I'm trying to find the calcs for determining where the CG should be on a triplane. Should I use the LE of the top wing & the TE of the bottom wing (they are different spans) and then calculate as if it's a monoplane? The plane is a Ziroli DR1.

Thanks for any help
Steve-S

Yes.
Look at the plan view. The chord for the c.g. computation is from the leading edge of the top wing to the trailing edge of the bottom wing.

Thanks for the reply. That's the method I used and hope to maiden it tomorrow if I can get the engine to run.

Steve-S

Calculating the CG on a plane is similar to balancing a fat kid and a skinny kid on a seesaw.
First, figure out the total area of your wing(s) and your horizontal stabilizer. For example's sake, let's say you have 1560 sq. in. of wing and 120 sq. in. of stab.
Then measure the distance from the center of lift of the wing to the center of lift of the stab. The center of lift of each surface is about 30% of the distance back from the leading edge, for a straight wing. (Tapered and delta wings use slightly different calculations, but approximations will do for now.)
Let's say that distance is 30 inches.
So, you have a seesaw that's 30" long, the fat kid "weighs" 1560, and the skinny kid "un-weighs" 120. That's because the horizontal stabilizer is actually pushing down, not lifting, so we just reverse the sign on the stab's area. Now you set up a ratio to establish where the pivot point would be. The equations are:
1560 x A = 120 x B and |A| + |B| = 30 , so
|B| = 30 - |A|
1560 x |A| = -120 x (30 - |A|) = -3600 + 120x |A|
1560 x |A| - 120 x |A| =3600
1440 x |A| = -3600
|A| = 3600/1440 = 2.5 inches
B = 27.5 inches
So the "pivot" or center of lift is 2.5 inches back (toward the stab) from the center of lift of the wing. Or 27.5 inches forward of the center of lift of the stab.
But the center of MASS (balance) should be 10% - 12% (of the distance between wing lift centers) FORWARD of the center of lift, which means it should be placed 3.0 inches forward of the point that was 2.5 inches aft of the main wing center of lift.
That means the CG should be approx 0.5 inches forward of the 30% mark of the main wing, for this example. As you can tell, the exact center of lift of any airfoil will depend entirely on the specific airfoil in question, but this method gives a good place to start the maiden flight. [8D]

And you thought you'd never need Algebra, didn't you? [X(]

All Bad_Daddy said is true however.....................

Go to geistware.com with your measurements and type them in. Hit the "DO ALL THE FIGURING FOR ME" button and Taa Daa!!!!!!!!

You need a couple of very easy to measure measurements. Look in the light purple box on geistware's page for what they are.
And it's blinding fast to plug in a couple more stability margin values and almost instantly have everything refigured for you. You plug in 20% SM to see where the forward CG would be, then a 10% to see where the rear would be, and you got the CG range for your airplane. In a heartbeat.

http://www.geistware.com/rcmodeling/cg_super_calc.htm

da Rock. Thanks! I never knew that calculator was there, and it's now one among my hundreds of bookmarks.
By the way, if one were calculating CG for a canard plane, I believe entering the LE / LE distance as a negative number produces the correct results. Do you concur?

Oh, never mind the silly question. I just found a Canard CG calculator at [link=http://adamone.rchomepage.com/cg_canard.htm]http://adamone.rchomepage.com/cg_canard.htm[/link]