The first, and only HP-21 4-stroke I ever saw run was on a Tiger Moth that had been converted from a large rubber powered kit. Man, what a sweet flying little model. I seem to remember the wingspan being around 30". It was covered in Monokote in a semi-camo scheme. It looked, and flew, fantastic. The only complaint the owner had about the engine was that the idle was fairly high 3K+ so landings were a bit tough with the engine running.