CG for flat wing

[gearup]

well once again I am living proof of an idiot and his plans are son parted.. I have a 30" flat wing spitfire I built out of a pull out mag plans. I modified it a wee bit ( ok a LOT!!) and now its finished, I need some imput for locating cg for initial flight. 17 oz auw, mean chord of 7", ws of 30. I am guessing 25/28% of MAC for initial cg.. ( am I too far aft ?) FS aircraft usually run this, but this IS an elliptical wing... MR has said I need to be at 20% ( thats 1.3 ") and I feel thats a little too far fwd ( but he DOES design great flyers sooo..)


thanks

JM

[Salto]

JM,

If you've calculated the MAC correctly, the fact that it is an elliptical wing won't matter. How far behind 25% MAC you can go will depend on the stab area and tail moment. You could search here for the formula to calculate tail moment co-efficient and put your numbers in there to see how the design looks. Then make a judgement how far back to place the C/G.

Graham.