ORIGINAL: modawg73
COG help please!
I was trying to get her ready to maiden this weekend, but had an unexpected thing happen with the center of gravity. I put her on a great planes balancer at 3 3/4" like specified, and she was significantly nose heavy. I had to add 5 1/2 oz. of lead to the tail to get her to balance. Since I had a temporary setup (i didn't have the final mufflers, so was using the factory muffler for the saito 62s) i didn't feel like adding the lead until I was at the final setup.
A couple of quick questions that I would love your perspectives on.
First, I balanced her with retracts in the up position and with gas tanks empty. Is this the best way to do it? (first model with retracts that fold front to back) The manual doesn't specify whether to do it with the retracts up or down.
Yes, you balance with the wheels up and tanks empty.
Second, i have 2 5-cell AA packs in the nose. Has anyone mounted the batteries anywhere else. I am thinking of moving the packs back to the wing center section. I am thinking it is best to balance without adding weight if you can.
Move the packs as far back as needed. Use heavy wire extensions. You're absolutely correct about dead weight.
Third, I am of the opinion that a COG too far forward is a livable situation, but too far back makes for hairy flights. How far would I have to go in front of the specified COG before I would be in trouble? Is it possible to have it too far forward to be flyable?
Ken
Find out where the CG range goes. Best way to do that is to measure the model, plug the measurements into the application listed, and run the application a couple of times. Using 5% margin will tell you how far back the CG can go, and 20% will tell you how far forward. The CG location affects the elevator sensitivity. If you choose to move the CG forward of the mfg's suggested, then rig your elevator to have more throw than the mfg's suggested throw. If you move the CG aft, then rig the elevator to have less throw. As long as the CG is in the range the airplane should fly reasonably well. But in your case, moving the batteries back will help quite a bit. Using geistware will reliably predict what can be done to help also.
http://www.geistware.com/rcmodeling/cg_super_calc.htm