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Thread: W&B calculations

  1. #1
    N5246H Guest

    Default W&B calculations

    I recently re-weighed my Piper Clipper. I have the weight at each of the three landing gear positions. In order to complete the cg calculation, I need to know the position of the main gear and tailwheel relative to the datum. I was not able to find that from perusing the drawings CD. The TCDS showed the position of items like the seats, baggage area, fuel tanks, etc. but not the landing gear. Where would I find this data?

    BTW, the empty weight is about 150 lb heavier than the TCDS says. I expected it to have gained weight, but was surprised by the magnitude of the increase. How much weight do airplanes typically gain in 60 years?

  2. #2
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    Look at you original pilots handbook, it has the data you are looking for . It has all of the locations and arm lengths. If you don't have that page with the original aircraft weighing diagram you need to get the POH from Piper as it is required to be in the airplane.
    The center of the main gear is aligned with the wing leading edge. That is your 60" datum point.. The tailwheel is 165.5" behind the center of the main gear.

  3. #3
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    The best source I've seen for W&B info comes right from the FAA in their 1999 "Aircraft Weight and Balance Handbook." FAA-H-8083-1. I purchased my copy from Jeppesen Training Products.

    Level your plane as per the TC then measure the distance from the datum to the landing gear. That will give you the arms to use in determining the CG. The aircraft needs to be perfectly level to get the correct CG.

    Here is the url for the PA-16 TCDS:

    http://rgl.faa.gov/Regulatory_and_Gu...B?OpenDocument

  4. #4
    N5246H Guest

    Default

    Thanks for the info. I do have the POH in the airplane. I was attempting to do the calculation at home.

  5. #5
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    Default

    It is on the TCDS. Look further down where it says "Data Pertinent to All Models."

  6. #6
    J.J. Guest

    Default

    Quote Originally Posted by Gilbert Pierce
    The center of the main gear is aligned with the wing leading edge. That is your 60" datum point..
    Gilbert,
    To me this is confusing. Why would the Datum be 60"? Shouldn't it be 0"? All measurements and CG envelopes I have seen on the Shortwing TCDS's measure from the wing leading edge. Where did the 60" come from if the main gear axle is in line with the published Datum? Is the front seat at 18 inches like the TCDS says or is it at 78 inches like you say?

    It really alarms me when I see outdated or made-up information passed on to people who aren't 100% sure of what they are doing in the first place. The ONLY place you might see a reference to 60" and the main gear is on an original shortwing W&B sheet from the factory which should carry ZERO relevance to anyone making a CURRENT W&B sheet because all CURRENT information indicates the Datum is at the wing leading edge and not some imaginary point in front of the spinner. I submit the possiblity that the original Piper W&B sheets were incorrect on some aircraft because the PA-20 handbook published in 1950 in front of me references the WLE as the Datum which happens to match the legally binding document commonly refered to as the TCDS.

    If you use 60" for the mains then 60 must be added to ALL measurements or the W&B will be off enough for potentially tragic results. Personally, I'd stick with information from the TCDS.

    Jeff

  7. #7
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    Look at the original W&B. Piper used the 60 inches on most all the ragwings I have seen. The original W&B sheet has all the load points. Can't see how this is "outdated or made up information". Been being used on Clippers for 58 years.

  8. #8
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    Jeff

    I am not quoting made up information. Never have and never will. If I don't know I say so.

    On my Clipper POH is a w&b drawing of the airplane ( loading diagram), all of the arms for the oil, fuel, front and back seats, baggage compartment, propellor etc are on the drawing. Additionally the drawing shows the aircraft W&B datum 60 inches in front of the wing leading edge. All of the arms are specified in the factory W&B as positive numbers starting at that 60 inch point. It is real simple, you multiply each arm by the weight at that point, total up all of the positive numbers, divide by the gross wt. and subtract 60 from that CG arm. You now have your center of gravity. You did not have to deal with adding positive and negative numbers which stretches some pilots math skills. Piper did this on all of the ragwing Pipers. I suspect they did this so when you loaded your aircraft and did a weight and balance all of the numbers were positive and you did not have to add positive and negative numbers.

    How do you do a weight and balance when you load your Pacer? I find the loading diagram much easier to use then to decipher the TCDS which does not give the information in a straightforward manner. Do you carry a TCDS in your airplane so you can do a weight and balance. I doubt most Short Wing owners have ever seen the TCDS for their airplane much less keep in the airplane to do a weight and balance. One reason the POH is required in the airplane so you will have this information plus the empty weight and empty wt. cg. from your Weighing form to work you weight and balance.

    The arms on the loading diagram are all given as a positive number referenced to the 60 inch datum. Most military aircraft I have been involved with are done the same way with an imaginary datum in front of the airplane. It is done on military transport aircraft so all of the numbers are positive when you add up the loads at the numerous stations.

