Miniature UAV

This is part of a larger article, on A History of UAV's.

Another growth field in UAVs are miniature UAVs, ranging from "micro air vehicles (MAVs)" that can be carried by an infantryman to man-portable UAVs that can be carried and launched like an infantry anti-aircraft missile. This chapter outlines developments in miniature UAVs.

MAVs & MESICOPTERs

The notion that small, even very small, UAVs might have practical uses arose in the early 1990s. In 1992, DARPA conducted a workshop titled "Future Technology-Driven Revolutions In Military Operations". One of the topics in the workshop was "mobile microrobots". The idea of using very small "microdrones" was discussed, and after initial skepticism the idea started to gain momentum.

The RAND Corporation released a paper on the microdrone concept in 1994 that was widely circulated. DARPA conducted a series of "paper studies" and workshops on the concept in 1995 and 1996, leading to early engineering studies by the Lincoln Laboratories at the Massachusetts Institute of Technology (MIT), and the U.S. Naval Research Laboratory (NRL) in Washington DC.

The studies demonstrated that the concept was feasible. In 1997, DARPA then began a multi-year, $35 million USD development program to develop "micro air vehicles (MAVs)". The MAV project's goals was to develop a microdrone whose largest dimension was no more than 15 centimeters (6 inches); would carry a day-night imager; have an endurance of about two hours; and be very low cost. It would operate with a high degree of autonomy to be used in the squad-level combat environment. MAVs capable of hovering and vertical flight would be used to scout out buildings for urban combat and counter terrorist operations. A MAV could be included in a pilot's survival kit. A downed pilot could use it to keep track of enemy search parties, or relay communications to search and rescue units.

Phase-two MAVs

This phase-one DARPA study ended in 2001, and was followed by a phase-two study that focused on particular vendors with an intent to develop MAVs closer to operational specification. A number of different MAVs were developed as part of these DARPA efforts:

Lockheed Sanders "Microstar": The Lockheed Sanders MicroSTAR series of prototypes. The battery-operated MicroSTAR designs resembled kid's toys. An initial design had a fat teardrop body with stubby cropped-delta wings running along most of the body, along with a single vertical tailplane and a pusher propeller. A later version had winglets instead of the single vertical tailplane, and a nose mounted propeller. The MicroSTAR featured a five-gram navigation system that could be given directions by the ground station, but could also automatically keep on a heading or orbit a target.

CIT, AeroVironment and UCLA "MicroBat" ornithopter: The MicroBat ornithopter from the California Institute of Technology (CalTech), working with AeroVironment and the University of California, Los Angeles. The ornithopter design concept followed experiments conducted in the mid-1990s by Charles Ellington, a zoologist at the University of Cambridge, and his colleagues, in which mechanical analogues of insect wings were tested in a wind tunnel. The group was only interested in studying the biomechanics of insects and was extremely surprised that somebody seemed interested in them. The CalTech / AeroVironment MicroBat ornithopter was test-flown for short distances under battery power. Researchers performing flight tests with the MicroBat said it tended to attract small birds when it ran low on power and fell to the ground. The birds clustered near the floundering ornithopter in what seemed to be a desire to help.

Other research groups also worked on ornithopters. A Georgia Tech group built a rubber-band powered ornithopter and also did research on a chemically-powered "artificial muscle" propulsion system.

Lutronix Corporation "Kolibri" micro-helicopter: The Kolibri micro-helicopter built by Lutronix Corporation of Del Mar, California. The Kolibri (German for "Hummingbird") was larger than the other DARPA MAV prototypes, with a weight of about 300 grams. The Kolibri was built as a cylinder with rotors at one or both ends, using vanes moved through the rotor airflow by piezoelectric actuators for flight control. It was powered by electric motors or a tiny, highly-efficient multi-fuel engine developed by a company named D-STAR.

Micro Craft "SLADF" ducted fan micro-helicopter: The Small Lift Augmented Ducted Fan (SLADF) ducted-fan micro-helicopter, built by Micro Craft of San Diego, California, and Ontario, Canada. The SLADF was a ducted fan helicopter with a diameter of about 15 centimeters (6 inches) and a weight of 1.8 kilograms (4 pounds), with payload. Interestingly, the SLADF did not appear to use a contra-rotating rotor design, using a single rotor with aerodynamic deflection surfaces inside the duct to cancel torque. First flight test of the SLADF was in late 2000. The SLADF could be fitted with an optional wing to provide useful lift to increase loiter time, and also provided additional fuel storage.

