Thank you, Cheif Invertigator Erik. I figured it wouldn't fly well without an airframe behind it. Same prop pretty much, maybe opt for 8" if it's a big model with lots of drag.

Free-flight! That's even better. I used to build rubber-motored free flight models, but when It came to the investment of an engine I wanted it on the end of strings so I could get it back. My neighborhood field had a river on one side and a busy highway on the other.