RE: GSP Spitfire ARF build
Haywood,
I don't know anything about those TT engines, but I will say that my finished GSP spitfire weighs about 12.5 lbs, fueled on takeoff. The engine I have (YS120) makes easy work of flying it, but I shutter to think how disaterous this plane would fly if greatly underpowered. As Richard L. said, it might fly scale-like, but if you get into a bit of trouble taking off or landing, that engine will not be able to save you. Moreover, even with the YS120 I have on the front (which I believe is ~35 oz), and even though my plane balances on the recommended CG, it still flys tail-heavy. You would have to add a huge amount of dead weight on the nose to get it to fly properly, further compounding the issue if using a lighter, underpowered engine. Someone else (might have been Travis) said that they were having trouble with a sensitive elevator and porpoising when slowing for landing. To me this indicates the plane is too tail heavy. I can't remember what engine he had on it, or how much weight he had to add to the nose, but I do know it was larger than a 91.
Sorry to have to tell you this, but I think that this Spitfire would not do well with a TT91.
Noah