Originally Posted by
smcharg
Fagox to be clear, I was NOT disagreeing with your CG position. It was just interesting to me. By your video, your CG is wonderful...for you. The CG range on the Rebels is amazing and I agree with Dave and this discussion came up in my MAX thread. I suggested doing CG with landing fuel and caught a bunch of crap for it haha. But, to be fair, you have to look at the possibilities, not just the probabilities and there is a possibility that one could run the tanks dry which would be worst case and the most aft CG. Therefore, a check there is still a great idea. If one were to find the AC (aerodynamic center) of this aircraft, I think you find that our SM (static margin) is probably around 6-10% at the center of the tube which is why you can go 20mm in front or behind the back of the tube. The bounce in the nose is purely due to the position of the mains in relation to the CG and nothing to do with how it behaves in the air. I believe Andreas purposefully put the mains in that position to allow the aircraft to rotate effortlessly. You've done a great job with your Rebel.
Jan Roskam says place the mains 15 degrees aft of the CG. I'd say it's spot on.
Scott, I would agree with the nose bouncing , my field is very sparse with patches of dirt and grass. We have to get the CG with fuel still in the tank, what would be left after say 9:30 of flying? When and if I burn the whole amount I would still would be in the CG window just probably a couple of mm behind the rear tube. The way mine is balanced it's the most forgiving plane I have flown since my racing days three decades ago. I have a couple 4mm of elevator down trim mix in with takeoff flaps, and it just rotates effortlessly off the runway, it seems to be in a sweet spot of the Rebels CG. Have installed a CMjets smoke tank and will recheck the CG. It's installed been the wing tube and should only marginally move the CG back.