of the 5 programs I tried for c/g ,tail volume one gives biggest change. than plans !!- 2 inches. 8% static margin . lost . compufoil also not the same a tail volume specs.

What model are you interested in? Dan.

turbinator 81 span wing

Do you have the plans? Dan.

yes

If it's on the plans, then that is the correct one to use. Why are you interested in the cg generated by some other software? Dan.

Lol

At least one spreadsheet out on the web uses a tail efficiency value that is more suitable to full size planes. So the choice made by the person that wrote up any given spreadsheet or online calculator would affect the results.

I'm going to guess that the reason for a higher tail efficiency value would stem from the higher Reynolds numbers of full size aircraft. So the ones that use smaller values for tail efficiency and generate a more conservative CG location would be the one to go with if in any doubt.

But I have to agree that other than perhaps serious 3D models where a given CG on the plans might be the sort of thing an expert pilot would choose I'd just go with what the designer indicates. If you have some concern over not being as good a pilot then shift it 2 to 3% of the chord forward as a starting point.

Mr. Matthews makes some excellent points, but one which I'd like to emphasize is in the quote above. You are under no obligation to make your initial test flights with the "final CG". You can ballast the model a bit nose heavy at first for a greatly increased safety margin. You may pay a slight price in performance, but that's of no great consequence at first. The objective of the first flight is to get up and get down in one piece and learn something about how well the machine functions. You don't need stability or handling problems to complicate your task.

Once you become comfortable with all the other operational aspects of flying the model, you can evaluate the pitch stability and then begin to sneak the CG rearward in careful increments.

Dick