Rule of thumb for center of gravity

[spark363]

What is the rule of thumb for determining the center of gravity for any given plane? (Aside from this problem, I'd like to know, there has to be a formula for determining the CG on any given plane.)) We were putting floats on an Avistar yesterday and set them up per manufacturers instructions and the CG was different than it was before. We called the manufaturer tech line and was told it was fine. My thought was " Why would the plane fly as it did before the Cg was changed? Wouldn't any CG change result in a difference in flight characteristics?" The tech said no. Hmm. I'm not convinced yet. We have yet to fly it also. Thanks.


PS, they were 33" Bee Gee floats.

[Sparky Paul]

Your suspicions are correct!
The advice you received is wrong.
Place the c.g. where it was, without the floats when adding floats.

[Andy W]

What Sparky said. They were wrong.
..a

[mkirsch1]

The techs are SO WRONG!

The CG of an airplane depends on the wing design. Did the wing design change when you added the floats? Did it somehow magically change shape? Nope. Why would the CG be any different?

[spark363]

Thanks all for the replies. I didn't think the tech was right. Didn't make any sense to me. It's not my plane, but I helped to put them on. I'll make a call and make sure that the CG is correct before he flies it. Drat...(sorry about the language)...now to drill more holes in the fuselage. Oh well, better that than building a new plane. Thanks again all.
Fred.