AeroVironment "Black Widow" flying-wing: The AeroVironment Black Widow MAV. Developed by a team led by Matt Keenon, the Black Widow was powered by electric motor driving a small propeller in the nose, with a lithium battery permitting about 20 minutes of flight. It carried an off-the-shelf camera chip giving it a color video resolution of 510 by 492 pixels. While the first Black Widow prototype was a flat disk with a single vertical stabilizer and a propeller in the front, it was followed by an improved Black Widow that looked a little like a thin portable CD player with tapered edges and cut-off corners; a propeller in front; and three fins on the back. It did not have autonomous navigation capabilities, and was controlled essentially like a hobbyist's RC airplane.

Subsystems design

Along with the flight prototypes, the DARPA effort considered subsystems design. A useful operational MAV would need a lightweight, highly efficient engine with a power source with high energy density. Electric motors were becoming available that met the requirement, but power sources were more troublesome. Lithium batteries were marginal. New compact fuel cells were in development but weren't expected to be available for several years.

One particularly intriguing option for both propulsion and power was a button-sized silicon microturbine ("jet") engine developed by Dr. Al Epstein at MIT during the 1990s. Silicon was actually a good structural material at such scales, though increasing operating temperature would have dictated use of silicon carbide.

A production device was envisioned as a centrifugal-flow engine about two centimeters across burning natural gas, with a single turbine disk for compression and a single disk for exhaust rotation. The design didn't look much like a conventional turbojet, resembling more a tiny flat cylindrical box with an inlet hole on one side and an exhaust hole on the other. It was expected to have a thrust-to-weight ratio of about 100 -- incredible compared to any "macroscale" engine but a logical consequence of scaling the technology down in size -- and run at about 1.2 million RPM, making bearings a tricky issue. Since it could "spool up" in about a millisecond, it was envisioned as operating in a pulsed mode to conserve fuel and also provide a throttling scheme. A functioning gas turbine was never successfully implemented at this scale after years of development.

Other tricky issues were control systems, since a MAV couldn't be flown like a model airplane and would have to be able to tolerate turbulence and wind gusts, and miniaturizing navigation, communications, and sensor systems, as well as ensuring that they didn't interfere with each other. DARPA specified that the payload would be no more than 15 grams.

Stanford "Mesicopter"

As extreme as MAV specifications were, a team under Ilan Kroo at Stanford University worked on an even more extreme design in the form of a centimeter-wide four-rotor mesicopter using microcircuit fabrication techniques. The work was funded by NASA. Design of such a small aircraft was constrained by the fact that at it such scales, the air becomes a highly viscous medium, or in aerodynamic terms a mesicopter had a low Reynolds number. Basic aerodynamics of the mesicopter were defined by a cycle of computer simulation, followed by tests of model components. The research led to mesicopter rotor designs where the rotor looked much more like the blades of an ordinary room fan than the rotor of a conventional helicopter. Propeller designs did not achieve desired efficiency and the Mesicopter was never able to loft the weight of its own energy source.

MAVs RETHOUGHT

The DARPA MAV effort ended in 2000 and the results of the effort were somewhat negative, demonstrating that a 15 centimeter UAV was simply too small to be useful or even workable, at least over the short run. However, though the size was unrealistic, the basic concept seemed valid even if it was recognized that a larger machine was needed.

DARPA did begin a follow-on effort in the spring of 2002, working with the US Army on a larger ducted fan vehicle as a follow-on to SLADF under the "Organic Air Vehicle (OAV)" program. Allied Aerospace, which had bought out Micro Craft, demonstrated a scaled-up SLADF, while Honeywell performed tests with their own ducted-fan vehicle, named iSTAR. However, neither vehicle seemed particularly promising and the program was cut short.

As an example of how defense research programs tend to disappear and reappear, it was revived as OAV-2 in 2004, with DARPA specifying a diesel-powered ducted-fan vertical-takeoff UAV with a weight of 51 kilograms (112 pounds), including a payload of 10 kilograms; a range of 10 kilometers (6.2 miles); a top speed of 92 km/h (50 knots); the ability to hover in a 37 km/h (20 knots) wind; an endurance of two hours; and a ceiling of 3,350 meters (11,000 ft).

The OAV was be carried, launched, and recovered on a Humvee truck, using a crew of two soldiers, who would be able to get it flying in five minutes. Its sensor systems will be able to provide targeting data to within 10 meters (33 feet) to support non-line-of-sight weapons. The UAV would have autonomous flight capabilities with the ability to maneuver in cluttered terrain using an all-weather obstacle-avoidance system, and DARPA wanted it to have the ability to land and conduct observations from its landing site. Other possibilities were use of the UAV for communications relay, SIGINT, countermeasures, or even armed attack. The Army was interested in the program, but its current status is uncertain. It may have disappeared again; and if so it may reappear once more.

