How do I find where CG should be?
The best way obviously is to follow the manufacturer recommendations, but these are not always spot on. Do a google search for forums with threads on your model, and find what others have found to work best too.
If you can’t find any info, or the model is scratch built and has no info, you’ll need to calculate where your CG should be (Rule of thumb is 25-30% MAC). There are decent calculators available here and here. Once you have your ‘best guess’, you just have to fly and see. But if it doesn’t feel right, how do you know what to adjust?
Well, if your plane porpoises up and down, is hard to control and keeps stalling at low speed, it is likely tail heavy. If you are running out of up elevator trim it might be nose heavy. But this is a good way to check:
A nose heavy plane will need up elevator to compensate. A tail heavy plane will need down elevator, to hold up the tail. Make sense?
So, trim for level flight at about half throttle. Get some height and push the nose down into go into a 45 degree dive. don’t change the throttle or trims, and let go of the sticks for a moment.
As the model increases in speed, the elevator gets more authority, and the trim you put in to get level flight (and compensate for CG) increases.
If a tail heavy state was compensated for by down elevator, the model will tuck down further into the dive as speed increases. The reverse will occur for nose heavy and the model will suddenly climb out of the dive as speed increases.
If the model dives straight, and slowly begins to climb out, your CG is pretty spot on.
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