    If a person can not deal with the 60 inch datum point when weighing their airplane they probably are not qualified to be working on it themselves anyway.
    Last edited by Gilbert Pierce; 07-08-2007 at 07:55 PM.

  9. #9
    Wayne Guest

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    Having the datum 60" forward of the leading edge of the wing makes all calks. positive. This makes the calks. much easier to do and less chance of errors. Most aircraft have their datum points somewhere in the area of the tip of the spinner for this reason.

  10. #10
    J.J. Guest

    Default

    The W&B information for my Pacer is based on the TCDS and my owners handbook specifying wing leading edge as the datum with with charts and info using that datum. Looking at the TCDS for all other Shortwings led me to believe they were all the same (I also checked Cubs) or I wouldn't have ever brought it up. To me it makes a lot more sense and is a lot easier to make all computations from the same point than to switch back and forth, especially for someone not used to making the computaions. Assuming an airplane has been recovered and weighed, the factory W&B sheet no longer applies anymore than a 337 from 2 covering jobs past. Some of this reminds me of the time I was told you can't compute W&B if you use zero inches as an arm somewhere.

    Wayne, I wouldn't say most. My experience is with Pipers and Cessnas and there are several out there that use the wing leading edge or the firewall according to the TCDS and the manuals. I don't know which is more prone to errors, hitting an occasional negative number or performing all the computations nose then converting to a different point to use the loading graph. I did the computations from the tailwheel once just to prove it could be done just as accurately and as fast as having a nose datum because some "guru" said it couldn't be done.

    Jeff

  11. #11
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    It doesn't matter where the datum is, what matters is the sign in front of the number for the moment arm (i.e. (+) aft of the datum or (-) forward of the datum) Piper probably picked the tip of the spinner as the datum reference so all the moment arms would be positive. After you arrive at your CG location aft of the datum, it is easier to use the wing leading edge for seating and cargo positioning and gives the pilot a better reference.

    Boeing and most other airframe manufacturers use datums at distances very forward of any physical part on the airplane. They do this because when they stretch the airplane, they don't have to monkey with the datum reference and all the moment arms stay positive.
    Ben V.

  12. #12
    Wayne Guest

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    Yes Jeff, I guess it may be 6 of one, a half dozen of the other. There is the advantage that the moment numbers are significantly smaller with the shorter arm lengths when measured from the leading edge. Smaller numbers= less chance of errors.

    My plane gained weight as well. 1168 lbs. empty. The upholstery and a lot more radios account for a lot of it. Aerothane paint on Stits fabric might account for some as well. The tail wheel is almost a wash.
    Last edited by Wayne; 07-16-2007 at 09:16 AM.

  13. #13
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    Negative numbers are just part of life when doing W&B calculations. After doing them a few times you get the hang of it.

    Our short wing Pipers were certified in an age when engine oil was not part of the empty weight. (In aircraft certified under Part 23, which came after our short wings, engine oil IS considered part of empty weight) So, when our aircraft are weighed the mechanic has to either drain the oil, or subtract it mathematically.

    For example: If a mechanic weighs a short wing without draining the oil, (using the leading edge of the wing as the datum,as per the TC) he will have to use a negative number to subtract the oil. Since the oil is forward of the datum, the moment will be a positive value. Then when the pilot does a W&B calc before a flight, he/she must add the oil to the empty weight, which will be a positive number, but the moment will be negative.

  14. #14
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    I really have problems when the tip of the spinner is used as the datum. It is really hard to calculate when the spinner is sitting in the corner of the hangar.

  15. #15
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    JD
    Just start at the tail when nose is in the cornor of the hangar and make all of the numbers negative (you are going backwards) and you will be alright.

  16. #16
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    I find the original W&B very helpful especially when removing some of the original equipment and not knowing the exact station the factory used.

  17. #17
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    Quote Originally Posted by J.J.
    Gilbert,
    To me this is confusing. Why would the Datum be 60"? Shouldn't it be 0"? All measurements and CG envelopes I have seen on the Shortwing TCDS's measure from the wing leading edge. Where did the 60" come from if the main gear axle is in line with the published Datum? Is the front seat at 18 inches like the TCDS says or is it at 78 inches like you say?

    It really alarms me when I see outdated or made-up information passed on to people who aren't 100% sure of what they are doing in the first place. The ONLY place you might see a reference to 60" and the main gear is on an original shortwing W&B sheet from the factory which should carry ZERO relevance to anyone making a CURRENT W&B sheet because all CURRENT information indicates the Datum is at the wing leading edge and not some imaginary point in front of the spinner. I submit the possiblity that the original Piper W&B sheets were incorrect on some aircraft because the PA-20 handbook published in 1950 in front of me references the WLE as the Datum which happens to match the legally binding document commonly refered to as the TCDS.

