Want Best Technique for Accurate Weight & Balance

[Walt]

Originally Posted by az_gila

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The approx. 5 degree slope of a -6A would give a very significant error if it was not leveled. The plump Bob mark from a non-leveled LE would be about 2 inches off.

I know the 6A has a little bit more deck angle than the 7A sitting on the floor. I have done the 7A both ways (level and not level) and found the change in datum to be negligible, certainly no where near 2", so I can't imagine the 6A being that much different is all.

[MeAndMyToys]

I think what he is trying to say is that if you think of the plane level and the string for the plumb bob is perpendicular to the cord of the wing, if you rotate the wing 5 deg and the string were to remain perpendicular to the cord, the point that it would touch would change by more than 2 inches. Probably 5 inches or something.

[SMO]

Make sure the tires are centered on the scales, both fore and aft as well as side to side. Maybe if I had a better set of scales it wouldn't be critical, but I found this can make a significant difference in the readouts.

[Walt]

Originally Posted by MeAndMyToys

I think what he is trying to say is that if you think of the plane level and the string for the plumb bob is perpendicular to the cord of the wing, if you rotate the wing 5 deg and the string were to remain perpendicular to the cord, the point that it would touch would change by more than 2 inches. Probably 5 inches or something.

I can see that, but that's not how it works in real life.

[az_gila]

Originally Posted by MeAndMyToys

I think what he is trying to say is that if you think of the plane level and the string for the plumb bob is perpendicular to the cord of the wing, if you rotate the wing 5 deg and the string were to remain perpendicular to the cord, the point that it would touch would change by more than 2 inches. Probably 5 inches or something.

Exactly..... I simply estimated the -6A LE at 24 inches above the floor and that gives about a 2 inch error.

In reality the error would be affected by the vertical heights of the C of G and would be less, but still measurable.

Just for fun I'll do it both ways when I do my final W & B.

[Walt]

Dusting off some math cobwebs here... using good ole Pythagoras theorem to find the difference in the datum arm, for the 6A raising the mains 3" would cause the datum to move from 60" to 59.93", less than a tenth of an inch.

According to those same calculation raising the 7A MLG 2" would cause the datum to move from 70" to 69.97"

Someone please correct me if my math looks incorrect.

[bret]

if you jack the plane up and set it on the scales instead of rolling up on them, you may be putting a lot of side load on the scales when the gear wants to load and flex outward, a side load on some scales may be inaccurate?

[Bill Boyd]

Originally Posted by az_gila

Exactly..... I simply estimated the -6A LE at 24 inches above the floor and that gives about a 2 inch error.

In reality the error would be affected by the vertical heights of the C of G and would be less, but still measurable.

Just for fun I'll do it both ways when I do my final W & B.

Take into account the plumb bob suspended from the spinner tip will be at the same "wrong angle" if airplane not leveled, which helps reduce the overall arm measurement error somewhat, but by no means completely.

[AlphaCharlieBravo]

Using my sheetmetal calculator.

A 24" run the (vertical part) and 5 degree pitch will give you a rise (the horizontal part) of (2") inches. This would be the difference where the plumb bob lands, plane being level vs.5 degree tail low. Im in Gils camp.

[Nedimbek]

Hello all,

In my math this trigonometry is not that simple. The plum bob will always be perpendicular to a level surface. What matters is: how much the leading edge, axle centers etc move forward when you raise the mains to compansete 5 degrees off level. Hence, in my calculations that deviation for 100" of arm will be equal to 100-cos5*100=.38". Also remember your datum will shift as well as your front wheel and everything else. Therefore the error is not anywhere close to 2 inches but still needs to be taken into account if you dont want a very rough cg calculation with an error of like +/- 3/8 of an inch.
After all if you are putting your life in risk, you will definitely want to do it accurately, complied with the recommended proper procedure IMHO.
Another heads up: An "arm" measurement on a non-level floor ( with some slope for drainage) also has a similar effect but much subtle! Here we go again!

Nedim