In general... (Off-Topic)

by Mid7night , Rocket BSCHSHCSHSHCCHGGH!!!!!!, Thursday, September 10, 2015, 13:54 (3449 days ago) @ cheapLEY
edited by Mid7night, Thursday, September 10, 2015, 14:15

How hard is the math for calculating CG?

I was in the Air Force for a time, working on the C-5, C-17, and KC-10. We had to figure CG when jacking the aircraft, but we just had a handy chart and plum line that did most of the work for us. Never had to do it from scratch before.

AH, I see what you're asking now. The real thing would have a similar loading chart and CG range limits, but there really aren't any big moving masses on an RC plane, especially a glider, and every build is going to weigh a little different.

For kits, the CG location is usually already calculated for you and given as a simple distance measurement from the wing root leading edge. You just build and then ballast to balance on the CG.

If it's not specified, the rule of thumb starting point is to put the CG at the 25% point of the Mean Aerodynamic Chord (MAC). This sounds more complicated than it is, and there are several online calculators that will do the math for you based on your wing dimensions.

Once you mark the desired CG location, it's just a matter of moving mass around to balance the plane on that point.

But after you've built a few (hundred) planes, you get an eye for where the CG ought to go. Most of the time that's good enough, because even if the CG is where it mathematically "should" be, you usually want to move it somewhat to tailor the flying qualities. But you've got to throw it to know it!