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Thread: Weight and Balance 101

  1. #1
    Join Date
    Jun 2008
    Location
    Roseville, CA
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    440

    Default Weight and Balance 101

    Ok, here's another question for you guys, one I'm almost embarrassed to ask it should be so basic a piece of knowledge.

    Wt x Arm = Moment

    We know this, it's how it all works.

    We know from basic math that anything times zero is zero.

    So, Wt 3.5 X Arm 0 should = a moment of 0 or no change in CG.

    The wt and balance info for my airplane, the current form and all others I can find in the logbooks don't agree. They have Wt 3.5 X Arm 0 = Moment 3.5

    Which is correct, and why? I've scoured my FAA Wt and Balance Handbook and can't come up with an answer since none of their examples have an arm of zero.

    Kurt
    Ex-Vice President

    63' Colt

  2. #2
    JohnW Guest

    Default

    ANY weight multiplied by an arm of "zero" has a moment of "zero". In order for 3.5 times the arm to equal a moment of 3.5, the arm must be ONE. A simple, "I guess I never THOUGHT ABOUT IT" mistake.

    Adding or subtracting weight from the "zero arm station" results in ONLY a weight change, NOT a balance change.

  3. #3
    Join Date
    Apr 2009
    Location
    Tehachapi, CA
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    58

    Default

    Kurt,
    First my disclaimer: I'm not an A&P, just a pilot and so this is really just my educated opinion. If you use the see-saw example when looking at weight and balance, the 0 arm is right where the two sides teeter. No matter how much weight you add at that point, it doesn't change how the see-saw teeters. So using this visualization in the airplane, weight added at a 0 arm would not change the moment either. It goes without saying however that it does add to your total weight so at some point even though the plane is balanced, it will eventually become overweight by continually adding weight to the 0 arm. I believe your aircraft documents are incorrectly calculated.

    Looks like John beat me to the answer!
    Last edited by av8ing; 06-17-2009 at 09:23 AM.

  4. #4
    Join Date
    Jun 2008
    Location
    Roseville, CA
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    Default

    Well thanks for validating my meager math skills. I find it interesting that over the years that mistake has been made multiple times by multiple different mechanics. I guess I could go back and recalculate each entry but probably to be exact I should just have it weighed during the next annual.

    Thanks!

    Kurt
    Ex-Vice President

    63' Colt

  5. #5
    Wayne Guest

    Default

    On some planes the datum, or zero for measuring the arm is the leading edge of the wing. The instrument panel is a couple inches behind this point. Therefore an instrument 4 or 5 inches deep has it's center of gravity located on this zero point. In this case it is true that the c/g is unchanged only the weight changes. WAyne

  6. #6
    Wayne Guest

    Default

    If in fact someone did enter a moment of 3.5 instead of zero this is incorrect but also insignificant. It would move the center of gravity of your airplane approximately the thickness of a human hair.

  7. #7
    Join Date
    Jun 2008
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    Roseville, CA
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    Default

    Very interesting! This was indeed an entry for the removal of a 3.5 lb panel mounted piece, an old King marine Loran. But I also find an older wt and balance sheet with the removal of a 6 lb Mark 12B (arm 0 but moment -6.0) and the Nav head where a similar error was made, and the addition of a VAL Com which added 3.0 lbs at arm 0 and increased the moment 3.0. Odd this kept happening throughout the years.

    For grins I'll go back through the logs and see if I can make it all correct.

    Thanks for all the good replies!

    Kurt
    Ex-Vice President

    63' Colt

  8. #8
    Join Date
    Jun 2008
    Location
    Roseville, CA
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    Default

    Going through the logbook binder tonight, in addition to the original Piper Wt & Balance document, I found one done in 1995. It is attached below.

    First error I found is -.5 x -10 = -5 (this isn't right, a negative times a negative is a positive)
    The second is 5 instances of a wt x Arm 0 = something other than zero
    The third is that if you do his math, the total weight x arm = moment calculation is wrong.

    Two subsequent wt & bal sheets were done using the incorrect numbers as starting points.

    This has all been corrected now.

    Kurt
    Attached Files Attached Files
    Last edited by Kurts; 06-18-2009 at 02:21 AM.
    Ex-Vice President

    63' Colt

  9. #9
    Join Date
    Nov 2005
    Location
    Marysville, OH
    Posts
    176

    Default

    Kurt,

    Don't keep us in suspense. How much did the weight and CG change?

