Drives for drones: engines designed for vehicles as diverse as snowmobiles and regional jets have been adapted for drones. Their special needs are reductions in engine weight, fuel consumption and detectability, and the use of battlefront fuels.

Availability and development cost (rather than technical) considerations dictate that large drones are powered by off-the-shelf turbine engines. Examples include the 36.9-kN Rolls-Royce AE3007 turbofan used in the Northrop Grumman Global Hawk, the 520-kW Honeywell TPE331-10T turbo-prop of the General Atomics Predator B and the 478-kW Pratt & Whitney Canada PW200/55 turboshaft used in the Bell TR918 Eagle Eye.

Aside from those used for small-arms practice, target drones are generally powered by simple turbojets to provide realistic speeds. Safran/Microturbo produces the 1.6-kN TRS-18-1 for the Meteor Mirach 100/5 and the 4.4-kN TRI-60-5 for the Raytheon MQM-107E.

In the 'sub-kN' category, the Pentagon is developing (relatively) low-cost turbojets under its Small Business Innovative Research programme. The principal result to date is the 13.3-daN Technical Directions J45 (see title photograph) for the Lockheed Martin Loitering Attack Missile. It has an SFC of 0.13 kg/N.hr, around 25% less than for the best aeromodeller's engine.

Cheaper hobbyist turbojets in the thrust range 4.5 to 23 daN are available. JetCat Germany's engines are used in the Eads-Dornier twin-engined targets: 16dan P160s in the Do-DT35, and 20 daN P200s in the Do-DT25. AMT Netherlands is working on a 67-daN unit, while Spain's Aries Jet Microturbines is developing the 35-daN A J-307, both for military use.

South Africa's Baird Micro Turbines (BMT) has produced 12-daN and 16 daN units featuring advanced electronic controls. BMT is now developing the 20-daN BMT-200-KS (kerosene starting) for possible target applications.

Rotaries

The Wankel-type rotary is widely used in drones weighing up to one tonne. It has far fewer parts than a conventional piston engine, is cheaper to produce and generates a lot of power for its size. Less prone to hot spots, it is more tolerant of low-octane fuels.

A rotary engine is vibration-free, but runs at high revs, increasing the need for silencing and a reduction gearbox. Its slender chambers result in less complete combustion, and consequently higher fuel consumption and emissions.

UAV Engines (UEL), owned by Silver Arrow/Elbit Systems, has provided mogas/avgas-burning rotaries for over 30 drone types, based on developments by Norton Motors between 1969 and 1992.

The bottom of the UEL range is the air-cooled AR731, which produces 28.3 kW at 7800 rpm. Designed for limited life, it powers various target and attack drones, including the IAI Harpy. Weighing only 9.9 kg, it probably has the highest power/ weight ratio (2.86 kW/kg) in its class. In a smaller category, the aero-modellers' Graupner/OS 49-PI produces 0.95 kW at 17,000 rpm and weighs 0.335 kg.

The longer-life (AR741) version powers the AAI Shadow 200/400, Aurora GoldenEye, the BAI Aerosystems Isis and the Elbit Hermes 180. It is also to replace the 19.4-kW Sachs SF350 two-stroke in the RQ-2B Pioneer. UEL's midrange 38-kW ARS01 is used in the AA1 Shadow 600, BAE Systems' SkyEye and the Elbit Hermes 450.

At the top of the UEL range is the liquid-cooled, twin-rotor AR682R, which produces 67.1 kW at 8000 rpm and weighs 56.5 kg with reduction gearbox and generator. Typical specific fuel consumption (SFC) in cruise at 70% power is 0.33 kg/kW.hr.

A relative newcomer to this field is Diamond Engines, a subsidiary of Austria's Diamond Aircraft. Its liquid-cooled IAE 50R rotary has electronic fuel injection and produces 40.4 kW at 7750 rpm. With radiator and coolant it weighs 30.8 kg. The IAE 50R powers the Camcopter S-100 drone helicopter.

