Thanks guys, there isn't anything on the market that appeals to me so I might have a look into building something myself (no experience and niaive ) Does anyone have any ideas what the smallest engine could be used (what about a Stihl chainsaw engine and its sprag clutch) at 46cm2 it produces 3.6bhp and 2.4kw of useable power. My GCSE calcualtions suggest that I would only need 1.6kw to maintain straigt and level flight (don't ask how I came to that, correct me if I am wrong) mechanical energy required would deplete 10-15% of the 2.4KW (what is a normal prop efficiency 70%+?) leaving me with 55% overall efficiency. Oh dear, I already see a shortfall. Help me out please.