POWER PLANTS
NEW ENGINE TECHNOLOGY
KEY TO LIGHTWEIGHT
LONG ENDURANCE
The requirement for new long-endurance UAVs, the conversion of kerosene
engines to heavy fuels and the development of electric power-plants for mini and
micro UAVs are some of the key technical challenges facing UAV engine designers.
The ten-second, Mach 7 burst of Scramjet-powered flight,
by NASA’s X-43A on March 27 2004, across the Pacific sky
demonstrated to the world the importance of UAVs in
developing next-generation propulsion technology.
But the technological questions which the X-43A’s
engine’s developers seek to answer are, in principle, the
same as those facing every UAV engine manufacturer: how
can we extract the most efficient combustion process from
the fuels currently available? The X-43A may have grabbed
the headlines for speed, but elsewhere, more modestly
powered UAVs are breaking records for endurance,
efficiency and low costs.
The X-43A’s engine works by burning fuel (hydrogen) in
a stream of air compressed by the forward speed of the
aircraft. As the air enters the intake at supersonic speeds it
is compressed and ignited with the hydrogen – leading to
a rapid expansion of hot air out of the exhaust nozzle to
produce thrust.
At first sight the Wankel rotary and pusher engines, the
simple two-stroke piston power plants driving wooden
propellers, which power many current UAVs, seem several
aircraft generations distant from the X-43A. But by using
these raw materials, some UAV manufacturers are winning
the race to develop a new generation of efficient low-cost
engine, airframe and payload combinations – thereby
exploiting a host of new civil and military markets.
One of the first of these remarkable new low-cost,
long-endurance lightweight UAVs is the Australian
Aerosonde, powered by a 0.75 kW (1.0 hp) ES & S Enya
R120, single-cylinder, air-cooled, four-stroke piston engine
in the Mark One version and a 1.0 kW (1.3 hp) Aerosonde
powerplant in Marks Two and Three. In August 1998,
Aerosonde “Laima” crossed 3,270 km of the Atlantic in 26
hours 45 minutes on a gallon and a half (7 litres) of fuel.
Since then, Aerosonde has entered a three-year agreement
with NASA to see whether such UAVs could fill the gap
between satellites and surface networks in an integrated
global observing system.
Meanwhile Boeing and InSitu Group are working on
the ScanEagle B, a development of the long-endurance
ScanEagle/Seascan piston-engine-powered range of UAVs
being co-produced with Insitu, and which has been aimed
primarily at the maritime market. ScanEagle B will be fitted
with a newly-developed four-stroke engine that will allow
it to fly for three days and approximately 4,344 n miles
(8.046 km); military, homeland security and other
commercial arenas are also being targeted. In August
2003, ScanEagle A completed a 15.2-hour flight at the
Boeing Boardman test range – now the team is working on
variants with 40 hours of endurance and more.
The key technical challenges for UAV engine designers
are to obtain the most efficient power-to-weight ratio and
maximise fuel consumption figures – while keeping
reliability rates high and maintenance costs low.
Using small, adapted piston-engine designs for longendurance UAV concepts is not new: the US National
Science Foundation in the early 1990s funded the Pegasus
range of proof-of-concept UAVs featuring a BombardierRotax gasoline engine fitted with its own liquid oxygen
supply, allowing it to maintain sea-level power at high
altitudes. Later versions featured three stages of
turbocharging for high-altitude operations. BombardierRotax now builds some of the most ubiquitous ranges of
small UAV piston engines – the 912/914 air/liquid-cooled,
horizontally opposed four-cylinder, four-stroke powerplants drive the I-Gnat, Perseus, Predator, Raptor
Demonstrator Theseus, Altus, Hermes 1500 and Heron
UAVs. Meanwhile the 582/586 in-line water-cooled two-
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POWER PLANTS
Pointer. AeroVironment. USA
Aerosonde. Aerosonde Robotic Aircraft. Australia
cylinder two-stroke engines power the Gnat 750, ARCH50, Prowler II, Sperwer and Ugglan.
The Bombardier-Rotax 582 features a four-blades
composite-wood 100cm propeller, made by the French
company Hélices Halter; this company claims an 85%
market share of the UAV propeller market, from all types of
structures: composite-wood, metal and all-composite.
Hélices Halter not only produce a standard range of
propellers, but has also developed a unique method to
design and produce the optimum propeller for a given UAV
with clearly defined performance characteristics.
