I have a Saito .91 and a 100, I originally bought the .91 to put on a Kellett AutoGyro, then Saito came out with the 100 which I couldn't stand not having. The Kellett needs a very large diameter prop because of its dummy radial engine, plus it requires about 8 lbs. of thrust to fly it. The .91 turns an 18x5 at 8,800 rpm and the 100 turns the same prop at 9,200 and the 100 runs much smoother than the .91. I would think that anything approaching a scale sized prop for your plane would eliminate the YS .63 and I've never tried the 18x5 on my YS .91FZ. I don't think the YS .91 would be happy working that hard. I'll have to try it.