Steroids for drones.

Apart from the larger high altitude and long endurance drones that use turbojet engines 'borrowed' from existing classical aeroplanes, the tactical and medium-altitude types use either drone- specific or microlight aviation-derived powerplants.

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The general starting rules to define the ideal engine in this context are lightness and compactness. Then other considerations like power/torque output can cut in, but maximum output should reach its peak at relatively low revolutions per minute, although this may depend on the diameter of the propeller used. Although reducers are available, there is at any rate a limiting factor in the rpm extremes since the smaller tactical drones usually do not have variable pitch propellers (though these are common on the much larger male types). Not that this would not be technically feasible, but it would not only impose a weight penalty because of the necessary control mechanism, but most importantly would also imply the installation of an automatic air speed and temperature management system running in symbiosis with the autopilot.

Boxers

Flat-twin or flat-four designs have traditionally been preferred in aviation when they became available because they offer a much lower centre of gravity than their 'in line' counterparts, they are also easier to cool by air and are lighter. However, as we shall see, technological advances have brought alternatives into the scenario. As is the case for motorbike engines, new materials have enabled lighter liquid-cooled powerplants to become eligible. Liquid cooling offers the advantage of a better spread of the cooling effect around all cylinders and the water jacket also acts as a sound muffler.

We mentioned above that drone engines were specifically designed for aviation applications. Here too, it appears that some changes are appearing and that where larger cylinder capacities are required, an automotive engine could do the job. Indeed, the engine of the Subaru GL Wagon is now being retained by Denel Aviation as a possible solution to power its new Bateleur male drone (see Armada 6/2004, page 31). This 1781 cc EA-82T turbocharged flat-four delivers 165 horsepower at about 3000 rpm. The other candidate for the new South African bird is the Rotax 914, which is also turbocharged. During an interview with the author, the manager of the Bateleur programme said that the reason for looking into the Subaru engine was inspired by the fact that these engines had started to be popular in home-built aircraft circles, particularly in Australia. In the case of the Bateleur, however, the engines would be stripped and reconditioned by Denel Aviation specialists themselves. <<They are very reliable engines, and are even used in one of their 4 X 4s, and they are probably able to provide 5000 flight hours no problem>>. There is also the cost factor, since the Subaru piece of muscle, built in hundreds of thousands, is cheap on the second-hand market (probably half the cost of the Rotax), relatively light (just over 100 kg) and parts are plentiful. Asked about the airworthiness certificate, the Denel official said that, in a first stage in any case, the aircraft would be flying in a restricted airspace environment.

Other Subaru engine 'aero-conversion' firms include NSI Aero in Arlington, Virginia. Rotax is also a major player in the drone engine field, but very sadly systematically turned down all invitations to talk to the author about this firm's activities in the defence field, an attitude that sharply contrasts with one manufacturer who went out of his way to specially take a series of photos of his engines for this issue of Armada International.

Not very well known by the public at large, but increasingly popular with drone companies, is an amazing manufacturer from Italy called Zanzottera. Driven by Guido Zanzottera himself, the company started about 20 years ago producing engine components for a German company, but some ten years later started to produce its own engines for ultralight aircraft. This caught the attention of Meteor, which started ordering engines for target drones. For some reason, the small factory migrated to Canada but returned to Northern Italy about four years ago with the aim of focussing primarily on development. However, success is success, and Zanzottera found itself producing again. Growth has been constant since and the company now churns out some 150 engines of all types per year. In the reconnaissance drone sector, ATE from South Africa was the first to take a sharp look at the very special technology applied by Zanzottera for its Vulture, this move was followed by AAI, IAI, Aeronautics, Meggitt and Galileo (for the Falco) to name but a few (in passing, it is worth noting here that Aeronautics is using the Zanzottera engine in its joint venture with the Russian Ministry of Defence to improve the performance of Russian drones). It is not Armada's tradition to mention website addresses in articles, but the exception confirms the rule as the Italian firm's site--www.zanzotteraengines.com--is an example of clarity and comprehensiveness, giving full technical specifications for each type of engine. The site also reveals that Zanzottera responds to very special requests.

All Zanzottera engines basically use the same technology, i.e. fuel injected two-stroke flat-twin or flat-four boxers, but the current models for drones are the 498ia and the 630a. The engines all have an integrated brushless alternator and electronic fuel and engine management system with an RS232 interface. This means that all engine parameters (instant fuel consumption, temperature, constant internal system diagnosis, ignition, injection, vibration, etc) can be monitored by the ground crew, which can in turn tweak the engine performance depending on temperature and altitude. Zanzottera engine blocks are milled from solid billet and finished through spark machining to avoid porosity problems that are inherent to cast blocks. They are then nickel finished. All other parts, nuts, bolts and exhaust, for example, are made of titanium.