Pistons

At the upper end, UEL competes with the four-stroke pistons of Austria-based BRP-Rotax, a subsidiary of Bombardier Recreational Products. The principal Rotax drone engine is the 1211cc 914F, which gives 85.5 kW at 5800 rpm and an SFC of 0.276 kg/kW.hr. It weighs 64 kg with reduction gearbox. Applications include the General Atomics Predator and the Saic Vigilante 502.

In developing the Predator for the US Army Extended Range/Multi-Purpose requirement, General Atomics adopted for the Sky Warrior the Thielert Centurion 1.7, which burns diesel or turbine fuel. The Centurion 1.7 is a turbocharged, 1689-cc, four-cylinder in-line engine, producing 99 kW at 2300 rpm. It weighs 134 kg and has an SFC of 0.212 kg/kW.hr.

The Sky Warrior (provisional service designation MQ-12A) is also being considered as the Class IVB drone for the US Army Future Combat System. The Centurion 1.7 is to be tested with the Mmist SnowGoose parachute delivery system and may power the Vertivision VSR-700 Orka drone helicopter.

Britain's RCV Engines was established in 1997 to exploit the Rotating Cylinder Valve patents of Keith Lawes. In an RCV engine the cylinder is gear-driven to rotate around the piston at half crankshaft speed. A single port in the rotating cylinder passes fixed inlet and exhaust ports, providing the valve function. The RCV runs well on heavy fuels, due to high chamber turbulence.

A single-cylinder 35 cc engine developed by RCV under US funding produces 2.54 kW at 9500 rpm, burning 0.36 kg/kW.hr at full throttle. The company is being funded by Darpa to improve part-throttle SFC, and believes that 0.33 kg/ kW.hr is achievable at 25%. Future projects include a twin-cylinder 25-cc engine for long-endurance drones.

Italy's Zanzottera Technologies began with powerplants for Meteor targets, and now specialises in fuel-injected, air-cooled, horizontally opposed two-strokes. Its principal product is the two-cylinder, half-litre 498ia, which generates 29 kW at 6700 rpm and weighs 18.6 kg with alternator/starter. SFC is 0.44 kg/kW.hr at full power. The 498ia is used in the Aeronautics Defense Systems Aerostar and South Africa's ATE Vulture.

Zanzottera is also developing a water-cooled, flat-four, heavy fuel engine (HFE), designated 998 and producing 60 kW. The company is studying various two-cylinder operating modes for the 998, to reduce fuel consumption in cruise and survive battle damage. Zanzottera engines have a constant 'engine health' monitoring system datalink and heavily redundant ancillaries.

Another two-stroke specialist is Hirth (Gobler-Hirth Motoren). Its fan-cooled two-cylinder 625 cc H32 produces 45 kW at 6400 rpm, and the three-cylinder 939cc H37 reaches 66.6 kW at 5200 rpm. Both are in-line engines, weighing 31.7 and 45 kg respectively. The horizontally opposed 1042-cc four-cylinder H30 produces 75 kW at 6500 rpm and weighs 42 kg. Hirth is developing a 20 to 80 kW family of two-stroke HFEs, using air-assisted fuel injectors and burning JP5/8.

Advanced Target Systems of Abu Dhabi is developing two-strokes for its drones. The range includes the horizontally opposed 26 kW two-cylinder AD 500 and the 79-kW, four-cylinder AD 1000. The 104-kW ME998 will have four in-line cylinders.

More than 5500 examples of the horizontally opposed mogas/avgas-burning piston engines of Limbach Flugmotoren have been produced for general aviation. The lower end is represented by the 274cc two-cylinder air-cooled two-stroke L275E, which produces 15 kW at 7200 rpm and weighs 7.2 kg. The 548-cc four-cylinder two-stroke L550E is rated at 37 kW and weighs 16 kg. Modified by Denel, it is used in that company's Seeker II.

The top-end Limbach engine is the liquid-cooled, four-cylinder 2424-cc L2400DT/ET, which weighs 86 kg and reaches 96 kW at 3000 rpm with an SFC of 0.255 kg/KW.hr. The company is developing the turbocharged L2400DT.X, to produce 118 kW at 3000 rpm.

Meggitt Defence Systems produces engines for its targets, exemplified by a 60-cc unit for the Snipe Mk 15, a 342-cc for the Spectre 2, a 520-cc for the Banshee, and a 955-cc for the Voodoo.