Black Widow "Wasp" and "Hornet"

AeroVironment has also worked on follow-ons to its Black Widow, named the Wasp and the Hornet. The Wasp is a flying wing, with the wing in the form of a rectangle with a slightly swept leading edge. It is propeller driven, with the propeller in front. The Wasp's main improvement over the Black Widow is that the lithium-ion battery and wing structures are one and the same, allowing maximum battery capacity relative to MAV size. The Wasp has a wingspan of 33 centimeters (13 inches) and a weight of grams 210 grams (6 ounces). Like the Black Widow, the Wasp is radio controlled.

In the spring of 2003, AeroVironment performed the first flight of the Hornet, which is similar to the Wasp but has a straight rectangular wing with a slightly greater span of 38 centimeters (15 inches) and, more significantly, is powered by fuel cells. The fuel cells are built into the top of the wing, where they combine oxygen in the ambient air with hydrogen produced internally by the MAV through reaction of a hydride material with water.

The fuel cell system is expected to provide three times the endurance of batteries of comparable weight, though early flights were limited by the tendency of the fuel cells to dry out. DARPA is actually more interested in the battery powered Wasp, but other interested parties in the US defense establishment, particularly the NRL, are very intrigued by fuel cells, and so DARPA is hedging its bets. Ultimately, AeroVironment engineers want to fit their MAVs with an autopilot and a color video camera.

French "Mirador"

The French have done work along similar lines, with the French Ministry of Defense (DGA in its French acronym) sponsoring a flight demonstrator, the Mirador. It was a fixed-wing, propeller-driven aircraft 25 centimeters (9.84 inches) long and was powered by miniature fuel cells that gave it an endurance of about 20 minutes. It was built by the French defense aerospace research agency ONERA, working with the Royal Military Academy of Brussels, and is primarily intended to be a testbed for miniature sensor technologies.

The DGA envisions an operational MAV as about 40 centimeters (16 inches) long, with a weight of less than 1.5 kilograms (3.3 pounds), an endurance of 15 minutes or more, a ceiling of 100 meters (330 feet) and an operating radius of a kilometer (0.6 mile). For the moment, the concept seems strictly experimental.

Future smaller MAVs

The notion of bird-sized or even insect-sized MAVs hasn't disappeared, it's just that it is now seen as a project for a future generation. The DARPA effort did get a lot of people thinking about tiny aircraft. MAVs have attracted a hobbyist and amateur community, somewhat along the lines of the "robot war" competitions that make it onto TV, and yearly competitive events have been conducted. These home-built MAVs are of course relatively unsophisticated, but have demonstrated a great deal of ingenuity. Possibly one of these days somebody is going to come up with an idea that will catch on.

Recent research (as of 2005) includes a model utilizing ground effect at NPS([1]), DelFly at TUDelft, etc. Some also consider using a Reciprocating Chemical Muscle for actuating flapping wing MAVs such as the Entomopter pioneered by Prof. Robert C. Michelson at the Georgia Tech Research Institute).

GUN-LAUNCHED & PARASITE UAVS

MIT "WASP"

The US Army has been interested in developing MAVs that could be deployed as munitions, fired from artillery or unguided rocket launcher pods. A research team at the Massachusetts Institute of Technology (MIT) has developed a prototype artillery-launched UAV. The UAV, named the Wide Area Surveillance Projectile (WASP), no relation to the AeroVironment Wasp, is fired out of a 127 millimeter (5 inch) naval gun.

The MIT group modified a standard illumination flare round to serve as the external case. After firing, the shell popped out six fins to keep it from tumbling. Once the shell was 20 kilometers (12.4 miles) downrange, a parachute popped out the tail to extract the drone. The parachute slowed the drone, which then unfolded into flight configuration. The WASP had a folding vee tail, a folding two-blade propeller up front, and two straight folding wings. The wings were folded into six sections and unfolded into a total span of 94.5 centimeters (3.1 feet). Once unfolded, the right wing was higher on the fuselage than the left, a result of the packaging scheme.

The WASP drone had a flight endurance of fifteen minutes, including ten minutes of powered flight and five minutes of glide. It had a tiny camera in its lower fuselage, and relayed both imagery and its own current GPS coordinates back to the warship or artillery battery that fired it. At least two WASP prototypes were built and tested. After initial announcements of the effort, the whole thing went quiet, but it certainly remains an interested possibility.

