UAVs are opening new doors for old ideas as developers strive to find the most efficient vertical take-off and landing systems
Science fiction often portends things to come, but in the case of the unmanned air vehicles on view at the recent AUVSI convention in Anaheim, California the creations appeared so far ahead of their time it would have been no surprise to find movie-makers from nearby Hollywood searching for inspiration at the event.
Despite often radical appearances however, the truth is that from insect-like micro-UAVs to futuristic unmanned combat armed rotorcraft (UCAR), there are few - if any - completely new concepts. What is new, and what makes UAVs an increasingly attractive arena for pioneers, is the fresh opportunities they represent for the rebirth and refinement of configurations that have either been tried and abandoned or, on a larger scale, were simply impractical in their day.
Predictably, this applies most in the vertical and short take-off and landing (V/STOL) category, where the limited size and weight of UAVs favours the more outlandish or ambitious concepts. At the lower end of the scale, one of these is the Samara, a stop-rotor hybrid micro-air vehicle (MAV) under development by the US Naval Research Laboratory (NRL) as a possible lightweight platform for intelligence-gathering and radar-jamming sensors. Pictured on video in awkward, stuttering flight around a laboratory, the flimsy vehicle looks more akin to a large insect than the hummingbird that inspired its creator, MAV pioneer David Cylinder, a researcher at the US Department of Energy's Princeton Plasma Physics Laboratory.
Constructed of carbonfibre-reinforced balsa wood and plastic, the vehicle is less than 0.71m (2.33ft) in span and flies using counter-rotating rotors that resemble the winged (or "samara") seed of the maple tree. Similar seed- or feather-shaped wings or rotors inspired the earliest vertical flight concepts, from Leonardo da Vinci's helix to the helicopter of Frenchman Paul Cornu, whose aircraft achieved the first vertical "free flight" in November 1907, but Samara and other MAVs are the first to come close to successfully mimicking the instinctive flight of insects and birds.
Counter-rotating
Weighing around 300g (10.6oz), the Samara lifts off vertically using two single-bladed contra-rotating rotors that pass over one another and align spanwise when stopped to provide lift in forward flight. Despite its small size, the current version of Samara is equipped with two motors - one to power the rotors and one for the pusher propeller. The brushless DC electric lift motor takes over powering the propeller after conversion to forward flight. "The rotors are deliberately out of balance," says NRL MAV principal investigator James Kellogg. "This thing shakes, but at this scale it is OK, and when you get it up to flight RPM it gets above the resonance of the rest of the vehicle, so it is not shaking itself to pieces. It is only something you could get away with on the micro-scale."
The NRL is developing a next-generation Samara that may not have a pusher propeller, although details are still to be determined, says Kellogg. "We are changing its configuration a little bit, which we hope will eliminate some of the problems." Assuming further development is successful, the NRL hopes the Samara will front up a whole family of "perch and stare" MAVs that could ultimately perform various roles for the US Navy and Marine Corps, ranging from carrying sensors for electronic, acoustical, magnetic, nuclear, chemical and motion detection to providing platforms for secure transmitters. Disguised as birds or even insects, Samaras could be sent out on covert surveillance missions, says the NRL.
Backpackable
Another emerging MAV is an unusual family of barrel-like UAVs being studied under a US Defense Advanced Research Projects Agency (DARPA) advanced concepttechnology demonstration. These MAVs are intended as backpackable battlefield reconnaissance tools and are miniature versions of larger relatives under development for possible use with the US Army's Future Combat Systems (FCS) and a wide range of other roles. All the designs rely on ducted propellers to provide thrust and manoeuvrability, including the capability of transitioning to forward flight - all without concerns over torque and at relatively low disc loading.
Again the concept of ducted propellers is not entirely new - the French company Nord (later part of Aerospatiale), for example, designed a VTOL aircraft powered by two large five-bladed ducted propellers driven by Allison T63-5A engines. The duct exit contained four control vanes in a diamond shape which controlled pitch (collectively) and yaw (differentially) in an attempt to expand the airflow for vertical lift and compress it for horizontal flight. Although it made a tethered hover flight in July 1968, the Nord 500 Cadet was cancelled, with no further tests taking place.
Today's ducted fan concepts include some that use the "ring-wing" lift of the fan duct to transition to horizontal flight, others that have wings to augment the lift from the duct, and distant descendants of the Nord 500 that support a fuselage or payload platform between paired ducted fans. All bring to bear advanced fly-by-wire stability augmentation and control systems and improved drive-train and gearbox technologies that make manned, remote and autonomous control feasible.
Virginia-based Allied Aerospace is in the forefront of ducted fan VTOL UAV development, having successfully demonstrated fully autonomous missions using its patented iSTAR (intelligence, surveillance, target acquisition and reconnaissance) vehicle. The lift-augmented ducted fan design recently undertook a series of trials at the Space and Naval Warfare Systems Center Robotics Testing Range in Point Loma, California, and at the Camp Pendleton Marine base in the same state. The 0.73m-diameter vehicle, which lands on its own landing support ring, needs no launch or recovery equipment and demonstrated completely autonomous flights that included hovering pauses, as well as "unattended ground sensor" missions in which it landed, shut its engine down, surveyed the area with a video camera, restarted its engine, took off and returned to base.
Honeywell, working with Allied Aerospace, is developing a family of iSTAR-related ducted-fan UAVs to meet the varied size requirements of an organic air vehicle for the FCS. In co-operation with DARPA, a related organic adverse-weather air vehicle programme is also under way to develop ducted-fan UAVs that can provide FCS direct and indirect weapons targeting in all weather conditions.
