I hope this is the correct section for this question. I had to rebuild my UltraStick 120 light. They are no longer made available by Hanger 9. I re-built the fuse using 1/8" door skin from a local lumber yard. I extended the tail end a couple of inches and increased the lenth forward of the LE of the wing by 3-4 inches. I have installed a CRRC 26ei gas engine for power. Cut a few lightening holes in the tail end just like the original. The wing was o.k. but I recovered it to match my color scheme of the fuse. The original CG from Hanger 9 calls for 4 1/8" aft of the L.E. Well, I am tail heavy even adding extra nose area. The CRRC is not a heavy engine...probably similiar to a Saito 120. I could add weight to the nose, but got wondering if by changing the dimensions of the fuse fore and aft, perhaps my CG needs to be recalculated. I know some of you out there know of a formula(ae) for computing that. If you could share that, it would be appreciated.
Tim in Tacoma

Look up at the sticky thread at the top of the thread title listings.

Please visit www.TailwindGliders.com and in the Articles/Files page and read about "Sailplane Calc" although the name is for gliders it'll work fine for your model.

I'd write more but it's 3am and I'm heading off to work.

Best of luck.
Curtis
Montana

You've lengthened the tail and a few other things, right? The start location for a CG is a function of the size, location and shape of the horizontal tail (the part of the airplane "responsible" for maintaining pitch) and the size and shape of the wing. Measure those features, plug the numbers in and within seconds the application will spit out your numbers http://www.geistware.com/rcmodeling/cg_super_calc.htm

It's quick and easy and can give you a range for the CG that's safe and reliable and considers what the flying world uses day in and day out. Plug in 5% static margin and run the application. Note the CG, then plug in 20% and run again. You'll have a range that'll show you where to get the CG plus the size of the range will indicate how sensitive your plane is to CG. An amazing amount of excellent info for almost zero work.

And, in the end, as the balance is always related to some point on the wing chord, and as the position and size of the tailplane only shifts things a small percentage, the balance won't actually change much. Use the same as before and it will fly much the same. Only thing is, you do what is needed to get it back where is was, add balance weights or remove/shift moveable weight (batteries?)as needed.
Evan, WB#12.

It's definitely worth knowing how much lengthening the tail affects the CG location, and as Rock notes, it's easy to do.

It's also possible that the previous location was more conservative than it needed to be. Model designers do this all the time. People set the CG and don't experiment, so they never find out. Before the changes, perhaps the plane was easy to balance, and it flew well, so there was no issue. Now there IS an issue.

I do this calculation for every plane I build. It often saves me the hassle of trying to move equipment around or finding places to add weights. Why add more weight to the nose than you need to?

Jim