Recent unmanned aerial vehicle (UAV) combat successes and US military plans for a multi-billion dollar UAV fleet by 2010 have made composites-intensive UAVs a key growth market for advanced materials. Potential UAV civilian applications include monitoring and controlling vehicle traffic flow, search/rescue operations, surveillance in earthquakes, floods, forest fires, and seaport security for ships. The US Department of Homeland Security is considering using UAVs to patrol the US/Mexico border.
The European military UAV budget is expected to reach around €5.5 billion between 2003 and 2012.
The Predator
The best-known UAV is the US Air Force's Predator, which comes in two versions, from General Atomics Aeronautical Systems. Both are medium-altitude, long-endurance unmanned aerial vehicle systems. The MQ-1's primary mission is interdiction and armed reconnaissance against critical mobile targets. To do this, it carries two laser-guided Hellfire anti-tank missiles. The RQ-1 is used for reconnaissance, surveillance and target acquisition but not armed attack.
The Predator is 8.22 m (27 ft) long, 2.1 m (6.9 ft) high with a wingspan of 14.8 m (48.7 ft). Its four-cylinder engine produces 101 horsepower, which provides a cruising speed of 84 miles/hour and higher speed bursts of up to 135 miles/hour. The craft weighs 512 kg (1130 lbs) empty and has a maximum takeoff weight of 1020 kg (2250 lbs). It can carry a payload of 204 kg (450 lbs).
Despite its small size, the Predator needs 5000 ft by 125 ft (1524 m by 38 m) of hard surface runway. Fuel capacity is 665 lbs (100 US gallons). Each costs about $47 million. The first military use was over Bosnia in 1995.
Other UAVs
Northrop Grumman's Global Hawk high-altitude UAV for the US Air Force can perform surveillance day or night, under any weather conditions. Prototypes have logged more than 3000 hours flight time and made successful flights during Middle East military operations. The Global Hawk is quite large, having a wingspan greater than that of a Boeing 747. More than half the take-off weight of 25 600 lbs is fuel providing a flight time of 34 hours.
Boeing is working on the ScanEagle, a long-endurance, low-cost small surveillance UAV. It is 1.2 m (4 ft) long with a 3 m (10 ft) wingspan platform and weighs 15 kg (33 lbs). Launched by pneumatic catapult, its two-stroke engine can fly the craft for up to 15 hours and has a range of 1500 miles. A new four-stroke engine extends flight time to 60 hours and the range to 5000 miles.
Northrop Grumman's Fire Scout vertical takeoff and landing tactical un-manned helicopter is being developed for the US Navy. The US Coast Guard has selected Bell Helicopter Textron's Eagle Eye unmanned tilt-rotor craft, a direct competitor to the Fire Scout, for surveillance duties. The Eagle Eye has a wingspan of 7.2 m (23.6 ft) and a length of 5.2 m (17 ft).
France has purchased the Predator and calls it the Horus. Both France and Germany used UAVs over Bosnia for reconnaissance. They have jointly funded development of the Brevel. The UK's Phoenix has served in Kosovo and mounts a mission pod under the fuselage.
Some of the leading UAV manufacturers such as Elbit, IAI, Sagem SA, EADS and Dassault Aviation are located in France, Germany and Israel. The European military UAV budget is expected to reach around €5.5 billion between 2003 and 2012. Besides military uses, the growing focus on homeland security is an important revenue driver, as the European Union expands and new reconnaissance needs emerge.
Beyond reconnaissance
Some UAV proponents believe that the current fighter planes under development will be the last to use human pilots. They claim that pilot limitations due to the gravity forces they can endure without losing consciousness (about 9-Gs) and other factors make unmanned combat aerial vehicles (UCAVs) inevitable. While UCAVs are under development, it remains to be seen if they will join UAVs in military use.
Because of their small size and lack of pilot interfaces and training requirements, UCAVs are projected to cost up to 65% less to produce than future manned fighter aircraft and up to 75% less to operate and maintain than current fighters. Boeing's X-45A UCAV demonstrator air vehicle has a tail-less 26.3 ft-long airframe with a 33.8 ft wingspan. Vehicle weight is 8000 lbs (empty) and it can carry a 3000 lb payload. The demonstrator's fuselage length is 32 ft and the wingspan is 47 ft. The X-45B operational UCAV will be slightly larger, incorporate stealth technologies and carry precision guided missiles and bombs.
