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Thread: Weight and Balance question

  1. #1
    01Delta's Avatar
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    Default Weight and Balance question

    So our PA-22 is weighed and found that the CG is 69".
    All other items in the weight and balance have the arm measured from +/- W.L.E.
    How to I figure out the arm of the airplane from the W.L.E.?
    Trying to create a W&B spreadsheet, and for some reason I am completely stumped.

  2. #2

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    Default Re: Weight and Balance question

    Quote Originally Posted by 01Delta View Post
    So our PA-22 is weighed and found that the CG is 69".
    All other items in the weight and balance have the arm measured from +/- W.L.E.
    How to I figure out the arm of the airplane from the W.L.E.?
    Trying to create a W&B spreadsheet, and for some reason I am completely stumped.
    When it was weighed, what was the datum that was used? Do you have the W&B report that should be attached to your flight manual from when the airplane left Piper, it has how to figure the W&AB.


    Sent from my iPhone using Tapatalk

  3. #3
    Gilbert Pierce's Avatar
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    Default Re: Weight and Balance question

    Piper confused the issue by putting a datum 60” in front of the leading edge and after the final calculations are made they subtracted the 60 inches and quoted the CG limits at the leading edge.

    I think they did this to appease people who cannot due math with both positive and negative numbers as any arm forward of the leading edge would be negative.

  4. #4
    Pacerfgoe's Avatar
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    Default Re: Weight and Balance question

    To be honest, I find it much easier doing the balance calculations adding the 60" on at the beginning and then subtracting it at the end....My 2 cents
    Forrest Miller

  5. #5

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    Default Re: Weight and Balance question

    I never have understood why people have trouble with negative arm lengths. I guess they never remove weight from an airplane because that weight would be a negative number too. I converted my Thorp to a wing leading edge datum. It gives me a fixed point to measure from when making changes and eliminates the need for any conversion.

    I don’t think Piper set out to cause confusion. I think one person at the factory set the datum at station 0.0 and another person computed the W&B from the wing leading edge to keep the numbers manageable for loading (alterations probably weren’t a factor). In those days they didn’t have W&B calculators available and a person actually had to have a clue how to do math. I don’t do it much anymore but I use to do a lot of w&b manually. If I am adding 10 pounds 5 inches in front of the wle, it is easier for me to compute -50 than 550. If I remove the 10 pounds instead, then the moments are 50 and -550. Again, I find it easier to compute the 50. Now we have computers to do the brain work but I still like smaller numbers.
    "Good judgement comes from experience, and experience comes from bad judgement." - Fred Brooks

  6. #6
    01Delta's Avatar
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    Default Re: Weight and Balance question

    So is it as simple as subtracting 60" from the CG arm to get the arm from WLE? Or adding 60 to all the other items to get to a common arm?

  7. #7
    Clayton Harper's Avatar
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    Default Re: Weight and Balance question

    A simple way to think about it.... The cord of the wing is about 63", center of lift is about a 1/3 to a 1/2 of the way back from the LE. The center of gravity has to be forward of that or the aircraft could not be controlled. The balance point have to make sense with that given.
    The only true wisdom is in knowing you know nothing.
    Socrates

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