Limbach’s L 275E, which powered the EADS DCS Fox,
runs on a petrol/oil mixture (Avgas 100 LL or 90 RON fuel).
The company also produced engines for IAI’s Seeker 1,
Searcher 1 – the latter is powered by a flat-four and threeblade propeller – and other UAVs.
So small, piston-engined, long-range UAVs may have
already demonstrated significant range and payload
capabilities, but the challenge facing many UAV engine
manufacturers is to generate the most efficient
combustion regime for long-endurance UAVs from heavy
kerosene-type fuels such as Jet A, Jet A-1, JP8, JP5, AVTUR
(the military variant) and even diesel – the most common
fuel types stored and used by military operators (See
“UAVs – A Vision of the Future”, Euro UVS 2003, Page
151).
“We can now achieve a combustion efficiency using
heavy fuels to within 5% of gasoline,” according to Peter
Hooper of Bernard Hooper Engineering Ltd, supplier of
UAV engine systems to the UK research agency QinetiQ.
Under a UK Ministry of Defence contract the company has
developed the SPV580 90-degree V4 “stepped piston”
UAV engine operating with two banks of paired crosscharging cylinders – operating essentially as a two-cylinder
engine but without the traditional durability problems of
conventional two-cylinder units. “The engine has also
demonstrated the feasibility of operation on heavy fuel
(kerosene), achieving power output within 5-10% of
gasoline levels. Further kerosene fuel consumption
benefits could be realised with direct fuel injection,”
according to Hooper.
According to Tony Fitzgerald of Orbital Engine
Corporation Ltd, speaking at the June 2003 Euro UVS Paris
conference, by using an air-assist direct injection system
heavy fuel can be successfully used in gasoline engines.
“Fuel consumption and emission benefits associated with
direct injected gasoline engines are maintained; an average
of 30% fuel consumption improvement is possible which
rises to 70% if operation is biased to the light load areas
of the duty cycle,” he says.
Autoflug’s solution is to fit a smart computerised fuel
gauge transmitter, which automatically computes the fuel
density and determines the fuel-type being used; fuel mass
flow is altered accordingly.
“The fuel mass information of the smart transmitter,
together with the no-moving parts design, will enhance
UAV mission reliability and will allow range extensions,
avoiding UAV loss or mission interruption caused by
blockage of the float-type sensors generally being used on
UAVs,” said the company’s Hans Dietrich Schnell, speaking
at the same Euro UVS symposium.
Sonex has developed a patented SCS starting system
and modified combustion chamber design to convert the
spark-ignited (SI), two-stroke, gasoline engine to start and
operate on JP-5/JP-8 standard military fuels. The SCS
modification comprises redesigned cylinder heads and
special combustion chamber inserts housing the
proprietary technology. Under a 1998 demonstration
contract with the United States Marine Corps (USMC)
Sonex delivered five SI 100cc single cylinder two-stroke
gasoline UAV engines converted from gasoline to heavy
fuel operation for use in the DragonDrone UAV. Following
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POWER PLANTS
EagleEye. Bell Helicopter Textron. USA
Aladin. EMT. Germany
the initial demonstration, Sonex converted an additional
40 of the 100cc gasoline engines, used in the DragonDrone UAV, to heavy fuel operation.
The DragonDrone UAVs with Sonex HFEs have been
deployed aboard ship and on land.
The number of UAV manufacturers working on potential
diesel power plants – Zoche, Continental, Textron Lycoming,
and General Atomics Aeronautical Systems and the Advanced
Technologies Group – is growing. Meteor of Italy is now
planning to introduce a 60 kW (80 hp) fuel-injected engine to
run diesel for its Falco tactical UAV; Italy’s Zanzottera – which
supplies, among others, engines to ATE in South Africa for the
Vulture UAV – is working to develop a two-stroke engine with
high-pressure direct injection and heavy fuel
But the technical challenges to converting gasoline
engines to heavy fuel operations remain considerable,
requiring an almost complete reworking of the engine
design. Almost all UAV engines have to be modified from
other applications, there are very few purpose-built UAV
power plants available. For manufacturers of UAV power
plants the technical challenges are almost outweighed by
the industrial issues. Production numbers are small,
research and development costs high and no two UAV
applications are the same. Attrition rates are also high:
lifetime cycles for UAV piston engines are often only 250
hours – though some reach 1,000 hours.
