Variable blade-incidence rotorlifts commercial gyroplane

Salt Lake City--Ever try to design something without legacy data on which to base it? That was the challenge facing Mark Woolsey, chief engineer for Groen Brothers Aviation, in developing the company's Hawk 4 commercial gyroplane. Add to that a two-year deadline to first flight (last year) and you can see the need for efficient design tools for his small design team.

While gyroplanes, or autogyros, have been around since the late 1920's, their development stopped during WWII. Since then, development has mostly been in the homebuilt, sport aircraft market. Most rotor data available related to helicopters, and specific autogyro data from the '30s were for less efficient blade structural configurations than have been developed since. Woolsey and his team used what existing data they could, combining it with that derived from fundamental principles and empirical information from the company's three previous smaller prototype designs flown over the last ten years.

Aerodynamics. The patented Groen Hawk 4 rotor is unique in that its blade angle can be trimmed at any value between zero and its maximum. Previous autogyro designs only had two-position blades, minimum and maximum. During takeoff, the rotor is first brought up to speed, and the small transmission disengaged. The aircraft then rolls forward and the rotor is "popped" to the lifting angle in a jump take off. With a variable angle setting, the pilot has more control of rotor speed and energy, and can tailor flight to a given aircraft weight and desired takeoff dynamics. Takeoffs can even be done vertically with no ground roll, and a landing run shorter than 10 ft is possible.

The design team used the variable blade-incidence rotor for takeoff and lift versatility along with a proprietary blade profile. The latter "optimizes lift and minimizes drag in the range of lift coefficients flown," says Woolsey. "It is not a laminar flow section but it behaves well if dirty, say with bugs." The resulting rotor has low and smooth-transitioning control forces, he adds.

Gyroplane explained. While it looks like a helicopter, a gyroplane is simpler and therefore more economical in price and operation. A helicopter has a rotor linked by a transmission to its engine. The rotor furnishes lift and propels the vehicle by changing the blade pitch angle to the air (collectively for lift) and tilting the lift plane of the rotor (varying blade angle cyclically as the rotor turns).

A gyroplane rotor is not powered--it turns freely, generating lift. Thrust from the propeller overcomes the drag generated by the lift-producing rotor. Since the rotor is free turning, no anti-torque rotor is needed as on a helicopter. Once the small trans- mission spins the rotor for takeoff and is disengaged for flight, the aircraft relies on airflow to keep the lifting rotor spinning. Not having a power-transferring transmission and tail rotor simplifies the design.

By not having a loaded rotor, a gyroplane is always in "autorotation" mode should an engine fail, says Woolsey. If an engine quits, there is a short time to disengage the rotor from the engine, and change the collective pitch, before it slows too much--allowing a controlled, autorotating touchdown or flight on a second engine. Woolsey adds that by not having to change rotor collective pitch constantly, "A gyroplane is easier to fly, more like a fixed-wing plane."

Tools of the trade. Groen engineers consolidated the design information they developed into the resulting Hawk 4 configuration via SolidWorks software from Dassault (Concord, MA), generating a virtual 3D mockup and engineering drawings. And designers were also able to check for interferences, by, for example, putting control inputs into the rotor system. The team only made a physical mockup of the cockpit to check ergonomics.

Another software key was COSMOS from Structural Research & Analysis Corp. (SRAC, Los Angeles) for structural finite element (FE) analysis. Woolsey says the package is powerful for what they need, yet simpler to use than closed-form solutions, a helpful feature since the Hawk 4 needed to be certificated to more stringent, updated FAA regulations. For instance, cabin structure must now withstand a 12g vertical impact, while the seat restraints within the cabin must hold up to 16g. He adds, "All FE results were checked with closed-form methods to verify the analysis."

Structure. Material-wise, the Hawk 4 is mostly aluminum and steel. "Only non-structural features, such as the nose, canopy frame, and engine cowling are made of composites," says Woolsey. "It is easier to certificate conventional structures to FAA requirements, and less testing and quality assurance are needed than for composites. We wanted to use conventional materials and methods for ease of certification to existing, accepted standards."

Thus, within a year, gyroplanes may become more numerous in the skies.

For more information on the Hawk 4 Gyroplane, Contact Hank Parry, Groen Brothers Aviation, 2640 W. California Ave., Suite A, Salt Lake City, UT 84104; Tel: (801) 973-0177; Fax: (401) 973-4027; Internet: www.gbagyros.com.