ORIGINAL: Reaper26
For all the planes I built i used the manuals recomended points for the CG. I recently built a 33% Yak54 without a manual. I've heard that the rule of thumb in finding the CG is 1/3rd the measurement of the widest part of the cord of the wing. I'd like to know if this is true for all planes. Thank you.
Since the initial CG is to give the model good pitch stability and solid elevator performance, no the CG isn't figured from just the wing dimensions.
Pitch stability has at least 4 major contributors. The size of the horizontal tail and how far back it is from the wing contribute tremendously as most would understand. The other two details that affect pitch are the wing area and chord. And then there are a number of other details. Considering only the wing planform doesn't get it by a long shot.
What is true for most planes (and that considers all the full-size flying right now) is to consider all the details that matter. The easiest way for us modelers to do that is to use the application that
proptop linked to in his post above. It is easy and quick. Take 9 measurements and plug 'em in. Run the application with 5% as the StaticMargin and record the CG location. Then run it with 18% SM and record that CG location. The two locations give you the range your model can balance in for a safe and flyable 1st flight.