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What is the correct formula to calculate the CG position as a percentage of the mean aerodynamic chord if the CG is at a distance L from the datum?

MAC: the length of the mean aerodynamic chord
LEMAC: the distance of the mean aerodynamic chord leading edge from the datum
TEMAC: the distance of the mean aerodynamic chord trailing edge from the datum

  • A

    ((TEMAC - L)/MAC) x 100

  • B

    ((L - LEMAC)/MAC) x 100

  • C

    ((TEMAC - MAC)/L) x 100

  • D

    ((LEMAC - L)/MAC) x 100

Refer to figure.
Generally, the Centre of Gravity (CG)is expressed as a distance (L) relative to a datum or reference point, but CG is usually expressed in swept wing aircraft as a distance relative to the Mean Aerodyanmic Chord (MAC) of a wing. To be more specific, the CG is expressed as a percentage of its position along the MAC (%MAC) from the leading edge (Le), also called LEMAC.

The MAC is the chord at the aerodynamic centre of the wing, not exactly half way to the tip, but close to it.

If the CG is at a distance L from the datum, then the correct formula to calculate the CG position as a percentage of the MAC is: ((L - LEMAC)/MAC) x 100.

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