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Old Oct 05, 2012, 12:10 PM
BMatthews is offline
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The 'Wack, BC, Canada
Joined Oct 2002
12,824 Posts
It's a common mistake to think that these older designs need a more forward CG location when you convert to RC. But as the replies above indicate it just ain't so.

One trick I will add is that with the common yet still stable rearward CG's as mentioned comes added sensitivity to the elevator. Don't confuse this with any instability. You'll simply need to reduce the amount of elevator throw by using the outermost holes in the control horn to get back to a harmonius elevator response.

One way to fine tune your CG position after the first few flights is to perform dive tests. This is where you trim for a nice floating glide and then push forward and hold a 30 degree dive for a second or so then release the stick. If the model raises the nose through level in a short time and distance then you are still overly "stable" and can shift the CG back more and re-trim the elevators for a good glide. When it gets to where it almost seems hesitant to recover then you've gone too far the other way. What you want is a nice positive but LOOONG recovery where from stick release to the nose coming through level for the first pullup it takes about 100 feet of altitude. That sort of stability is close to optimum for glide performance and lowers the amount of downthrust needed for power on.

What you're doing is playing with moving the CG back and forth relative to the overall aircraft neutral point and then compensating with the amount of wing to stab decalage to re-trim. The idea is to end up with the least decalage that still gives you a positive stability value. The CG location simply needs to be matched to this decalage setting to ensure a good glide. But we set the CG first than alter the decalage via the elevator trim since it's easier to do so.
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