The Wright brothers managed to make their plane fly with an engine that produced 12hp and weighed 80kg, their propellers had about 60% efficiency, magnificent for being the first ones of their kind, modern propellers have about 80% efficiency, so here's a question for you, what would it take to use pic related to power a manned airplane? It weighs 15kg and produces 4.8hp stock, a bit more if you tweak the governor, there are aftermarket camshafts and such that up the power by a lot but lets not get into that.

So basically, can you power a manned airplane with an engine weighing 15kg and producing only 4.8hp? and if you can, what design specifications would it have, ie. what would it weigh? what wingspan? what aspect ratio? Could you make it out of conventional materials like wood or would you need some more advanced materials like carbon fiber and so on?