In truth, the formulas cut out the guesswork.

You want to know how much Horizontal Tail Area you need, right. You know your MAC. Lets say it's 11" for example. You can easily figure out your WingArea. Let's say it's 1100sq.in. And for the example, let's say the TailMomentArm is 36".

(3 x MAC x .20 x WA) / TailMomentArm = HTA

so..........

3 x 11 = 33
33 x .20 = 6.6
6.6 x 1100 = 7260
7260 / 36 = 202

If you wish to have an average size horizontal tail, then it'll need to be about 202sq.in.

Now all the numbers used in the example were guesses, because your parts weren't available to measure. You've got the old airplane size to work from, and most of the parts. No guessing there other than what your old TailMomentArm was if the fuselage (or parts) aren't still around. In fact, if you have all the dimensions from the original, you can plug them into software like geistware, and it'll give you some figures to start with that make it easy to recreate the performance of the original wreck with just changing the TMA length. That does take a touch of understanding of the formulas, but it's not really hard at all. Get a couple of measurements from the old bird and you can easily work out what changes would be needed with the new fuselage.

Tell ya what..... if you'll provide the numbers, I'll show you how to use something like the above formula and geistware to give you reliable numbers for the new bird.

What