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Center of Lift for Biplanes with a stagger and unequal wing chords

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?

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  • $\begingroup$ The graphic came from Model Airplane News. modelairplanenews.com/balancing-biplanes $\endgroup$
    – Wally
    Feb 16 at 23:42
  • $\begingroup$ 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. $\endgroup$
    – sophit
    Feb 17 at 9:28
  • $\begingroup$ 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. $\endgroup$
    – Wally
    Feb 17 at 17:19
  • $\begingroup$ 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. $\endgroup$
    – Wally
    Feb 17 at 17:28
  • $\begingroup$ Well I was almost right: it's the average indeed but based on the wing areas instead of the chords 😉 $\endgroup$
    – sophit
    Feb 17 at 18:57

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I think I found the answer to my own question. I mentioned it in the comments above. It's found on Quora by Mr. Kishna Kumar Subramanian.

This graphic seems to show the more complicated variables used to locate the gap and combined MAC calculated

This graphic shows CG after finding MAC

Source: https://www.quora.com/How-are-the-calculations-for-a-biplane-different-from-a-conventional-monoplane

This graphic allows estimation for "e" used in above formulas

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  • $\begingroup$ I completed these calculations and was left with the problem of locating the MAC by distance or by the sweep at MACd. I'm looking for this small detail. In the meantime can anybody direct me to calculations for Biplane MAC distance from root and sweep from root datum line at the MAC location? Thanks Wally $\endgroup$
    – Wally
    Mar 4 at 18:33

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