Similar concepts are being explored by BAE Systems, as well as Virginia-based Aurora Flight Sciences, which is preparing to start flight tests of the GoldenEye-50, a smaller variant of the winged, ducted fan GoldenEye-100. Originally developed under DARPA's Clandestine UAV programme, the larger GoldenEye variant can carry payloads of up to 10kg (22lb), and has a gross take-off weight of up to 68kg. Unlike the alternative ducted fan designs, the GoldenEye concept includes torsionally disconnected wings that provide lift and enhanced stability in horizontal flight mode, without interfering in vertical flight operations.
Bond-themed
Perhaps the most extraordinary VTOL concepts on display at AUVSI - at least visually - were the Trek Aerospace ducted fan-powered Springtail EFV-4 and Dragonfly UMR-1. Although both are capable of being manned, the two vehicles are also being developed for autonomous operation as UAVs. Though every bit as James Bond-themed as the GoldenEye UAV, the dual ducted-fan designs are rooted in the dim past somewhere between the early Focke-Achgelis Fa-269 and LePage V/STOL tiltrotor studies of the Second World War and the Nord 500. The EFV-4 and UMR-1 use identical drive-train, gearbox and engine designs originally developed under a 38-month DARPA "core technology" research effort completed in February 2004.
Flight tests of the EFV-4, which achieved its first free hover and forward flight in October and November 2003, respectively, were expected to resume at Trek Aerospace's Los Altos, California base as Flight International went to press. The craft is fitted with an improved 118hp (88kW) rotary diesel engine and additional fan cowl vanes for improved roll control, and the company is optimistic it will have "a better power curve response, and better control - we are also bidding for a number of US military roles and requirements".
The US Navy and Special Forces Command are said to be interested in both the EFV-4 and the UMR-1, and Trek hopes to fly the UMR-1 for the first time by the end of the year. Special Operations has expressed interest in deploying the UMR-1 by parafoil from a Lockheed Martin C-130. On touching down, the vehicle would be able to conduct manned or autonomous operations, providing the capability to transport a payload of up to 200kg across distances of more than 1,000km (540nm). With a predicted rate of climb of up to 5,500ft/min (27.9m/s), a maximum speed of 200kt and a hover ceiling up to 12,900ft, Trek believes the UMR-1 provides the potential for attack helicopter-like performance for tactical special-operations transport requirements.
Traditional tiltrotor
A more familiar tiltrotor VTOL UAV is Bell's Eagle Eye project, recently boosted by the formation of Team Eagle Eye, including Lockheed Martin, AAI and Textron Systems. Aimed initially at the US Coast Guard's Deepwater programme, the twin-propeller UAV clearly takes its heritage from Bell's XV-3, XV-15 and later Bell-Boeing V-22 Osprey. Production of the first three full-scale Eagle Eye air vehicles is scheduled to begin later this year, with flight tests starting in early 2006.
Bell believes the UAV, fitted with a larger 3.05m-diameter rotor and a dynamic systems upgrade, will also be suitable for shipboard use aboard the US Navy's littoral combat ship and forthcoming DDX destroyer, as well as in possible international markets. The Eagle Eye is also likely to be offered to the US Marine Corps, which is expected to issue a formal request for proposals for a Pioneer UAV replacement by the end of 2004.
Another advanced concept - seen only in minute model form on the Boeing stand by the sharpest-eyed of attendees - is the X-50 canard rotor/wing (CRW). Although the project is behind schedule after a crash during hover tests this year, Boeing still fervently believes it is on the threshold of the long-awaited leap in efficiency promised by the CRW concept, which theoretically combines the best of rotary and fixed-wing flight. "To us, this is the breakthrough in rotorcraft," says Don Wolfinger, director of advanced ground vehicles, who was involved in the earliest days of the DARPA supported effort.
With test flights due to restart in the next few months, the breakthrough could be in sight thanks to two significant differences between the X-50 and all previous stoppable rotor/wing designs. Everything from the pioneering Herrick HV-2A convertiplane in the 1930s to the more recent X-Wing concept used the rotor/wing to provide lift at all times.
The X-50 is designed to unload the rotor/wing during conversion, with the vehicle's lift provided by the canard foreplane and the tailplane. "That makes it easier to maintain positive control during conversion," says Wolfinger.
This transition is vital to the concept in which the rotor/wing acts in helicopter mode as a two-blade rotor, and the aircraft mode in which the rotor/wing is locked perpendicular to the fuselage. "What also makes the CRW different is it has got a simpler, cooler reaction drive which uses a turbofan [rather than a turbojet] and is therefore more practical," he adds.
Previous reaction-drive designs, such as the Hughes XV-9A "hot cycle" research helicopter of 1964, required special alloys in the ducting such as Inconel to handle the extremely hot gas temperature. These features should help Boeing outdo the abortive X-Wing, which "required a number of miracles to happen in quick succession for it to really work", says Wolfinger, referring to several challenges, including a convertible fan-shaft engine, a ballistic pull-up conversion mode and a complex array of mechanical vales to control circulation around the fixed rotor.
So it seems UAVs offer the newest and perhaps best opportunity for frustrated innovators yearning to close the seemingly unbridgeable efficiency gap between helicopters and their fixed-wing brethren.
GUY NORRIS / LOS ANGELES
Source: Flight International
