Tuesday, 8 June 2010

Alternative engines

There was a time when almost every aeroplane pictured on the cover of FLYER was powered by either a Lycoming or Continental. Now, most are powered by either a Rotax 912, or by one of the diesel engines - and knowing what's coming down the cover pipeline I can't see that changing any time soon.

It's interesting to compare the mogas (Rotax) engined aircraft with the Jet A fueled aircraft. Almost all of the Rotax-powered machines have been designed around the engine. The advantage here is that the designer can conjure with the full set of compromises - speed/range/weight & balance/space etc. when setting out the basic elements of the design and configuration of the airframe. By contrast, the engineers adding a diesel engine to an existing design have to work with the compromises and assumptions made by the aeroplane's original designer - as an example, Diamond's DA40 was originally designed with 180hp Lycoming - when that was first replaced by the 135hp Thielert, the airframe had to cope with a heavier engine producing less power. Now that the Thielert has been replaced by the Austro Engine, the power has (almost) been restored, but there's even more weight hanging on the front - giving the engineers some weight & balance issues (dealt with by putting extra eight in the back, I understand). The installation in the Cessna C172 brings other challenges, the wet-wing fuel tanks were originally specified at a size suitable to provide enough avgas for a decent range/weight compromise - jet fuel is heavier than avgas, but it contains more energy, and the engine consumes less of it. It's not practical to reduce the size of the tanks, and too much useful load is taken up by filling them, so the POH supplement that is part of the STC limits the fuel contents.

Things will be improved when someone designs an airframe around the engine. That will require a large chunk of money, particularly in the certificated world, but before risking that much cash, manufacturers will want to know for sure that the engines are reliable, affordable and here to stay, and that there's a market large enough to justify the expense.

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