"Wing-store UAV" and Raytheon "SilentEyes"

The Army has also worked on a UAV that can be launched out of a 70 millimeter (2.75 inch) unguided rocket pod mounted on a helicopter and could also be carried by larger UAVs. This wing-store UAV is 1.8 meters (6 ft) long, and is fired out of the launch tube with a solid-rocket booster. It then deploys wings, tail, and propeller, and cruises for up to two hours on electric power at a speed of 185 km/h (100 knots). It can carry a small daylight or infrared camera.

Details of the wing-store UAV are unclear, but it may have some resemblance to the Raytheon SilentEyes UAV. SilentEyes looks like a simple metal cylinder with a rounded cap, straight folding wings mounted in the middle of the UAV and with a noticeable dihedral, and a folding inverted-vee tail. The UAV is 46 centimeters (18 inches) long and less than 7 centimeters (2.75 inches) in diameter.

Raytheon calls SilentEyes a "parasite" UAV, as it would be dispensed from a larger UAV, like a Predator; a gliding submunitions dispenser; or a cruise missile. The baseline version of the SilentEyes would be strictly a glider, but its glide ratio of 11:1 would allow it to stay in the air for a half hour if released from typical Predator operational altitudes. It would be used for close-up examination of targets spotted by SAR to ensure that they are valid targets, or for post-strike target damage assessment.

The little UAV can carry a gimbaled infrared or color TV camera, with the video compressed for transmission by a UHF communications link over line-of-sight ranges. It could also carry a jammer payload, or a small warhead. Since multiple SilentEyes would be deployed at the same time, each could be assigned a different code or "telephone number" to minimize confusion in communications.

Raytheon is shooting for a target price of about $5,000 to $10,000 USD. The company is considered a powered version of SilentEyes with a microjet engine, as well as "stretched" versions of the UAV.

Italian "MALP"

Galileo Avionica of Italy is currently working on their own "parasite" UAV, called simply the Miniature Air Launched Payload (MALP), to be carried on a Falco or similar UAV. The MALP has large cruciform tailfins, small cruciform nosefins, and "switchblade" wings stowed back along the fuselage that pop out straight when the UAV is released. It is intended to carry imaging or other sensors to probe dangerous targets.

MAN-PORTABLE UAVs

There is actually a great deal of activity in the small UAV field, with a number of systems now being acquired and some being used in combat.

AeroVironment "Pointer" and "Raven"

In 1999, the US Army bought four AeroVironment Pointer small UAVs for testing in the service's "Military Operations In Urban Terrain" and was enthusiastic about the usefulness of the Pointer. The Pointer system is too large to be conveniently carried by soldiers and is normally hauled around in a Hummer vehicle or the like, and so the Army asked AeroVironment if the company could come up with a more portable solution. AeroVironment agreeably developed a half-sized control system and a cut-down version of the Pointer called the Raven (no relationship to the Flight Refueling Raven).

The Raven has the same configuration and central pod of the Pointer, but a shorter tail and a wing reduced to a 52% span of 1.34 meters (4 feet 5 inches). The Raven has an endurance of 80 minutes. The entire system can be carried by two soldiers. Following the Afghanistan campaign in 2001:2002, the US SOCOM ordered 80 Ravens from AeroVironment, which was more than the total number of Pointers that had been sold to that time. The US Army also placed orders for up to 105 Ravens in the late summer of 2003 after the US occupation of Iraq led to persistent insurgent attacks on US forces. Encouraged by such successes, AeroVironment is also working on a larger version of the Pointer, named the "Puma", with greater endurance and payload.

Lockheed Martin "Desert Hawk"

US forces are also using another mini-UAV in Iraq, the Lockheed Martin Desert Hawk. It weighs 3.2 kilograms (7 pounds), has a wingspan of 1.32 meters (52 inches) and a length of 86.4 centimeters (34 inches). It is made mostly of plastic foam, suggesting something like a Nerf toy, and uses an electric motor driving a pusher propeller as a powerplant, making it very quiet. It is launched with a bungee cord, carries three small CCD cameras, has an endurance of about an hour. It flies mostly under autonomous control, with the "pilot" keeping track of what's going on with a laptop computer.

The Desert Hawk was designed by Lockheed Martin for the Air Force on a quick-reaction contract issued late in the winter of 2002, with the first system delivered in the early summer. It was designed quickly because it leveraged heavily off of technology and design studies developed for the MicroStar MAVs.

