I need building advice!

This can be a pretty complex design issue. If you're not interested in designing airplanes then the answer is -- wherever the designer put it. If you _are_ interested in designing airplanes then do a web search on "tail volume coefficient" -- I know this was in magazines, I dunno if it's made its way to the web yet. The idea is to find a plane who's pitch stability you like, then copy its TVC and CG placement.

You generally have quite a bit of latitude with the wing location, as long as it balances. If you have the wing at the design location and it balances -- go fly.

On most model aircraft with a conventional layout you want the CG to be

25 to 30% back from the leading edge. With a longer tail moment or larger tail you can go back farther, with a shorter tail moment or smaller tail you must go forward more (a straight-chord flying wing needs the CG to be between 19 and 22%, depending on whether it's high wing or low). Do a web search on "Heintz" and "Stability" and you'll find a paper on finding the CG for full-scale light aircraft written by Chris Heintz. It's for homebuilders, so it's pretty accessible by modelers.

I think you will find that for a flying wing, its as low as 16% of chord..for conventional arrangements it can go much further..well aft of the trailing edge..in cases where there is a large or remote tailplane. Continue in this way towards two equal wings, or a canard, and its a moot point as to which is the main wing anyway.

The rule of thumb 'between 20% and 35% of chord' only suits more or less conventional-looking planes.

What I usually do with a new airplane is initially balance it at, or a little in front of, the recommended CG range noted on the plans/instruction sheet. This follows the reasoning that it is better to be a little nose heavy than a little tail heavy. Then I slowly adjust the CG aft in 1/4 inch increments over several flights to the aft point of the recommended CG range.

If you didn't get any instructions when you bought it on ebay you should do a search for it on Yahoo, Google or some other search engine. Most manufacturers list the manual on their site.

Here is Duraplanes site:

and they do show the manuals are available with each airplane they make.

Chris Heintz's paper on stability gives a formula that's valid for anything from conventional to canard to flying wing.

I can believe the 16% rule for flying wings on a model at least. It would increase the speed at which you could keep the nose up, however.

If you want the info Tim gave you - and more - in a book you can sit in your easy chair and read, check out "Basics of R/C Model Aircraft Design" by Andy Lennon, published by Air Age Media, the same folks who publish Model Airplane News. Once you absorb what's in it, you'll be able to actually design models.

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