Unmanned aerial vehicles spur composites use

Unmanned aerial vehicles
spur composites use
Unmanned aerial vehicles or UAVs are of growing interest to military operations,
but they can also be used in a variety of civilian applications such as monitoring
and controlling traffic flow and search/rescue operations. John K. Borchardt reviews
some of the latest composites-intensive UAV projects.
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 antitank
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
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The Global Hawk.
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
0034-3617/04 ©2004 Elsevier Ltd. All rights reserved.

engine extends flight time to 60 hours
and the range to 5000 miles.
Northrop Grumman’s Fire Scout vertical
takeoff and landing tactical unmanned
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 (magnesiaalumina-silicate
glass with high tensile
strength), high electrical resistivity glass
Unmanned aerial vehicles spur composites use
The US Air Force’s Predator. (Picture courtesy of
US Air Force.)
(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 cocure
or co-consolidation composite
manufacturing processes;
Boeing’s X-45 A unmanned combat aerial vehicle is intended to replace conventionally piloted fighter
planes. This is the ‘first generation’ demonstration version of the aircraft. (Picture courtesy of Boeing
Corp.)
April 2004 REINFORCEDplastics
29

Unmanned aerial vehicles spur composites use
The Franco-German Brevel UAV is
manufactured by GIE Eurodrone. The dark
circle on the nose is the location of the
reconnaissance instrument package. (Picture
courtesy of GIE Eurodrone.)
· 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.
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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
(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 semistressed
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 undercarriage
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 Establish-ment 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.
Unmanned aerial vehicles spur composites use
AAI Corp’s Shadow can carry a payload of 50 lbs, which is considerably less than some existing small
UAVs can carry. However, it can carry the