There are cost and reliability attractions in adapting an existing car engine. In the case of the Denel Bateleur drone, one alternative to the Rotax 914F is a modified Subaru EA-82T, a 1781-cc, turbocharged flat-four, producing 123 kW at 3000 rpm. In that instance Denel would convert the Subaru engine. Others performing car engine conversions for aviation include NSI Aero of Arlington, Virginia.

For lightweight drones, hobbyist engines are available. Germany's 3W Modellmotoren manufactures two-strokes, from the single-cylinder, 24-cc, 1.75-kW 3W-24i (adopted for the Boeing/ Insitu ScanEagle drone) to the horizontally opposed four-cylinder, 684-cc, 44 kW 3W-684i. Insitu is working with Sonex Research to modify the 3W-24i to burn diesel or turbine fuel.

The Czech Republic's ZDZ sells engines ranging from a single-cylinder 40 cc unit up to a 420-cc four-cylinder. New Chinese manufacturers such as GMS and JBA are increasing their market share at the expense of old established companies such as Japan's OS Engines and Italy's SuperTigre.

Desert Aircraft of Tucson, Arizona produces the single-cylinder 50-cc DA50R of 3.7 kW, and the twin-cylinder 100 cc DA-100 of 7.3 kW and 150-cc DA-150 of 12.3 kW. The DA-150 is used in the Raytheon Cobra.

Boeing is studying the use of piston engines burning liquid hydrogen fuel, which involves the development of lightweight composite tanks.

Electrics

Small electrically powered drones became feasible cost-wise only in the late 1980s. There has since been rapid progress, with brushless motors and major advances in rechargeable batteries, driven by commercial applications such as cell phones, PDAs and low-emission cars.

In the brushless DC (Bldc) motor, the switching function of the traditional commutator/brush combination is performed by an electronic controller. The Bldc is more efficient, reliable and longer lasting, and produces less electronic interference, but costs more.

Israel's Bental produces the brushless B-103 for micro drones such as the IAI Mosquito. The motor weighs only 56 gm, but produces 60 W for take-off. Other manufacturers include AXI from the Czech Republic, China's Hyperion and America's E-flite.

Batteries

Nickel-cadmium (NiCd) batteries are expensive but robust. However, by the latest standards they are large and heavy, with energy storage figures of only 60 Wh/kg and 150 Wh/litre. Until recently they suffered from 'memory effect' or 'voltage depression' (not taking a full charge) if not fully discharged prior to recharging.

Nickel-metal hydride (NiMH I batteries have a cathode of hydrogen-absorbing alloy (rather than cadmium). Providing 300 Wh/litre, they have around twice the capacity of a similar-sized NiCd. High-profile applications include the Toyota Prius hybrid car and the Honda Asimo humanoid.

The advent of Lithium-ion (Li-ion) batteries brought a breakthrough in weight savings, with energy storage of up to 180 Wh/kg and a power/weight ratio of 1800 W/kg, which is double the NiMH figures. However, they are expensive. have a shorter life and can be dangerous if mistreated. They are nonetheless widely used in camera phones and laptops. A Li-ion battery developed by Saic's Telcordia Technologies forms the wing structure of the AeroVironment Wasp micro drone.

The lithium polymer (LiPo battery, which is the current slandard for hobbyists' model aircraft, is less hazardous than Li-ion and has energy storage figures of up to 200 Wh/kg and 300 Wh/litre. Manufacturers include the UK-based Flight-Power.

Near-term battery developments include lithium sulphur dioxide for 200 Wh/kg, bi-polar lithium-ion for 250 Wh/kg and lithium sulphur for 350 Wh/kg (as recently demonstrated by Arizona-based Sion Power in a Qinetiq drone).

Bental is developing hybrid engines for drones. An internal combustion engine is used for take-off and cruise, during which time it charges the batteries. On reaching its patrol station, the thermal engine closes down and the electric motor takes over to minimise signature, later acting as a starter for the engine, used in the recovery phase.

For the longer term and drone operations requiring extreme endurance, fuel cells and solar power are likely to dominate the drone scene. Such developments are planned for discussion in our next issue (Armada 2/2007).