[dbuds2]

Has anyone done any actual measurement of the effect of adding/removing weight at the engine location and how that shifts empy CG?

I'm looking at a custom mount of a slightly heavier engine (+20lbf) and trying to figure out how much to move it aft to keep the empty CG in the same location.

 

[David-aviator]

Has anyone done any actual measurement of the effect of adding/removing weight at the engine location and how that shifts empy CG?

I'm looking at a custom mount of a slightly heavier engine (+20lbf) and trying to figure out how much to move it aft to keep the empty CG in the same location.

I've done quite a bit of CG calculation. It's easy with a spread sheet.

For sure if you add 20 lbs in the engine compartment the CG will move forward. Moving the entire mass toward the firewall will minimize it, but it will move forward.

You really won't know what's going on until the airplane is built. Pre build calcuations are a best guestimate. Five to ten pounds of lead at the HS will go a long way toward putting the CG where you want it with a heavy engine. With a light weight power plant, you may have to install a monster battery on the firewall to get it comfortable.

 

[Kevin Horton]

If you add or remove a weight at a given arm, the effect on the empty CG is equal to:

(weight added * distance from CG)/empty weight

If you move an existing item fore or aft, the effect on the empty CG is equal to:

(weight of item * fore or aft distance that item is moved)/empty weight

So, for example, if the 20 lb item that you add is 30" ahead of the CG, and you expect an 1100 lb empty weight, then the CG will move forward by (20 * 30)/1100 = 0.55".