http://www.chiefaircraft.com/da-100.html Would this engine work for a homebuilt ultralight aircraft? I am still in the research phase, so i don't know what the weight of the aircraft will be, but I would like to know what a good prop size and max weight that it will be able to fly.
You can use any engine in a homebuilt or ultralight, including one from a Model A Ford if you want. A 10 hp RC engine that runs at 8000 rpm? Even if you were building a single seat ultralight, in most cases you'd need at least 3 to 4 of them to have any kind of decent performance, if you could stand the racket they make. For something that can carry two people, 5 or 6 at least.
You'd have a pretty cool looking bank of throttles to control all those motors though!
That being said, I can think of one practical application; a Lazair ultralight has probably one of the lowest power requirements in the ultralight business and got by on two 10hp 2stroke forestry water pump motors, so two of those RC motors on a Lazair, of which used ones are easy to find, should work.
Scale up to the DA-150L, https://www.desertaircraft.com/products/da150l. It's more mature and widely available. With a 32x12" prop it gives 82 pounds of thrust at 5400 RPM. It puts out 16 hp. The "original ultralight" Easy Riser flew with just 11 hp.
So it's possible. Here's some broader discussion of more than just the powerplant: Can an ultralight aircraft fly with a 18hp engine?
Desert Air makes R/C aircraft engines. The 200 cc DA-200 puts out around 19 hp. I would go with 3 DA-100, two on the wing, one on the nose, giving it the Ford Trimotor look.
But these engines run around $ 1000.00 US dollars each, and are NOT designed for human flight.
It may be possible to time share a safer aircraft such as a Cessna 152 at a local airport for that kind of money.
But if you wish to proceed, the single cylinder 2 cycle 313 cc 28 hp Hirth F-33 is specifically designed for ultralights, yet would pass for an R/C engine any day.
And please do not believe the Wright Brothers got by on 12 hp. That version barely got off the ground. Weight reduction, however, is helpful.