Unmanned combat aerial vehicles are projected to cost up to 65% less to produce than future manned fighter aircraft.
Northrop Grumman's UCAV candidate is the X-47B Pegasus intended for flight operations from an aircraft carrier. The UCAV measures 27.9 ft long with a wingspan of 27.8 ft. Its operational radius is 1300 miles. Shaped like a kite, Pegasus is built largely with composite materials.
Materials
UAVs are no longer simple and inexpensive. Use of lightweight advanced composites is essential in increasing UAV flight time. Lear Astronics Corp Development Sciences Centre's composite capabilities for the design and fabrication of UAVs include high molecular weight polyethylene, S-glass (magnesia-alumina-silicate glass with high tensile strength), high electrical resistivity glass (E-glass), aramid, quartz, bismaleimide and graphite fibres reinforcing epoxy, polyester, vinyl ester, phenolic, and polyimide resins. Composite processing methods include compression moulding, resin transfer moulding (RTM), prepreg lay-up, wet lay-up and convolute winding with oven or autoclave curing.
These advantages of composites over metals are important in UAVs:
- low weight;
- excellent corrosion resistance;
- high resistance to fatigue;
- reduced machining;
- the ability to fabricate tapered sections and intricate contoured parts;
- the ability to orient reinforcement fibres in the direction of maximum stiffness and strength;
- the reduced number of assemblies and fasteners needed when using co-cure or co-consolidation composite manufacturing processes;
- low radar and microwave absorption of composites, which provides ‘stealth’ capabilities making radar detection difficult; and
- a very low thermal expansion reducing operational problems in high altitude flight.
However, composites also have disadvantages compared to metals:
- higher cost;
- relative lack of established design criteria;
- degradation of structural properties at high temperature or when wet;
- poor energy absorption and resulting impact damage in hard landings;
- the need for lightening strike protection;
- corrosion problems due to poor or incomplete adhesion to metals particularly when using carbon or graphite materials;
- reliable detection of substandard composite-metal bonds is difficult;
- the location of these sub-standard bond locations is often imprecise; and
- the expensive and complicated inspection procedures needed.
For weight reasons, aluminium is the only metal used in UAVs. Use of composites can reduce overall UAV weight by 15-45% depending on the extent of composite use. Above a 50% weight reduction requires improvements in composite economics at the moment. Composites have been used in modest load-bearing components such as elevators, which comprise about 20% of aircraft weight. For further weight reduction, composites must be used in higher load-bearing components such as the tail, wing and fuselage.
Thermosets are used more than thermoplastics because the resin readily impregnates fibres, making it possible to manufacture complex shaped parts. Thermosets provide high strength and high stiffness parts after curing. Epoxies are the most commonly used thermosets in UAVs. They provide good low-temperature (<93°C) properties, high chemical resistance, good fibre adhesion, excellent dimensional stability, good performance under wet conditions and high dielectric properties. Improved epoxies designed for higher-speed parts fabrication, greater toughness and higher use temperatures are under development. Blends of epoxies and thermoplastics have been used to provide increased impact damage resistance. Polyesters, phenolics, bismaleimide and polyimide have also been used. The most commonly used fibres are carbon and graphite. Kevlar and glass are also used. Among the thermoplastics used in UAV construction are polyethylene, polystyrene and polyetheretherketones (PEEK).
Organic fibres offer high strength and low weight and are used more in UAVs than ceramic and metallic fibres. Graphite (>95% carbon) and carbon (93-95% carbon) fibres are the most commonly used. Other organic polymers such as Kevlar are also used. Glass fibre is used occasionally for its low cost and is likely to be more common in civilian than military UAVs due to the former's less rigorous operating conditions.
Typical damage to composites that needs to be detected both after fabrication and after UAV flight are: cracks and delaminations in the skin; debondings between skin and core; and defects in the core (crushing), of which only a small part is visible from the outside. Ultrasonic detection can indicate internal defects. Detection of damage is essential to proper UAV maintenance and long service life.
