[airguy]

Does anyone know offhand what the location of the firewall on a 9A is in reference to the CG datum? I'm working up a spreadsheet to determine how much weight I need to move, and how far aft, to compensate for an added 17 pounds of engine/prop combo up front by using the IO360 and WW200RV prop (compared to standard O-320 and FP prop).

I have several items which can be mounted aft - like battery, transponder, ELT, strobe power pack, ADAHRS, oxygen bottle, etc - but the question is which items go where. I know their weights and I can calculate their moment offset, but I need a good reference to calculate the standard moment of the battery (on firewall) and the moment arm of the added engine weight. If I had the firewall arm location that would be a snap - otherwise I'll just have to go level the airplane and take a bunch of careful measurements. I'm hoping someone has already done this for me....

 

[Mel]

The datum is an arbitrary reference line. It can be anywhere you want it to be. You can use the firewall as the datum if you want.
Personally I use the leading edge of the wing as the datum.

 

[airguy]

Ok, I guess I see your point, I was looking at VANS factory numbers and they use the datum as a point 70" forward of the wing leading edge. I was going to calculate all moment arms based on that location, but from the math perspective you are correct, it's arbitrary.

Now, where did I put my pencil?

 

[DanH]

Ok, I guess I see your point, I was looking at VANS factory numbers and they use the datum as a point 70" forward of the wing leading edge. I was going to calculate all moment arms based on that location, but from the math perspective you are correct, it's arbitrary.

Now, where did I put my pencil?

So measure forward from the leading edge, to the firewall, subtract that from 70, and, well, Bob's your uncle......

Seriously, use the standard datum so all your numbers are positive.