pup cg mystory solved
Well I dug a little deeper on this cg issue with the Balsa USA Sopwith Pup and this is what I found: 5 1/4" from the leading edge of the top wing is the standard optimal cg for this model. The assembly manual, page 30, photo 164 shows the construction of the machine gun , this photo also shows the location of the original CG at 5 1/4". If you compare the photo to the plans that come with the kit the cg on the plan is not in the same place and is well forward of the cg in this photo.
I used the full size plans to find the proper cg by drawing the center lines on the wing cross sections, put a point 25% aft of the leading edge on both wings on these center lines, connected these two points, marked the bisection of this line, and finally brought this bisection point up perpendicular to the datum line and to the bottom of the top wing. The two wings are the same size, they have no sweep , and the incidence appear to equal. This point is at 5 1/4", and matches the photo on page 30 of the assembly manual.
My pup has a Magnum 91fs, 16/6 prop, and a heavy prop hub. I mounted the engine, battery and servos as far forward as possible. i used the full length of the cowling and the engine is mounted upright. I had to use a 120fs motor mount to mount the engine as far out as possible. I used no lead for balance.