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So my best friend’s son is a structural engineer, and he and I are talking about designing an ultralight. We were talking about which engine to use RE: power/weight ratios, engine weight, etc. and I mentioned that I see a lot of folks using the Rotax 503, but that I didn’t have enough information to offer an informed decision. Does anyone out there want to offer a few pro and con bullet points about this particular engine? Especially if you have experience using this. Thanks.
I don't believe Rotax makes the 503 anymore. You would have to find a used one and assure it's airworthy.
They required major servicing every 300 hrs or so.
It would make a lot more sense to design a new ultralight around a current engine in production. Hirth still makes some small two-strokes as do a few others, but I would come down to the 25-36 hp range to use any of several very light, two-stroke paramotor engines in the $2,500-$5,000 price range (Vittorazi, Simonini, Cors-Air, Polini, etc.) It will require a cleaner design than something like a Quicksilver, though.
The 503 is a good engine. Not made any more, but readily available used or freshly overhauled. But an aircraft with a 503 will likely be overweight and/or too fast to be a legal ultralight, a 447 (or Cuyuna) might be a better choice. But you need to work out the basic design before selecting an engine; successful ultralights are flying on as little as 25HP, half of what a 503 makes and much less than half the weight. What type of configuration did you have in mind?
If by "ultralight" you mean one that complies with Part 103, I agree a 503 will be too heavy and it will have too much power.
If I were building an ultralight, I'd go with the "Big Twin" from Valley Engineering. Four stroke, auto carb heat, PSRU, etc..
http://www.culverprops.com/big-twin.php
Rotax 582 is still made and sold. TBO is 300 hours. You can overhaul a 582 for less than what it costs to overhaul one Continental cylinder. I would explore the 582 or the half VW aero engines.
I do not know much about the topic, but i have seen a few videos of people doing it. I would explore electric power. I know the batteries are big and heavy, but you can get some pretty powerful motors (I think). I may be very wrong because i have not done that much research into it, but it may be worth checking out.
Christian L.