I'm at the point of balancing my Sig Extra 300xs 1/4 scale (upside down) and have a question. The instructions specify the CG by telling me to measure 4.34" back from the leading edge of the wing and balance at that point. My thought is that I should cut a piece of string 4.34" long, attach it exactly "tangent" to the leading edge and then contour the string back over the top of the airfoil and mark the end-point and then balance there. I'm not sure if I explained myself clear enough but that would be a more accurate way then to just use a straight edge to measure since it wouldn't take into account the contour of the top of the wing. Please let me know if my measurement method is correct.
Thanks

Your making it far more complicated than it is. Measure straight back from the leading edge and dont worry about the contuor of the wing. I use a Great Planes C.G. machine and thats exactly how that works. Mine was tail-heavy with a OS FX 1.60...just to let you know. Ended up putting dual batteries under cowl and still had to add a little lead.

Dan

The designers developed that CG recommendation by measuring straight back from the leading edge of the wing saddle along the side of the fuselage, parallel to the purple/white color break. If you do it your way, you will not get the same CG location. In fact you will probably be significantly noseheavy.

4.34 inches? In the real world that translates as 4 1/4" to 4 1/2". The CG of any conventional airplane has a range of acceptability.



Jim

Thank you guys for your help. I went out and bought a GP balancer today. As always, I learn something new every time I post a message.