Flying car ambitions lead to development of modular VTOL cargo UAV
What was once a Pentagon research program to demonstrate a flying jeep has been given a new name and a new direction. Formerly called Transformer, the Defense Advanced Research Project Agency's ( ) rechristened Aerial Reconfigurable Embedded Systems (ARES) program will now fly a modular, unmanned vertical-takeoff-and-landing (VTOL) delivery system.
While a setback for flying-car advocates, it is not unusual for Darpa to stop or rejig a program when the original idea is not working out. In this case, the original idea was to develop a highly automated military vehicle able to fly four people from ship to shore then drive on and off road, taking to the air when necessary to avoid ambushes or roadside bombs.
Under the original Transformer program, a/Piasecki Aircraft team was selected over AAI in 2012 to build a prototype fly/drive vehicle. Their winning design combined a manned vehicle with an unmanned ducted-fan flight module that could detach and operate independently.
Darpa reviewed the program early in 2013 and stopped work on the ground-vehicle portion to focus on demonstrating the flight module as a remotely piloted aircraft. “They took a relook at the flying-car concept,” says Kevin Renshaw, Lockheed Martin Skunk Works program manager. “They were not getting a great response [from the services], but they liked the modular VTOL UAV part.”
ARES is seen as a follow-on to the Lockheed/Kaman K-Max unmanned helicopter now being used operationally in Afghanistan to resupply U.S.forward operating bases. Based on the K-Max's success, the Marine Corps has plans for a cargo UAV program of record and the Army and Navy also have shown interest in unmanned resupply.
In January, Lockheed and Piasecki began work under Phase 3 of the ARES program, which will culminate in flight tests of the ducted-fan cargo UAV. First flight is planned for mid-2015, says Renshaw. Piasecki is building the flight module and its rotating machinery; Lockheed is team lead and responsible for the flight-control software.
For the prototype, the team has stayed with the flight module design developed for the Transformer. The tailless air vehicle has a pair of tilting ducted fans attached to a central wing section and outboard wing panels that tilt with the fans and fold against the ducts when stowed. The detachable payload module fits under the center section, between tall landing skids.
The thick center section houses two helicopter turboshafts that drive the fans mechanically via a combined gearbox and cross-shaft. Flight control is provided by the constant-speed, variable-pitch fans and movable vanes in the duct exhausts. A control surface at the trailing edge of the center section provides long-term steady-state pitch trim, says Renshaw.
The original Transformer requirements for a roadable aircraft limited the size of the flight module to 8.5 ft. wide and 30 ft. long when stowed on top of the vehicle in ground mode. The full-size ARES prototype will be the same size, with a span of 42 ft. with the wings unfolded. The ducts are 8.5 ft. in diameter, enclosing 7.5-ft.-dia. fans.
While a cargo UAV seems much less “Darpa hard” than a flying car, Renshaw says, the challenge is in developing the control system, in which the same effectors produce different results in vertical, transition and forward flight. In the hover, the duct vanes provide yaw control; in forward flight they provide pitch and roll.
The ARES fly-by-wire system employs the “dynamic inversion” control-law technique used on the short-takeoff-and-vertical-landingJoint Strike Fighter to blend vertical and forward flight control. “The pilot commands up, down, left or right and the flight-control system works it out,” he says.
Wind-tunnel tests to collect data on control effectiveness were conducted last October on a 1/3-scale half-span model of the flight module, with tilting duct, electrically driven fan and variable-pitch blades. “We pick up significant lift on the duct from air being pulled through by the fan,” Renshaw says. “We are still tweaking the duct design and will continue to use the model in the wind tunnel to tune the flight controls.”
The ARES prototype will be completed this year for initial ground tests at Piasecki early in 2015. “We will use the flight vehicle as a test stand,” he says. Flight tests on a government range are planned to begin in June/July 2015. For its graduation demo, the vehicle will take off vertically with payload pod attached, fly to another location, land vertically, disconnect the payload and fly back as just the flight module.
Advantages of a ducted-fan configuration over an unmanned helicopter like the K-Max include its compactness, performance and modularity, Renshaw says. The ARES can use smaller landing areas, while the ducts protect the vehicle and personnel. Speed ranges from 130-150 kt. for best fuel efficiency to a maximum of about 200 kt. “It's faster than a conventional helicopter with a sling load, which is the correct comparison,” he says.
An operational vehicle would have a gross weight in the 7,000-lb. class, about half of which would be available as payload. As an alternative to a cargo pod, the vehicle could carry an intelligence, surveillance and reconnaissance payload, a special-operations dune buggy or even a casualty-evacuation module.
Because it was originally intended for a manned vehicle, the flight module has redundant engines and hydraulics and triplex flight controls. “We do not expect this to be man-rated out of the box,” says Renshaw. “It is a show-me thing. We have to prove it works, it is reliable and it is safe. But we will show the core capability is there.”
AAI's design for the Transformer program was a 7,500-lb. vehicle with an unpowered rotor for VTOL, fold-out wing for cruise and ducted fan for propulsion. A single turboshaft engine powered electric wheel motors on the ground, spun up the rotor for a “jump” takeoff and drove the ducted fan in flight. Although AAI stopped work on the design after losing the Transformer program, its partner Carter Aviation Technologies has continued to flight-test the stopped rotor/compound (SR/C) configuration on which it was based.
Carter is now seekingapproval to demonstrate its SR/C prototype to potential customers as it designs a sea-based unmanned variant under Phase 1 of Darpa's Tactically Exploited Reconnaissance Node (TERN) program to demonstrate a Predator-class medium-altitude long-endurance UAV capable of operating from small ships.
The SR/C is a combination of autogyro and compound helicopter, with a rotor for vertical lift, a wing for forward flight and a propeller for propulsion. The unpowered rotor is slowed in flight to reduce drag and allow higher speed than a conventional helicopter. Although it cannot hover, energy stored in the autorotating high-inertia rotor allows a “zero-roll” landing.
In forward flight, the SR/C offloads almost all lift from the rotor 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 about six for a conventional helicopter So far, Carter's piston-powered 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.
Carter has asked 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, in a bid to drum up interest in the SR/C concept. “If we can raise the money, we plan to perform a shipboard recovery demonstration for Darpa,” Carter says, in support of the TERN program.
Under a previous Darpa program tied to Transformer, Aurora demonstrated an electrically driven ducted lift fan. The fan was driven by a ring motor and, rather than vary blade pitch as in the ARES, the design modulated thrust by controlling fan speed.'s unmanned rotor-blown wing concept for VTOL X-plane is a tailsitter with dual prop-rotors, but 's design has embedded ducted rotors and tilting ducted fans.
Lockheed Martin, meanwhile, is doing its own studies of where demonstration of the ARES modular VTOL delivery system could lead. “None of the services have invested yet; they want to see the demo. But everyone understands the modular idea,” says Renshaw. “We are talking to the Marine Corps, Army and special-operations forces. The next big step for Darpa and industry is to identify a transition partner.”