RC Specialties;

As you can see from your responses most modelers are satisfied with the trial and error method, probably because it's simple, and they're not so inclined to work with mathematics.

But there are some of us who do use math to get answers - and rather simple, just by knowing that for an object to balance about a point, the summation of moments about that point must be equal to zero.

Example: Say you airplane is tail heavy, weight 5 pounds and is off the desired CG by 1 inch. I have a place in the nose that is 8 inches forward of the desired CG, so how much weight do I need to put there? The tail heavy moment is 5 pounds x 1 inch = -5 inch-pounds. You need +5 inch-pounds to balance, so you will need X pounds at 8 inches to balance 5 inch-pounds. Then X = 5/8 pounds or 10 oz of lead.

As a model designer, I use weight and balance calculations during the design process so that the airplane will balance as close as possible at a desired CG without the battery installed - so that the battery can be moved around to fine tune the CG. About 2 years ago, I used this process for my first foamie and it worked out fine. Here is the weight and balance sheet from my notes that you might find interesting. BTW the Kaos has had many flights and is still flyworthy today.

feihu