CG for Rudderbug

The RudderBug has a constant wing chord if memory is working. Therefore, the normal formula should work. Just place the CG about 30% from the leading edge or at 30% of the area.

Close, but not exactly. The tips are tapered on the trailing edge. I have a Royal Rudder Bug and the plans somewhere. I'll take a look. BTW, there are two Rudder Bugs, the Walt Good one and the Royal version. The Royal version is a little smaller than Walt's.
You could goto the Yahoo Group VRCS (Vintage Radio Control Society) and ask the question. Several fellows there would know.
Dan Thompson

Dan, I know that tip shape effects stability and tip stall but nowhere have I ever read or heard of it effecting the CG. Now if you have a flying stab.. that would change things alittle.. general rule of thumb is balance between 28% and 33% of the wing area...

I agree with your thumb. However, I just put down the Berkley, Royal Rudder Bug plans which has the 62 inch wing. The wing cord at the fuselage is 10 inches and they show the CG at 2 inches from the LE. This is a 20% location. Boy, that blows my mind. It is shown as a rudder only, but all the rudder only planes I flew in the 60's had the CG real aft, like tail heavy. I have the plane and have not flown it as yet. I don't think I will balance mine at 20%. I like your figures better.

Like I said, if it has a full flying stab that would make a difference and also make some sense since the flying stab would move the CG forward.. I thinks???? But a Goldberg skylane does have a flying stab and the CG still falls within the 28-33% range.

Wing shape, including tip profiles, DO affect CG location. If the trailing edge only is tapered, the centre of lift (C/L) of the airfoil moves forward along with the forward-swept TE. If the CG isn't adjusted to accomodate the more forward C/L, the airplane will be tail heavy. Any tapered or elliptical wing, or any swept wing, for that matter, will present more hassle in determining CG location. Swept wings especially can be difficult, since the centre of lift moves outward at higher cruise speeds (due to smaller tip vortices) and therefore back somewhat, making the airplane act noseheavy. Airliners, since they have swept wings, have to be carefuly monitored when flying at max cruise because the C/L moves back far enough that the stabilizer may not be able to raise the nose if the speed got any higher, especially if the airplane is loaded with the CG near the forward limit. They call this speed range "coffin corner," for obvious reasons. Most models have rectangular wings, with no sweep, to make things easy to build and to balance. The 25-33% CG rule works ok for these. Like full-scale, though, if a guy wants to get more efficient or go faster, the rectangular wing isn't going to cut it.
Dan