# Is this method to calculate a biplane's center of gravity using 57% of the biplane's gap and 23% of each wing's MAC correct?

Once you have the MAC for both wings and the CG balance point as a percentage of the MAC. connect the two CG positions with a line. Measure up between the wing Gap distance and place your center of lift (CL) at the 57% reference line. Project this location up and balance your biplane at this postion (see red lines). A CG forward of the CL makes the model more stable. Placing it aft of the CL makes model less stable.

Source: Modelairplanenews.com

I'm trying to use this method for a Bucker Jungmeister model. The gap is 12.5 in. and the top wing MAC is 9.44 in. and the bottom wing MAC is 9.9 in. Is this the best method?

• The graphic came from Model Airplane News. modelairplanenews.com/balancing-biplanes Feb 16, 2023 at 23:42
• I think that that 57% is just the average of the two MACs. If both MACs are equal (let's say 1) then the average is 0.5 i.e. 50%. If the upper MAC is bigger then the average is closer to it, otherwise the other way around. In your case it should be less than 50% since the lower MAC is bigger. Feb 17, 2023 at 9:28
• I think I found the answer. It seems a little more complicated. I'm grappling with the variables and math found in this graphic on Quora I found. It's from a gentleman by the name of Krishna Kumar Subramanian. quora.com/…. The use of wing area seems similar to derive a Gap. I'll try to add the graphic somehow. I'd like to know the combined MAC value. Feb 17, 2023 at 17:19
• I also found this simplified calculation method that comes from "Balancing Biplanes by Dimitrios Katsoulis" rckavalaacroteam.com/balancing-biplanes I am suspecting that both simplified methods are sourced from the more complicated formula demonstrated by Mr. Krishna Kumar Subramanian. Hopefully someone more knowledgeable will confirm this. Feb 17, 2023 at 17:28
• Well I was almost right: it's the average indeed but based on the wing areas instead of the chords 😉 Feb 17, 2023 at 18:57