Instead, motors push the craft forward while freely spinning helicopter-like blades on top to provide lift as air from below rushes up.
Gyroplanes, also known as gyrocopters or autogyros, typically are unable to hover because there is no power going to the rotor. But Groen Bros.' heliplane-rotor design calls for the craft to be engineered so that "tip jets" on the end of the blades, when necessary, can draw power from the jet engines below and allow the craft to hover.
Moving from that concept to the engineering stage, though, has proved formidable. And the task has been made even more daunting because the government's Defense Advance Research Project Agency is requiring the aircraft use existing engine and fuselage designs.
If the heliplane development is successful, the agency wants to use the aircraft for combat search-and-rescue missions.
"We knew from the start of the program that this task would be challenging, but we underestimated the difficulty in achieving 400 mph cruise performance with an existing engine and airframe," said Jan Walker, an agency spokeswoman.
Still, Groen Bros. - which has teamed with the Georgia Institute of Technology for its aerospace engineering expertise, Williams International for its jet engines and Adam Aircraft for its composite airframes - has developed a design that may work, she said.
When the heliplane isn't using its rotors for hovering, takeoff and landings, it will rely on its wings for support, leaving the rotor above to spin freely.
That said, actual construction of a prototype heliplane is several years away, Groen said.
"We're just now completing Phase 1 of our contract that called for us to design the rotor system," he said. "And we should know within the next few months whether we'll be proceeding with Phase II."
If the government agency gives the go-ahead for the next phase, Groen Bros. will take its design from the drawing board to the fabrication stage so a full-sized rotor system can be tested in a NASA wind tunnel at 400 mph.
There is some uncertainty about whether the go-ahead will be forthcoming.
"Only about one in five projects that [the agency] initially authorizes gets a Phase II," Groen said. "What we do know is that everyone working on this project, including government personnel, really want it to go forward."
The federal government's Walker said the Groen Bros.-led team is working to resolve design issues that were uncovered during a preliminary design review conducted in late September. "Once that work is completed, we [DARPA] will make a decision on the next step for the program."
If the heliplane program is allowed to move forward, Colorado-based Adam Aircraft may be called upon to fabricate the body of the plane using carbon/composite structures and its existing airframe design.
Adam Aircraft is pioneering the development of a generation of business aircraft that it will assemble in a newly constructed facility in Ogden. Its A-500 twin-engine piston aircraft has been certified by the Federal Aviation Administration. The company's A-700 jet was flight tested for the first time in late September.
"We'll be able to build any kind of airframe that is needed," said John Knudsen, who leads Adam Aircraft's government group and also serves as the corporation's general counsel.
Knudsen said it is possible that as the engineering of the heliplane progresses, the airframe design may have to change. But that is one of the beauties of working with composites, he said, because they are easily adaptable to changing design needs.
At Georgia Tech, Professor of Aeronautics Dan Schrage concedes that in order to successfully build a heliplane to government specifications, developers will have to push the technology envelope of rotorcraft design.
The agency "is looking for a technological breakthrough, which is what they like to try and achieve with all their projects," Schrage said.
David Groen is convinced that the technology to make the heliplane a success is within reach. If it is, it will validate his long-held view that gyroplanes should have played a role in the aviation industry instead of disappearing in the late 1940s after Igor Sikorsky developed the first practical helicopter design.
What is Groen Bros. Aviation
* Since 1986, the company has been developing gyroplane technology from its headquarters in Salt Lake City. Groen Bros. sells a kit to build a two-seat gyrocopter known as the SparrowHawk II. It bills that aircraft as ideal for use by law enforcement and for recreational use.
* Groen Bros. employs approximately 100 people.
2 FJ-44 jet engines
Adam Aircraft A-700