Specific UAVs
Early UAVs and most prototypes for civilian applications use a simple, fixed-pitch wooden propeller protected with a varnish finish. Wood has a high fatigue strength-to-weight ratio, low material cost and excellent vibration damping properties. It also has a low radar signature making combat reconnaissance harder to detect.
For larger UAVs, operating conditions often require more propeller strength and durability than wood can provide. One solution to this problem is the use of synthetic fibre reinforcement over a laminated wood core. Kevlar and glass fibres with epoxy resin have been used for this, but using carbon semi-stressed fibre-reinforced epoxy resin over laminated wood has become more common. This construction optimizes airfoils and blade shape to provide rapid climb, while maintaining level flight performance, good damage tolerance and long service life.
Completely synthetic composite propellers are beginning to be used because they provide both high performance and durability. These are typically comprised of carbon/glass fabric impregnated with high-temperature epoxy resin. Their pitch may be adjusted before takeoff to provide flexibility for operating in different weather conditions.
Propellers must be resistant to rain erosion; a resistance varnished wood propellers lack. Urethane tapes applied to the propeller leading edges provide a short-term, replaceable solution to this problem while inlaid urethane resin edges provide increased durability and improved aerodynamics. Urethane and epoxy resins may also be applied to the entire propeller. Recently, bonded nickel erosion shields have been used with all-composite propeller blades.
The University of Sydney's UAV Brumby was developed for flight research. The main gear of the under-carriage is a carbon fibre/Kevlar fibre composite. The fuselage is constructed with a sandwich composite of glass fibre/Nomex resin, which provides a lightweight, stiff, strong structure. Originally the wings were foam cores covered with plywood and glass fibre but are now a composite of glass/Nomex resin.
Early prototype versions of the ScanEagle were made largely of aluminium and composite, but the newer versions are constructed of lighter-weight carbon fibre composites. While the Global Hawk's fuselage is mostly aluminium, some components are composites, as are the tail assembly and engine nacelles. Its long wings are built around four I-beam carbon/epoxy spars. The wing leading and trailing edges are Nomex® aramid honeycomb-cored sandwich laminates.
Other nations developing UAVs also rely extensively on composites for UAV construction. For example, Israel's Orlite uses glass, aramid, graphite, reinforced polyester, epoxy and phenolic resins in its UAVs. According to the Aeronautical Development Establishment of the Indian Ministry of Defence, its UAV has a laminated glass/carbon-reinforced fibre airframe.
The first versions of Boeing's X-45A UCAV had an internal aluminium structure with a low-radar-profile carbon composite being used for the eternal skin. Production aircraft are likely to be of an all-composite construction. The wing is built around a lightweight foam matrix core.
To accommodate the high mechanical stresses and thermal loads associated with maneuvering UCAVs at high speed (Mach 12-15), use of metal matrix composites and ceramic/metal composites is likely to become common.
Beyond the battlefield
Ohio State is leading a university consortium working on a US$600 000 a year project to determine how to use UAVs in traffic applications. Ohio transportation officials and university researchers believe that UAVs can monitor road traffic, direct police and emergency vehicles more quickly to accident scenes and hospitals, and control stoplights so traffic flows better. Two firms, GeoData Systems and MLB, have built small UAVs for traffic applications. However, these units are quite small and some models have operational problems in heavy rain. Cost is another concern.
GeoData Systems' UAV drone costs about $150 000 including the ground control station and software. MLBs costs about $50 000.
Insitu's Seascan, on which Boeing's ScanEagle is based, is a commercial ship-based UAV that can be used to monitor crowded sea lanes or look for survivors of ship sinkings.
Dr Peter Corke of Australia's CSIRO Complex Systems Integration says of their Mantis helicopter UAV: “Mantis makes it possible for fleets of small drone helicopters to do jobs now done by conventional aircraft. This could lead to a quantum leap in the speed of air sea rescue efforts – covering many square kilometres faster by having many small aircraft searching at the same time.”
The UAV market and perhaps the UCAV market will grow substantially as will the need for composites to fabricate these useful and versatile aircraft.
