Well i know this sounds silly but I think that impellers on the sides of a yoyo would give greater stability than the current hollow ones.( Just to give you an idea of the type of yoyo's I'm talking about: http://shop.yoyoexpert.com/product/162/Juggernaut [Broken]). I haven't started a cad model as I don't really have time to waste on a fruitless effort considering how close midterms are. Basically the idea is that the impeller would suck air in from the side face of the yoyo and blow out from the side. This is just a hypothesis and I am just a first year student in mech.eng so sorry for the ignorance.