Weight and Balance question

[fgayford]

As you all must know I have a JT2 and I am making enough small changes that I know that I could upset my CG.
Being I am mathematically challenged I constructed a balance platform to put the whole plane on. ( a titter totter) When I have it balanced, the weight of a wheel nut can make it tilt one way or the other.
I placed the plane on the balance before I did any modifications. With a full tank of fuel and myself aboard I got the whole set up to balance so that only throwing my head forward or backwards made the platform tilt forward or backwards. I locked the wheels and then using a plumbob held beneath the wing I brought the pointer over the pivot point of the titter totter axle. I could then see exactly where the CG was in real time.
I found the CG to be almost right on the mainspar rivet line.
I figure that with any modifications I do I can make sure I bring the CG back to that point with useful weight.( shifting the battery or fire extinguisher etc.)
So my question is. Does anyone know where the absolute sweet spot for the CG should be? Mark? Because I can now change it by the gram and by the 1/16 of a inch easily.
Thanks
Fred

[blaswichk]

That's quite interesting. Back in the early 90's, when I was flying fat ultralights, the designer of my Cadet has his shop right at the ultralight park at Arlington airport. After upgrading from a Rotax 277, to the 447, and adding way more stuff, the ultralight had to be weighed again. His method was to hang the plane with me and full fuel from a scale and rope right at the CG point, which was about 1/3 in from the leading edge. I'd say if your at the rivet line of the spar, your right there, and pretty ingenious for figuring that out. The Gull has so much trim authority that you would probably never notice the difference flying, but perfect CG will minimize any drag increase from trim changes. My .02.

kb

[fgayford]

Hi Kess
I got the idea from the RC aircraft I designed. That is how I balanced them and I could slowly and accurately move the CG and see how it affected flight.
Your right, a perfect CG at cruise would decrease drag so increasing range and speed. I hope Mark chimes in with some real knowledge and experience. I am just bush league.
Thanks
Fred