That's what I think they were made for, but I assumed you were talking about the hard plastic props. A CONQUEST is a pretty hot engine, and I would be leery about using nylon or rubber props on one. They [COX] would know a little bit more about it than I though. A 6x6 would be a good choice for a hot .15 in a REAL SMALL plane. Maybe they also worked well with the .09s too? I don't have any experience with the COX big blocks, but it seems that COX did a great job of matching props to their engines.