Someone had asked how to find what the CG would be at the root if they knew the CG at the wingtip already. However he gave an answer but I thought of something that I want to know if there would be a difference in the root CG that is determined by this method.

So in this example it is an airplane with removable wings and it has a fuse width of about 8 inches or so. This is the reponse the guy gave to finding the CG at root if the tip CG is known.

"The easiest way to get the CG at the root if you have the tip measurements is to remove the wings, place the wings on the tube, off the airplane, mark the tip measurements with "T" pin, run a string between the two pins and mark the root of the wings. There you have it. The CG at the wing/fuse. Also use this method to get a stright line across if you have to line up decals and stuff."

OK simple enough but then I got to thinking that is there any difference that the reading would get if the wings were put next to each other to include the width of the fuse. In his example he takes the wings off and when they are put next to one another it is a shorter span because the fuse is not factored in. So does the angle the string make at the root with and without the fuse change the root CG location? This is how I tried to explain but I don't know if it is right...

I am not saying this is incorrect because I do not know for certain but it has me considering this: If you do this you are eliminating the fuse width which is actually considered part of the wingspan. To me it seems the width should include the dx of the fuse, say 8 inches or whatever it is, when you get the root reading. The space of the fuse will alter the mark that gets made at the root if the wings are put up directly next to each other or if they are matched with the fuse with also because the string will be in a different place along the inner root chord, slightly forward when fuse length is not added. I believe it will alter the mark and may possibly may make a difference. It being a negligible difference is another point, but I think theoretically the fuse width should be included. Again, I'm not positive but I would think the same wingspan should be used with reference to tip and root cg chord positions.

I am estimating that upto a certain point if the center section is made wider, ie the fuse, then the string placed from wingtip to wingtip would slightly move forward along the root chord...upto a certian width of course. ie, where would the root cg be at the chord if there were a 10 foot wide section placed inbetween the wings. I would thing the CG would move forward along the chord line....

so what say you??

You're over analizying the issue. Yeah in theory a wide fuselage with a highly tapered wing should take into account the small difference between the blanketed wing and the total wing. But in fact it's not an issue If it bothers you then use the true root and tip chords for your MAC calculation and reference the balance point to that.

Seems kinda like measuring with a micrometer to cut with a chainsaw I agree with BMathhews that while theoretically a difference may exist, it is practcially insignificant...

Cheers!

Jim