NRL "Dragon Eye", "Swallow" and "Finder"

The U.S. Naval Research Laboratory (NRL) has developed a man-portable UAV of roughly the same size as the AeroVironment Raven, named the Dragon Eye (no relationship to the BAI Aerosystems Dragon). The Dragon Eye is a tailless design with a rectangular wing and twin props. It is designed to fit into a backpack, with a weight of 2.25 kilograms (5 pounds) and a span of 1.14 meters (3 feet 9 inches). It can be lanched by hand or bungee slingshot and has a GPS-INS-based waypoint navigation system.

One of the interesting features is that the operator monitors Dragon Eye operation through "video goggles" connected to a laptop computer. The control system weighs about 5.4 kilograms (12 pounds). The Dragon Eye's endurance is an hour. It has been deployed on an operational trial basis with the US Marines. The Marines would like to put it into production and have plans to order 300 of them.

The NRL has also built at least two other small UAVs. The Swallow is of more conventional configuration than the Dragon Eye, roughly comparable to the AeroVironment Pointer, with long sailplane wings and a tail-mounted propeller. Details are unclear, but it has been used in NRL experiments to develop anti-sniper sensors for base security applications.

The Finder (Flight Inserted Detector Expandable for Reconnaissance), with a weight of 26 kilograms (57 pounds), can carry a small imager, or an atmospheric sampling sensor to check for radiological / chemical / biological contaminants, and other sensor payloads are being considered. Other details of the Finder are unclear.

The Finder has been evaluated as a payload for the Predator UAV, with one Finder carried under each wing, acting as a parasite UAV like the Raytheon SilentEyes. Initial flight tests of the Finder with the Predator were performed in the summer of 2002.

International man-portable UAVs

Of course, man-portable UAVs are being developed or are being sold by other countries. French work in this field was mentioned earlier. Several others are shown below:

EADS "Tracker": European EADS organization has designed a small UAV named the Tracker, which features a wide-span wing, twin booms for payload and so on, and a central pod with tractor and pusher propellers. It has a weight of 7.5 kilograms (16.5 pounds), a span of 1.4 meters (4 feet 7 inches), and an endurance of an hour.

China "CATIC": CATIC of China is working on their own hand-launched man-portable UAV, the "ASN-15", with an endurance of an hour and a payload of 6.5 kilograms (14.3 pounds).

Elbit "Skylark" and "Seagull": In the spring of 2003 Elbit of Israel introduced two electrically-powered man-portable UAVs, the Skylark and the Seagull. Both of these UAVs have a launch weight of about 5.5 kilograms (12 pounds), a speed of from 35 to 70 km/h (20 to 40 knots), and can carry either a color daylight imager or an infrared imager. The Skylark is of conventional configuration, resembling nothing so much as a large kid's rubber-band airplane with a pod under the fuselage. It has an endurance of two hours.

The Seagull is much less conventional, in the form of a boomerang-shaped flying wing with wingtip fins and a pusher propeller. Size, performance, and payload details of the Seagull are similar to those of the Skylark, but the endurance is stretched to six hours.

Rafael "SkyLite": Rafael of Israel has built a man-portable UAV also named the SkyLite, which is fired out of a tube like an antitank missile, and has an endurance of about an hour. It can be launched from a vehicle mount or shoulder-launched by a soldier.

The SkyLite has a certain general resemblance to the Raytheon SilentEyes, being a tube a 110 centimeters (3 feet 7 inches) long with a glass sensor nose; a pusher propeller powered by an electric motor; pop-out straight wings with a span of 150 centimeters (4 feet 11 inches); and a cruciform pop-out tail. It has a launch weight of 6 kilograms (13.2 pounds). It was originally named "Skylark" but Rafael decided to change the name to avoid confusion with the Elbit Skylark.

IAI Malet "BirdEye"s and "Mosquito": IAI Malat has also introduced their own small UAV line, designated BirdEye, which includes the 5 kilogram (11 pound) BirdEye 500 and the 500 gram (1.1 pound) BirdEye 100. Sources also mention a Malat micro-UAV, the Mosquito, though this may be the same as the BirdEye 100. Malat has been promoting the BirdEye 500 for both military and civilian uses, with civilian uses including urban security, crime-fighting, and traffic observation.

Baykar Machine Inc. "Bayraktar": Baykar Machine Inc.'s (Turkey) Bayraktar Mini UAV system with 1.2m length and 1.6 m wing span body, can be hand launched and land on its body or wheels. Inverse Vtail, fixed wing platform has 5 kg maximum take off weight, can reach up to 95 km/hour airspeed and has a flight envelope of 10000 feet. Integrated with its own avionics and payload systems, it has an edurance of 1 hour.

References

This article contains material that originally came from the web article Unmanned Aerial Vehicles by Greg Goebel, which exists in the Public Domain.

External links

Mini UAV - Aeronautics Defense Systems