Lycoming’s One-Lunger
The smallest Lycoming will fit in a suitcase and the company appears to be making a whole bunch of them.
I suspect if you could somehow get a full overview of the number and type of unmanned aircraft in use today and the companies involved in their manufacture, the response would be: Wow, I never imagined. I am constantly running into people in the aviation business who are directly or indirectly involved in UAS projects.
Some of this has been explained to me off the record or I've been on plant tours where I could look at things, but not photograph or report on them. (One was a four-cylinder engine swinging a 10-foot prop. Huh?)
One I can report on is an interesting project at Lycoming which has been above ground for about a year. You can get a good look at the engine in this video. It's Lycoming's EL-005 which, for all intents and purposes, is sort of the best scale aircraft engine money can buy. It's similarly configured as a scale or RC aircraft engine in that it's a single cylinder with direct drive. But it has some tricks. The big one is that it's designed to run on Jet A or diesel, but it's an Otto cycle rather than a diesel cycle engine. That means it's spark ignited, not compression ignited, as is a conventional diesel.
This is obviously a design compromise in terms of efficiency and durability. Two cycle engines, which the EL-005 is, aren't as durable as four-cycle engines, nor does a low-compression, spark-ignited engine burning Jet A deliver the kind of fuel economy it would if it were compression ignition. On the other hand, two cycles are lighter and easier to maintain, at least in theory. The EL-005 is actually a multi-fuel engine that will also burn gasoline or, probably, any combustible liquid fuel you can stuff into it. The reason for that is that it's used in places-desert war theaters and shipboard operations-where gasoline either isn't available or is a hazard to have around.
Although the challenges of burning mid-distillate fuels in spark ignited engines are substantial, it's been done before when the market demanded it. Briggs and Stratton, Kohler, Clinton and others sold traditional gasoline engines modified for kerosene or diesel. The biggest problem is just getting the fuel to atomize and light off under low compression. Carburetors will work, but they require rejetting, heating of the fuel and probably starting the engine on gasoline or a blend of gas and diesel. A hotter-than-normal sparkplug also helps.
In the modern world of engine design, we have other choices and Lycoming used them on the EL-005. The "E" stands for electronic, so it has some kind of high energy ignition system that may not necessarily be just dumb. Lycoming declined to provide the details. The engine also has direct injection, which helps with atomizing the fuel, making it far easier to ignite. To keep the thing from blowing apart from detonation, the compression ratio is low, less than 8 to 1. Diesels typical have ratios in the 18- to 23-to-1 range. Thanks to the lower compression ratios, I suspect the cylinder pressures in the EL-005 aren't nearly what they are in a conventional diesel, so the structure can be lighter. (Even so, the engine has some pretty beefy cylinders.)
We aren't routinely invited to cover UAS aircraft or programs because many of them are mil-spec projects. I thought the EL-005 came out of that dark world, too, but it's actually not a mil-spec engine, according to Lycoming's Michael Kraft. The aircraft the EL-005 is used in, the AAI Mk4.7G, began life as a civil UAS used for atmospheric research and survey work. The Australian company that developed it, Aerosonde, was bought by AAI Corporation which was, in turn, bought by Textron, hence the Lycoming connection. Lycoming was asked to develop a new engine for the Mk4.7G because the previous powerplant wasn't delivering good service. The aircraft still finds use in the civil world, but it's fair to say it probably finds far more application in military work. The Mk4.7G is a class of drone known as a LEISR, for long-endurance intelligence, surveillance and reconnaissance. It also has communications relay capability, hence the need for long endurance-up to 40 hours.
But, it's still not strictly a military aircraft. Although the airplane, if not flocks of them, are in the air over Afghanistan and likely other places 24/7, they're operated by contractors who sell camera time to the military. I'm sure if we knew the scope of that business, we would be equally astonished.
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