ERCOUPE 415-E with O-200 field approval, no STC

Off subject but… “STC authorized field approval using a form 337” does not make sense. A “field approval” does not require an STC. And an “STC” does not require a field approval. But an STC does require a 337. With an STC you do not need the FAA to fill out the field approval section of the 337 block 3. Just an IA to inspect and approve the major alteration. Hence block 3 on the 337 should not be filled out with the FAA stamp.

One way to figure it is to take the old weight and balance, get the weights for the C-85 and prop, and the O-200 and prop, and the cg location for both, then just do the math and see what you come up with, just remember, anything forward of the firewall is a negative number, and your distances are in inches from the datum.

How should added weight forward of datum be calculated? Engine weight is 190lbs, and arm is 1 so moment is 190 for o-200. I added difference to total weight and to total moment. Divided total moment by total weight to get new CG.

don't forget the engine is forward of the datum,

I was able to find in maintenance records of the STC authorized field approval using a form 337, along with W&B change way in the back out of order. They subtracted 170lbs and added 170lbs as if both engines weighed the same. An A&P I talked to said o-200 is 20lbs heavier so I added that difference to weight, multiplied that by arm and added difference to moment. The new CG calculated shows way less aft and ultimately fixed the discrepancy I had. I can now confidently carry a passenger and 50 lbs of baggage if I use no more than 12 gallons of fuel as prescribed in current weight and balance. I still can’t carry 65lbs the max baggage weight with a passenger but ForeFlight assumes all tanks burn evenly. The nose tank would remain full though but no way to tell w&b on ForeFlight that. I haven’t had airplane weighed but I may on next oil change.

Have you checked the weight difference between the C-85 and O-200? have you had the plane weighed ? I would be suspect of any numbers until you weigh the plane yourself.