Zanzottera is now working on four new concepts. The first is diesel, and the second direct fuel injection, which is hoped to reduce fuel consumption by about 40 per cent, together with oil cooling of the engine. The third is to add a mechanical supercharger (two-stroke engines are incompatible with turbo-blowing), with a subtlety though: the charger will have a variable drive so that it will progressively cut in as the altitude augments up to 30.000 ft to maintain a constant power output. The fourth is a four-cylinder banger for heavier drones.

The Italian firm is also developing derivatives with dual direct-injection and dual ignition. Now this is a very important aspect since there is a growing need for drones to fly in civilian airspace in absolute safety. With such redundancy, and via the above-mentioned interface and datalink, it becomes possible for the ground operator to overfeed one injector in case of malfunction of one of the pair per cylinder or have advanced warning of a malfunction (vibrations are also monitored) and take necessary measures.

On the subject of diesel an interesting evolution is coming from Sonex in the United States. For diesel engines to provide the same output as their light-fuel counterparts they have always had a propensity to be heavier. Sonex, under a programme sponsored by Darpa, is specialising in the conversion of petrol engines, concentrating on the development of very specially designed combustion chambers to allow a full burn of kerosene-based fuels. This, in effect, would kill two birds with one stone in that the engines spit out cleaner gases (always a problem, particularly with diesels) and also come a step closer to a Department of Defense directive requiring a generalisation of the use of diesels.

Cam Spinners

The alternative to reciprocating pistons comes from the rotary engine, better known as the Wankel engine, thus named after its German inventor Felix Wankel. The Wankel engine first appeared in the mid-1950s in a record-breaking NSU automobile. Very quickly, Wankel's theories proved right, as this diminutive 50 cc engine managed to propel the car up to 196.3 km/hour. Much of the performance of the rotary engine is due to the absence of reciprocating parts, inertial forces and the enormous reduction of the number of moving components. In spite of remaining a four-cycle engine, there are no inlet or exhaust valves, for instance (see the diagram herewith). This alone translates as a formidable reduction in overall size and weight. Being rotary, Wankel engines are practically devoid of vibration. On the downside, the Wankel quickly showed its shortcomings in terms of wear, particularly in the domain of the small scrapers at the tip and sides of the cam, which play the role akin to that of the rings in a conventional engine. However, modern sintered materials came to the rescue enabling Mazda, who had bought the production patent (amongst other automotive companies) in 1960, honed the art to the point that its 787B took an outright victory at the 24-hour Le Mans race in 1991. The point was made.

In the meantime, the motorcycling industry had also taken an interest in the rotary piston engine in the 1970s, and amongst them was Norton with the F1. However, the British automotive industry had then slipped into a major economic disaster, which links us now to the current and highly successful UAV Engines company.

UEL, as this British firm it is also known, indeed finds its roots in former Norton Motors line of rotary engines, which was bought up by Alvis in 1992. This branch was then spun off as an independent company specialising in engines for unmanned aircraft only. The ancestry probably explains why some of the engines, the AR 801 in particular, are sometimes referred to as Alvis engines by certain drone manufacturers.

The British company, now understood to be owned by Elbit, has had an amazing growth in the past three years, its turnover having leapt from two million [pounds sterling] to five million [pounds sterling], and only caters to the defence tactical drone sector (hence its engines are rated as defence materiel). In the tactical drone sector with altitudes of up to 20,000 ft, UEL, a company spokesman told the author, enjoys about 80 per cent of the worldwide market.

While UEL will tailor its core engines to meet any particular requirement, the firm's catalogue includes two main product lines--the air-cooled engines up to 38 hp, and the water-cooled types spanning a 50 to 120 hp range. Asked whether UEL had any plans to convert its engines to diesel, the same company official jokingly answered that they would run on anything including whisky. Turning serious though, he said that although they were originally intended to run on petrol, it was found that they operated quite happily on JP8 jet fuel and still retain about 95 per cent of their efficiency. Due to their compact shape and high power-to-weight ratio the UEL rotary engines are perfect candidates for vertical take-off and landing drones, like the shrouded design adopted by Sagem and Singapore Technologies.

Target drones, as their name implies, are destined to have a relatively short life, and because the lighter rotary piston engines are remarkably light and cheaper to build, the simpler and cheaper entry-level engines are particularly well suited for that application. The AR731, for example, has a time-before-overhaul rating of only ten hours.