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> I'm using the tip of the spinner as my ref datum so each time I > move the engine fwd so moves the datum and I'm not sure if that is correct. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ I don't see a problem with that. Each time the engine moves forward so does the distance of your new "arm" for calculating the new moments (weight on the three scales). As long as you are using the new "arm" distance with the new weight on the scales, you should be getting an accurate measurement of the C.G. Using the most forward location will keep all your numbers positive and simple. Any future calculations will be valid as long as you don't change the "arm" dimension, i.e. change length of the spinner. If you pick a location 5 feet forward of the firewall, beyond the most forward aircraft component, and use the correct arm distance and moments (scale weights), you should get the same exact numbers (CG location of the aircraft). As always, your results may vary. :-) Larry Flesner
