#### Erik

##### Registered User

It appears that there is some inconsistency in Piper's publications regarding what they call the datum for their moment calculations. There is a document that came with my airplane that clearly shows the datum at 60" forward of the wing leading edge. Mechanic "A" added Bushwheels (and removed 6.00's) to my aircraft and used an arm of 62.25". That is consistent with such a datum. However, Mechanic "B" added a climb prop (and removed cruise prop) and used an arm of -54". This did not make sense to me, so when we discussed it, he told me that the "Type Certificate" for my plane shows the wing leading edge as the datum. That means Piper is inconsistent. Two mechanics used two different datums for weight changes, which makes for a meaningless aircraft empty c.g. when you think about it. Has anyone crossed this bridge before?

The c.g. envelope I have in my paper work shows a range of acceptable values "aft of wing leading edge". What that means is that if I use the wing leading edge as the datum point for all weights, then I simply use the chart for the overall arm calculated. BUT, if I use a datum 60" from the wing leading edge, that is fine too, I just need to make sure all weights use the same datum and then that the 60" is subtracted so the chart can be used. Sound confusing? It sort of is. Consistency is the key I think, but look what happened in the above example! There must be a lot of hosed-up w&b's out there.

Example:

10 lbs added at prop, 54" forward of wing leading edge. Mechanic "A" would say the moment added is 6 in*10lbs = 60 in*lbs. (remember, Mechanic "A" used a datum of 60" from W.L.E., so the arm for the new mass would be 60-54=6). Mechanic "B" would say the moment added is -54 in*10lbs = -540 in*lbs.

Say before the change, the aircraft has a weight of 1200 lbs and an arm of 12" aft of wing leading edge OR otherwise stated as 72" from datum 60" forward of wing leading edge:

Scenario #1: Use datum at wing leading edge (consistently) -

aircraft moment = 1200 lbs * 12 in = 14,400 in*lbs

added moment = 10 lbs * -54 in = -540 in*lbs

new moment = 14400-540=13,860 in*lbs

new weight = 1200+10 = 1210 lbs

new arm = 13,860/1210 lbs = 11.45 in aft of W.L.E.

Scenario #2: Use datum at 60" forward of wing leading edge (consistently) -

aircraft moment = 1200 lbs * 72 in = 86,400 in*lbs

added moment = 10 lbs * 6 in = 60 in*lbs

new moment = 86,400+60 = 86,460 in*lbs

new weight = 1200+10=1210 lbs

new arm = 86,460/1210 lbs = 71.45 in aft of datum OR 71.45 in-60 in = 11.45 in aft of W.L.E.

**Conclusion: Scenario #1 and #2 are the same and correct.**

Scenario #3: All previous weights use datum at W.L.E., but add weight using datum at 60" forward of wing leading edge -

aircraft moment = 1200 lbs * 12 in = 14,400 in*lbs

added moment = 10 lbs * 6 in = 60 in*lbs (

**this is where the error is induced**)

new moment = 14,400+60=14,460 in*lbs (error)

new weight = 1200+10=1210 lbs

new arm = 14,460/1210 lbs = 11.95 in (but from what datum? This is an error caused by mixing datums)

Scenario #4: All previous weights use datum at 60" forward of W.L.E., but add weight using datum at wing leading edge -

aircraft moment = 1200 lbs * 72" = 86,400 in*lbs

added moment = 10 lbs *-54 in = -540 in*lbs (

**this is where the error is induced**)

new moment = 86,400-540=85,860 in*lbs (error)

new weight = 1200+10=1210 lbs

new arm = 85,860/1210= 70.95 in (but from what datum? This is an error caused by mixing datums)

Conclusion: Scenario #3 and #4 both differ from each other and from the correct answers. Imagine the w&b accuracy of a plane with a lot of changes using different datums each time. The end result is a worthless aircraft C.G. #.

Thanks to those taking the journey with me!