“The power-plant is the UAV’s reliability weakest link –
but typical investments in power-plants are less than 3% of
platform cost,” according to Hanan Silverman of RSL
Avionics, speaking at the Paris Air Show’s June 2003 UAV
Awareness Forum.
“It’s not a high-volume market,” said David Cliff,
General Manager at UAV Engines Ltd, which supplies
customised engines to the AAI TUAV Shadow and IAI
Malat programmes. “You are building a niche product and
the emphasis is on research and development.
Considerable resources are put into the on-going research
and development which hopefully results in production
contracts downstream.”
Mr Cliff sees the future as small UAVs but heavy fuels.
“It’s clear that the demand will be for smaller and smaller
UAVs, down to the platoon or individual soldier
application. And this means using just a single fuel, such as
Jet A.”
The NASA Environmental Research Aircraft & Sensor
Technology (ERAST) programme is driving at least some of
the HALE-UAV propulsion technology thinking in the US.
Among some of the more esoteric propulsion systems the
programme has pioneered is the solar-powered
Raptor/Pathfinder, which in 1997 broke the world record
for high-altitude flight by a propeller-driven aircraft when
it reached over 21,650 m (71,000 ft). This was followed by
the Pathfinder Plus and the Centurion, the prototype of
AeroVironment’s Helios UAV, which uses solar cells to
power electric motors in the day and recharge regenerative
fuel cells that keep electricity flowing to the motors at
night. The Helios may be able to fly for up to six months.
Another major area of major UAV engine activity is in
the development of electric engines for mini- and microUAVs. The AeroVironment’s FQM-151A Pointer UAV – with
a 300 W electric (samarium cobalt) motor, powered by two
Li/SO2 primary or Ni/Cd rechargeable batteries – has been
in service with the US Marine Corps since 1988 and has
been delivered to the French Army; US Army, US Air Force,
Marine Corps and National Guard and several US civil law
enforcement and other agencies.
Since then, there has been a rapid mushrooming in the
development of other “backpackable” electric-powered
UAVs. In Israel, the Elbit Systems Skylark is entering a low
initial rate of production in 2004; featuring a quiet electric
motor that gives the UAV a 90-minute flight duration. The
electrically powered Aladin mini-UAV produced by the
German company EMT has been delivered to the German
Army and deployed by German troops to Afghanistan.
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POWER PLANTS
Heron TP. Israel Aircraft Industries Ltd. Israel
Nibbio. Galileo Avionica. Italy
The Lockheed Martin SentryOwl – propelled by a
battery-powered electric motor with a two-blade pusher
propeller – has also seen operations in Afghanistan and
Iraq. Even small micro-UAVs, such as the 6 inch BAE
Systems MicroSTAR – powered by a 10 W electric motor,
powered by lithium batteries and driving a two-blade
propeller – is driving the research into more esoteric
technologies such as fuel cells and micro turbines.
As reported in the 2003 edition of “UAVs: a Vision of
the Future”, a fuel cell operates like a battery but does not
run-down or require recharging. It will produce energy in
the form of electricity and heat as long as fuel is supplied.
It comprises two electrodes sandwiched around an
electrolyte. Oxygen passes over one electrode and
hydrogen over the other, generating electricity, water and
heat. Hydrogen fuel is fed into the “anode” of the fuel cell.
Oxygen (or air) enters the fuel cell through the cathode.
Encouraged by a catalyst, the hydrogen atom splits into a
proton and an electron, which take different paths to the
cathode. The proton passes through the electrolyte. The
electrons create a separate current that can be used before
they return to the cathode, to be reunited with the
hydrogen and oxygen in a molecule of water.
A fuel cell system that includes a “fuel reformer” can
use the hydrogen from any hydrocarbon fuel – from
natural gas to methanol, and even gasoline. Since the fuel
cell relies on chemistry and not combustion, emissions
from this type of a system would still be much smaller than
emissions from the cleanest fuel combustion processes.
But the road to lightweight, long-endurance UAVs has
been paved with esoteric and now derelict technologies –
such as the Stemme SX1500 “Pathfinder Hawk” with a
retractable, small-diameter propeller for low-altitudes, a
larger propeller for high altitudes, or the NASA Dryden
Centre Mini-Sniffer 111, powered by unstable hydrazine.
The benefits of developing the most appropriate
combination of engine, airframe and fuel are now
becoming increasingly tangible, however, and UAV powerplant technology is continuing to push performance
envelopes. ■
Global Hawk. Northrop Grumman. USA
Camcopter. Schiebel Technology, Inc. Germany
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