The datum line can be any point you want. It's just a reference.
I personally use the leading edge of the wing. In level flight attitude the leading edge of my wing is exactly in line with my wheels. Makes the math really easy. Anything multiplied by "0" is "0".
The datum line can be any point you want. It's just a reference.
I personally use the leading edge of the wing. In level flight attitude the leading edge of my wing is exactly in line with my wheels. Makes the math really easy. Anything multiplied by "0" is "0".
Note that the CG range is expressed in reference to the designers datum. If you change the datum, you must change the envelope reference dimensions similarly.
Also be careful to measure the actual landing gear axle distances from the datum, rather than just using the book numbers. Mine were different enough that it made a noticeable difference in the CG calculations.