At 07:55 PM 4/1/2016, you wrote: >Sure, it's a sum of moments either way. But why would you want to change >not only the changed weights, but every other distance in the spreadsheet >(or worse, your W&B sheet)? Now THAT's some bothersome math, adding >whatever your new arm is to all those moment arms, rather than just the >moment arm of the engine alone. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
I wouldn't be concerned with his spread sheet until he determines where he wants the motor. A blank sheet of paper will work to determine that. It's important to know the "arm" from your new datum to the location you want for your empty CG. That's not a problem, how far forward did you move the engine? Add that to the prior arm and you have the new arm. The only calculations he needs to determine is the weight on the three scales times the new arm, then divide the total moment by the new arm. Move the motor and do it again until he gets the CG where he wants it. Once he fixes the motor he can put the correct numbers on his spread sheet. It need not be all that complicated. Kids on a tetter todor do it without even thinking about it. :-) Someone mentioned using the firewall to keep the numbers positive. That only works on a taildragger. On a tri-gear, anything forward of the firewall is negative. Select a datum, even beyond the nose , spinner, whatever, of the aircraft and your numbers will always be positive, whether it's a Cub or a 747. Larry Flesner
