Carter Aviation Technologies is seeking FAA approval to demonstrate its slowed-rotor/compound (SR/C) prototype as it designs a sea-based, unmanned variant of the concept for the Defense Advanced Research Projects Agency (Darpa).

The SR/C is a combination of autogyro and compound helicopter, with a propeller for propulsion, rotor for vertical lift and wing for forward flight. The unpowered rotor is slowed in flight to reduce drag and allow higher speed than a conventional helicopter.

Carter is working on conceptual design of an SR/C unmanned aircraft system under Phase 1 of Darpa’s Tactically Exploited Reconnaissance Node (TERN) program to develop a Predator-class medium-altitude long-endurance UAS capable of operating from small ships.

The company previously worked with AAI on a SR/C design for Darpa’s Transformer “flying-jeep” program, which was won by Lockheed Martin and Piasecki Aircraft with a ducted-fan design. Carter also bid unsuccessfully for the agency’s VTOL X-Plane program.

Forward speed of a conventional helicopter is limited to prevent the advancing blade going supersonic and retreating blade stalling. A compound helicopter offloads lift from the rotor to a wing to delay stall and allow higher speed.

In forward flight, the SR/C offloads almost all lift to an efficient sailplane-like wing and slows the windmilling rotor to where its drag is minimal. “We’ve achieved a lift-to-drag ratio of 15,” says designer Jay Carter, compared with around 6 for a conventional helicopter.

So far, Carter’s prototype has reached a speed of 175 kt., slowed the rotor to 105 rpm and achieved an advance ratio (airspeed divided by tip speed) of 1.13. Powered by a 325-hp. piston engine, the 4,000-lb. aircraft is capable of exceeding 190 kt., Carter says.

Flight testing began in 2011, but was slowed by issues developing software to automatically control rotor rpm, critical to maintaining stability a low rotor speeds. “It was much more difficult than we thought,” Carter says.

Rpm of the autorotating rotor is controlled by tilting the rotor mast – tilt it back, more air flows through the disk and the rotor speeds up, and vice versa. “The issue we had was with the pilot’s ability to hold altitude or constant rate of climb,” he says.

“We installed an autopilot to free up the pilot,” he says, adding the automatic controller holds a pre-programmed rpm versus airspeed. The pilot can “beep” the rotor speed down in 5-rpm steps to cut drag and increase airspeed.

“When we slow the rotor, the aircraft becomes ultra-smooth, like a fixed-wing aircraft. There is not much lift or drag on the rotor and the tall, soft mast is like an airbag. If the rotor moves, you don’t feel it in the airframe,” he says.

Carter is to petition the FAA to change the prototype’s certification to demonstration, from research and development, so it can tour the U.S. this year, including visiting military bases. An international tour could follow if the aircraft is re-engined with a turbine.

“If we can raise the funding required, we plan to perform a shipboard recovery demonstration for Darpa,” Carter says. This private-venture demo would use the existing prototype in support of the TERN program.

“We want to demonstrate this technology, and make it available to other manufacturers,” he says, adding, “We would be their fast-build prototype shop.” Carter was building two aircraft for AAI, but the company ended its license after losing the Transformer program.

The SR/C can make vertical “jump” takeoffs by first spinning up the rotor and “zero-roll” landings using energy stored in the high-inertia rotor, but it cannot hover. The CH-45X concept, proposed for the VTOL X-Plane, has a gearbox and tailrotor providing full hover capability.