Model Forum / Radio Controlled / Air Models / August 2004

Of course, for a plane with a completely rectangular wing like the
Duraplane, you don't need any sort of calculator.  Just put the CoG
25-30% of the way back from the front of the wing.  You don't even
need to carefully measure it -- eyeballing it's usually good enough,
though using a ruler isn't a bad idea.  If it's a plane you've never
flown before, put it 25% forward, then fly and start thinking about
moving it back, depending on how it flies.  The further forward the
more stable the plane becomes, but the more elevator throw you need to
keep the nose up, and the less aerobatic the plane becomes.  If it's
too far forward, you won't even be able to get the nose up.  As the
CoG goes back, the plane becomes more nimble and more efficient, but
also becomes more unstable, and at some point it'll become unflyable,
going into uncorrectable spins at the slightest provocation.

This 25-30% rule of thumb applies to any rectangular wing.  If your
wing is swept or otherwise non-rectangular, you can use the
calculator, though a little experience will let you eyeball most of
those too.

It's only when you start dealing with precision pattern flying, or
flying wings that you need to be *really* precise about where the CoG
is.  The longer the distance between the wing and the tail, the less
critical exact placement is.  (Flying wings don't have a tail, which
is why it's so critical.)

25% is not a "more forward location" however. It is the standard recommended
starting point for most any typical plane regardless. I've also never had a
stability or snapping issues setting up a heavily wingloaded plane to 27% as
well. Now I request that you please keep responses on the NG only, and try
to restrain yourself from adding to the spam that already overloads my
inbox. Thanks Dr!