Re: ultralight/moped anyone?
I haven't read through all the replies to the original question, but I can provide some basic answer here, as I used to own and fly a powered paraglider. The small bicycle engines we use aren't powerful enough. It's not the size, but the power of the engine that counts, and as you know there are vast differences in power between different engines that are the same size. The other big factor is the type of wing, and how efficient it is and how much lift it can produce. I used a 172 CC, 20-25 HP engine to fly a 28 square meter wing and the combined weight with me and the motor was about 220 lbs, the wing another 15 pounds. Now that wing was extremely efficient, but it costs 3300 bucks. I also was going for thrust since that's what that sport requires, so I used a 4 foot propeller and a reduction drive.
So basically, it seems that what you have in mind is not feasible, and most importantly, and I say this from experience, not safe, unless you were to consult experienced pilots. I have known of several deaths in the circles I flew in, and those were people being very safe, and knew all about the weather, and the atmosphere(lots to know there, I thought I knew a lot about it until I started to fly). So if you ever do get anything with enough power and wing to get into the air, safety must be paramount. I hope it doesn't seem like I'm talking down or anything as we're all adults, it's just my experience that makes me cringe at the idea you have here. There are a lot of people who were do-it-yourselfers in my sport, and paid dearly with pain, injury, or death, for not training properly. Well I hope I was of at least some help. If you're serious about flying, there are plenty of enthusiasts to help you the right way. Good luck.