    If you use 60" for the mains then 60 must be added to ALL measurements or the W&B will be off enough for potentially tragic results. Personally, I'd stick with information from the TCDS.

    Jeff
    The fact remains that you acused someone of using made up data when in fact they did not and that has been proven. He spends a lot of time researching things and giving out information so that people are more informed about their airplanes. Appears an apology is in order to me.

  18. #18
    J.J. Guest

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    Quote Originally Posted by Steve Pierce
    The fact remains that you acused someone of using made up data when in fact they did not and that has been proven. He spends a lot of time researching things and giving out information so that people are more informed about their airplanes. Appears an apology is in order to me.

    The one thing that has been drilled into me over and over about airplanes is the TCDS is the bible for the airframe you are working. If you re-read the post I said outdated or made-up. The current TCDS for ALL shortwings lists WLE as the datum, therefore, the info was/is outdated in this case. Maybe I need to call the instructors, IA's and FSDO's I have worked with and over the years and tell them the TCDS doesn't mean as much as they insisted it did because there are some people who don't like to use a negative arm or because they have a 50 years old drawing that has never been revised.

    There is more than one person on this website that does research and tries to provide the most accurate information. Mine is accurate AND current. I find things like this because I take the time to look it up instead of relying on "it's the way it's always been" or "so and so said so". Just because a person doesn't post a message everyday doesn't mean he doesn't know what he is talking about. I can't believe all the flak I'm taking because I have the foresight to actually read the current data on our planes.

    Jeff

  19. #19
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    The flak isn't over the current data. The flak is over accusing someone of "it's the way it's always been" or "so and so said so" when that wasn't the case at all. The data is there along with the TCDS. If I were only to rely on the published data only it would be hard to know how to do a lot of things. Taking all the information out there and using what you can and throwing out the rest is the best way I have found to get things done the right way.

  20. #20
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    All:

    I have attached an excel file for computing the weight and balance of your tri pacers. If more interest exists and I can get an original W&B cover sheet, I'll make one for a Pacer also.

    All you have to do with this spreadsheet is complete the first sheet by inputting the weight data for the three wheels. After that, click on the tab for a tripacer without and aux tank or one with and aux tank to see your computed results and print the form for your mechanic.
    There is also room for an equipment list, but by adding stuff to it does not affect the weight and balance. This tool is specifically for calculating W&B after weighing it. However, by adding all the equipment items you can then subtract the moments and weights to arrive at a corrected W&B after weighing.
    Attached Files Attached Files
    Ben V.

  21. #21
    J.J. Guest

    Default

    Quote Originally Posted by Steve Pierce
    If I were only to rely on the published data only it would be hard to know how to do a lot of things. Taking all the information out there and using what you can and throwing out the rest is the best way I have found to get things done the right way.
    I agree with this statement but in this case there is current published data (the TCDS) and if everyone was to use the current data when it's available there would be a lot less to throw out. Even the factory W&B sheet for my bird contradicts itself. On one side there is a sketch showing a datum beyond the nose and on the other side there is an equipment list showing the prop at -50 inches and the main wheels at 0 inches with a loading graph based off of WLE. I follow the TCDS last revised in 1995 instead of the 50 year old sketch. For me it's not really a choice because the TCDS is the most current information available.

    I know I came into this discussion too strong and I apologize to everyone for that. I need to quit doing that because it automatically puts everyone on the defensive and that is not my intent as well as being counterproductive.

    By the Way - Andy has a pdf copy of the weight and balance form I made for Pacers on his website http://home.earthlink.net/~andy2220/id4.html
    It is complete with an equipment lists, loading graphs and airplane sketches. There are 4 pages; 2 for a nose datum and 2 for a WLE datum. The original file is an excel file but it is over 3 MB in size.

    Jeff

  22. #22
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    thanks, you did not even put the "use at your own risk" clause in. I will use it on my PDA phone.

    Patrick

  23. #23
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    We're all friends here, with the same goal - to get the safest most enjoyable experience out of our short wing Piper's.

    Sometimes it's easy to say one thing and have it misinterpreted by someone else. It's even easier to be misinterpreted when we write something, because the other person isn't seeing your body language, or hearing the intonations of your voice. I'm sure the intent of anything said in this discussion was to be helpful, not hurtful.

    I'm sort of a newby here, but I've learned a lot about short wings this past year, and I thank you all for sharing your knowledge.

    -Ed

  24. #24
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    Hey Jeff,

    could you give me that pdf for wt&b for the Pacer ??? Ist there an ecel sheet so I can use in on my computer ??

    Thanks

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