THE use of the C.G. as the datum to which all moments involved in the trim relations are referred is doubtless the result of a convention widely used in mechanics generally and having one advantage, i.e., that the moment due to gravity is zero. In general mechanical problems where the position of the C.G. is normally a fixed characteristic of the body whose motions are under consideration, the C.G. has a good claim to be a convenient datum. In the case of the aeroplane, where the C.G. of a particular aeroplane is rarely found to be in the position estimated for it—even under carefully defined conditions of loading—it is subject to considerable movement with variation in loading; and, in different aeroplanes, may occupy the most various positions relative to the organs which produce the moments involved—the use of the C.G. as a reference point for trim estimates has very serious disadvantages.
Sayers, W.H. (1936), "Longitudinal Trim Relations: A Plea for the Use of the Quarter Chord Point as the Datum in Place of the C.G.", Aircraft Engineering and Aerospace Technology, Vol. 8 No. 9, pp. 241-244. https://doi.org/10.1108/eb030090
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