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With a swept wing aircraft (specifically a MiG-25 I am modelling) where is the root chord taken from to calculate the location of CG? Is it the root chord of the wing (where it attaches to the fuselage) or a projected line extended from the the leading / trailing edge to the centreline of the fuselage?

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  • $\begingroup$ Hi and welcome to the site! I have edited your question a bit to make it clearer, you can roll back to edit if you meant it differently. $\endgroup$ – AEhere supports Monica May 13 '19 at 9:16
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It's expressed in Mean Aerodynamic Cord. On a swept back wing, you can construct it as follows.

From Torenbeek

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  • $\begingroup$ Hi. Many thanks. $\endgroup$ – Lee Cowen May 13 '19 at 13:23
  • $\begingroup$ The centre line of the aircraft is used regardless of the fuselage width? eg if the aircraft has a really wide body and stubby wings, the centreline of the fuselage is still used for the calculation? $\endgroup$ – Lee Cowen May 13 '19 at 13:24
  • $\begingroup$ @LeeCowen Not sure I understand your question. $\endgroup$ – Koyovis May 13 '19 at 13:26
  • $\begingroup$ Sorry - i'm not explain myself well. I will try to put it another way. If two aircraft have the same size and shape swept wings, but one aircraft has a wide fuselage so the wings are far apart, but the other has a narrow fuselage so the wings are closer togehter, would the CG be the same place on both? $\endgroup$ – Lee Cowen May 14 '19 at 7:43
  • $\begingroup$ Oh OK. CoG of the fuselage would be at the same (X,Y,Z) location. CoG of each wing would have the same X and Z location. Each wing would have an offset in Y-direction, one has $ \Delta Y $ and the other $- \Delta Y $ which cancel each other out...so yes, the total CoG would be at the same location. $\endgroup$ – Koyovis May 14 '19 at 9:37
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For CG calculations on swept wing aircraft you don't use root cord, you use mean aerodynamic chord or MAC. To calculate the MAC:

  1. Identify the center of gravity location, in inches from the datum.
  2. Identify the leading edge of the MAC (LEMAC), in inches from the datum.
  3. Subtract LEMAC from the CG location.
  4. Divide the difference by the length of the MAC.
  5. Convert the result in decimals to a percentage by multiplying by 100.
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