Installing lighter engine, how far forward should I move it?

I found a lighter engine for my plane that I would like to install. I know the weight and arm distance from the firewall to the CG of the old engine so I thought I would take that moment and divide it by the new engine weight to give me the arm for the new engine CG. Then to test my theory I moved the "firewall" back 10" and ran the same numbers and it gave me an answer much greater than 10" more for the new engine. Shouldn't the result been the same? Am I overthinking this?

Is there a simple formula to figure the new engine arm? Any help will be greatly appriciated.

Rick

Eh? What do you mean you moved the firewall back? You don't want to change the datum during the calculations. If the datum is the firewall, then you can divide the moment by the engine weight. This will give you what the arm needs to be for the new engine. Of course, you have to determine where the center of mass of the new engine is so you can place it on that arm.

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Originally Posted by Rick n Sharon

I found a lighter engine for my plane that I would like to install. I know the weight and arm distance from the firewall to the CG of the old engine so I thought I would take that moment and divide it by the new engine weight to give me the arm for the new engine CG. Then to test my theory I moved the "firewall" back 10" and ran the same numbers and it gave me an answer much greater than 10" more for the new engine. Shouldn't the result been the same? Am I overthinking this?

Is there a simple formula to figure the new engine arm? Any help will be greatly appriciated.

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Rick, is this a pusher installation, correct? What you did sounds correct, you want the moment to be unchanged to keep the aircraft c.g. unchanged. So it's weight x arm of old engine = moment and that moment / new engine weight should give you new arm. For this exercise, you can use any fixed location on the aircraft forward of the engine for an "arm"

Double check your arithmetic? What was the weight and arm of the old engine?

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Originally Posted by martymayes

Rick, is this a pusher installation, correct? What you did sounds correct, you want the moment to be unchanged to keep the aircraft c.g. unchanged. So it's weight x arm of old engine = moment and that moment / new engine weight should give you new arm. For this exercise, you can use any fixed location on the aircraft forward of the engine for an "arm"

Double check your arithmetic? What was the weight and arm of the old engine?

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It is a tractor config. and I figured it out using the tetter-totter example. When I changed the "firewall" (datum) point the ratio of the two stayed eaxctly the same. Blame it on staying up too late, in the morning it was very clear... ;-)

Thanks anyway for the reply.

Yes, I should have said datum point. I thought it over again this morning and it was clear my error.
Thanks anyway.