It sounds to me like ur doing Science Olympiad. I did it as well last year and the top five contetstants were all monoplanes (single wing). I also have seen some videos of the most incredible gliders in rubber band powered competitions, and the ones that win stay up for 35 minutes. Those are all single wing as well. I don't know the physics of all of it, but it seems that biplanes are not quite as good. Hope this helps.