    Dean
    Dean Dayton

  10. #10
    Join Date
    May 2006
    Location
    Shanty Bay, Ontario, Canada
    Posts
    619

    Default

    Kurt, with this huge problem/error, I would justify that new wt and balance be conducted.
    What do you think?? I had mine done after new fabric was put on. It is very straight forward to do and fun.

    Peter Lubig

  11. #11
    Join Date
    Jun 2008
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    Roseville, CA
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    Default

    You know, the funny thing is it didn't really change much. Wt was off less than a pound, the moment was a few numbers off, and the arm was about the same. Changes they'd made, changes I made, all seemed to pretty much cancel out in the end. Eh, at least I'm happy the numbers are right. We'll get it weighed next annual and see how close it really is.

    What's yours weigh Dean? Mine now says it weighs 1023 lbs.

    Kurt





    Quote Originally Posted by deandayton View Post
    Kurt,

    Don't keep us in suspense. How much did the weight and CG change?

    Dean
    Ex-Vice President

    63' Colt

  12. #12
    Join Date
    Nov 2005
    Location
    Marysville, OH
    Posts
    176

    Default

    Kurt,

    If I remember correctly (which seems to happen less and less often), my W&B sheet says mine is 1060 lbs. I have not done the exorcise of verifying this.

    Dean
    Dean Dayton

  13. #13
    Join Date
    Apr 2012
    Location
    Jamestown, NC
    Posts
    65

    Default

    since we're on this subject, I'll redirect slightly instead of starting a new thread. My Pacer seems to have a more forward CG than i think it should. For example, i cannot take more than about 320 pounds in the front seat with full fuel (36 gal) and still be in CG on the forward limit (no rear pax, 10 pds gear in baggage). i do have an O320 hanging out front. I'm interested in what others out there have for forward CGs limitations and how much you can take in the front with full fuel.

    It seems odd to me that a four-place airplane wouldn't be able to handle 2 adults in the front with full fuel.
    Slipping the surly bonds of Earth...

  14. #14
    Join Date
    Jun 2008
    Location
    Roseville, CA
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    Default

    My old thread, back from the dead! (Poet and didn't know it)

    My Colt has this affliction as well, always being at the forward part of the CG envelope. We were always told that airplanes fly more efficiently with a rearward CG though. My dads Fairchild 24 had the battery located in the aft fuselage. With the amount of nose-up trim these Short Wings require for normal flight, not to mention landing, I wonder if they might benefit from a similar battery placement.

    Kurt
    Ex-Vice President

    63' Colt

  15. #15
    Join Date
    Apr 2012
    Location
    Jamestown, NC
    Posts
    65

    Default

    Just came out of Annual and after discussing my W&B oddity with IA, we got the scales out and weighed the plane after leveling it, of course. I had predetermined using my W&BPro app on my iPhone that my CG paperwork numbers were about 2" off to the forward side. The actual results proved me correct. My paperwork, modified numerous times over the years since last weighing was off nearly 2" forward. Armed with real results now, my W&B is much more in line with 'reasonable man' thinking--I can put 400 lbs in the front seat, fill 'er up with fuel and still be inside the CG envelope.

    Seems 2 owners ago when they did the recover and other work, they arbitrarily decided to convert the Arm numbers to a different Datum. TCDS says 0 datum is the leading edge of the wing/center of wheel (PA20 here). These good folks wanted to use the spinner tip as the new Datum and 'converted' the other stations to match the change. Who knows why. Something got lost in translation that no one picked up on. So much for doing that required W&B prior to every flight, huh?! If they had done that, they would have caught the discrepancy. My new W&B sheet reflect the TCDS Datum line and station values and the numbers finally make sense again.
    Slipping the surly bonds of Earth...

  16. #16
    Join Date
    Mar 2012
    Location
    Searcy
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    66

    Default

    Interesting, the datum on all musketeer series airplanes are the tip of the spinner (or perhaps someplace out front of the tip). This makes all arms positive, and may be the reason that Beech chose to go that route when certifying the musketeer.

  17. #17
    Join Date
    Apr 2012
    Location
    Jamestown, NC
    Posts
    65

    Default

    Most every light GA plane I've flown has the datum at an intangible point forward of the aircraft. My Pacer is the first I've encountered where it is not only an actual point on the fuselage but also aft of the firewall. Guess Piper's engineers had their own ideas about things.
    Slipping the surly bonds of Earth...

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