So I finally got my plane back together enough to weigh today, and while the weight was in line with what I expected, the moment arms (and thus CG) were not. For reference, my -4 is on short legs, has the original non-swivel tailwheel, Concord battery in the box between the rudder pedals, heavy starter, automotive alternator, and a standard weight saber 4” extension with a three blade Catto on the front.
For context, the original W&B performed by the builder had the following measurements, taken with a two blade wood prop and without engine oil. This was on the old 50” datum form.
Right wheel 438.75 pounds, 49.375” aft
Left wheel 445, 49.5” aft
Tailwheel 59.5, 227” aft
Total 943.25 pounds empty, 60.64” empty CG
In my ownership period I’ve added some avionics and landing lights, and I also weighed the plane with my normal engine oil level. All the weight I added was at or slightly forward of the CG (instrument panel forward)
With the 60” datum and 7 quarts of oil and unusable fuel I measured the following:
Right wheel 450, 61.5”
Left wheel 461, 61.5”
Tailwheel 53, 238.5”
Total 964#, 71.18” empty CG.
The big difference is my main wheel moment arms. Where previously they were slightly forward of the wing LE, now they are 1.5” aft. I did remove the mount and legs prior to get cracks on my mount addressed, but ended up replacing it with basically the same shims that I took off (I aligned the axles by measuring distance from the LE). That doesn’t explain the large discrepancy.
Could I get a sanity check on my numbers? If I somehow mis-measured my moment arms I significantly threw off my CG. Conversely, perhaps the builder didn’t have the plane perfectly level? I noted when I aligned the gear during the engine mount reinstallation that with the tail down the axles are approximately 4.5” forward of the leading edge. Speaking with a couple other owners of short leg -4s it seems they also measured the axles slightly aft. However, I note that the sample W&B in the manual from the prototype also list wheel axles forward of the CG. Maybe it’s as simple as having rolled the plane forward onto the scale vs backwards?
For context, the original W&B performed by the builder had the following measurements, taken with a two blade wood prop and without engine oil. This was on the old 50” datum form.
Right wheel 438.75 pounds, 49.375” aft
Left wheel 445, 49.5” aft
Tailwheel 59.5, 227” aft
Total 943.25 pounds empty, 60.64” empty CG
In my ownership period I’ve added some avionics and landing lights, and I also weighed the plane with my normal engine oil level. All the weight I added was at or slightly forward of the CG (instrument panel forward)
With the 60” datum and 7 quarts of oil and unusable fuel I measured the following:
Right wheel 450, 61.5”
Left wheel 461, 61.5”
Tailwheel 53, 238.5”
Total 964#, 71.18” empty CG.
The big difference is my main wheel moment arms. Where previously they were slightly forward of the wing LE, now they are 1.5” aft. I did remove the mount and legs prior to get cracks on my mount addressed, but ended up replacing it with basically the same shims that I took off (I aligned the axles by measuring distance from the LE). That doesn’t explain the large discrepancy.
Could I get a sanity check on my numbers? If I somehow mis-measured my moment arms I significantly threw off my CG. Conversely, perhaps the builder didn’t have the plane perfectly level? I noted when I aligned the gear during the engine mount reinstallation that with the tail down the axles are approximately 4.5” forward of the leading edge. Speaking with a couple other owners of short leg -4s it seems they also measured the axles slightly aft. However, I note that the sample W&B in the manual from the prototype also list wheel axles forward of the CG. Maybe it’s as simple as having rolled the plane forward onto the scale vs backwards?