A weighty question: What kind of scales should I use?

Discussion in 'General Experimental Aviation Questions' started by bifft, Apr 18, 2017.

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  1. Apr 18, 2017 #1

    bifft

    bifft

    bifft

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    Getting to be time to weigh my RV. Looking up similarly equipped ones I expect a total weight of 1050-1150lbs or so. Should be fairly evenly distributed on all three wheels or 350-400lbs. Asking around for other builders, they don't have scales in this range. Asked the local FBO, they are set up to weigh jets from the jackpoints, and don't have wheel scales. So, need to buy something. Figure I can get one scale and then build blocks for the other wheels. Jack up and weigh one wheel at a time.

    I see three options:

    Bridge two bathroom scales (the ones I have only go up to 300lbs). Don't need to buy anything new, but accuracy is questionable. Have one that is at least repeatable, precision is 0.2kg. Another that is +/- 1kg weighing 250lbs of me if I stop off it and walk around the room. Another I haven't used as much, but prevision is 0.5kg. (all are metric only, but that math is easy).

    Buy a postal scale with 400+ range. Something like this: https://www.amazon.com/Accuteck-Digital-Industry-Shipping-ACB440/dp/B00LPUZP1I
    Cheap, plenty of precision. Accuracy is unknown. Amazon reviews are mixed.

    Buy a real certified aircraft scale. This seems to be the cheapest option: http://www.aircraftspruce.com/catalog/topages/scales4planes-std.php
    Goes to 2klbs, way more than I need. Precision is either 1lb or 0.5lb (it says both) either is less than 0.1% of total weight, so probably Ok. Certified accurate for a year, can be recalibrated (but where would I get calibrated weights?)

    I am by nature a total cheapskate, but weight and balance is a critical safety item. Thought I'd run it by the cheapskates here to see what people think.
     
  2. Apr 18, 2017 #2

    TFF

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    First try to borrow some real airplane scales. Next rent some from a shop. There is a pivot bar for bathroom scales for higher weights that some use.
     
  3. Apr 18, 2017 #3

    Mad MAC

    Mad MAC

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    You could try hiring a set of auto motive scales (they normally work with just three pads connected), should be able to get a set with a calibration certificate. It is better to weight all three points at the same time.

    Are you sure about it being a even split across the three wells
     
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  4. Apr 18, 2017 #4

    cdlwingnut

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    check with the local EAA chapter, and FBOs you may be able to borrow or rent cheaply a set.
     
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  5. Apr 18, 2017 #5

    Winginit

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  6. Apr 18, 2017 #6

    Little Scrapper

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    How about the RV forum? There's RV's being built everywhere, there's gotta be a bunch in Utah I'd imagine.

    I think I know a guy in Utah that has scales. Where abouts are you?
     
  7. Apr 19, 2017 #7

    lr27

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    Do you need this all the time, or just once? Around here, it appears there are many places you can rent scales of various kinds.

    There's probably a simple trick you can do that avoids using commercially made scales at all. Long i-beam, pivot, and sand bags?

    People used to take model airplanes down to the Post Office and weigh them. If they have 3 scales like the one in your first post, maybe you could do the same? Pick a slow day. ;-)
     
  8. Apr 19, 2017 #8

    bifft

    bifft

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    I'm in Provo. I've asked around, but no one in my (admittedly small) circle has any.

    I got the roughly even between all wheels based on calculations from all the RV-8A wgt&balance numbers I could find posted on-line. Probably should ask on the RV forum as well. Bunch of RV's in Utah, but nobody I know well enough to ask.
     
  9. Apr 19, 2017 #9

    Winginit

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    For the time involved in borrowing and returning them and possibly needing them more than once, it's better to have your own. Then you can make more friends by lending to them.It also allows you the option of weighing any future additions beforehand.
     
  10. Apr 19, 2017 #10

    Little Scrapper

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    I think there's a set 45 minutes from you. Working on it, hang tight for day.
     
  11. Apr 19, 2017 #11

    Little Scrapper

    Little Scrapper

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    Check your private messages.
     
  12. Apr 19, 2017 #12

    AdrianS

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    Like passengers?

    I dare you to weigh your wife before each flight - tell her it's for weight and balance calcs.
     
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  13. Apr 19, 2017 #13

    Winginit

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    The local club I was in used to offer cheap introductory flights. One day this guy weighing well over 300 lbs came in and wanted a flight. The airport owner took one look at him and told him, "NO".


    (Note: 1700 ft grass strip with electric wires at one end and trees at the other)
     
    Last edited: Apr 19, 2017
  14. Apr 19, 2017 #14

    Winginit

    Winginit

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    Its good to see members try to help one another, but think about how much time is going to be invested, not to mention gasoline. 3 hrs driving time and 8-10 gallons=$20+
    Like I said, its great to see HBAers try to help one another, but I'd rather just put that time and money into buying a scale.
     
  15. Apr 19, 2017 #15

    TFF

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    Thats why you will buy a set of $1000 scales, for one usage, but not ever use them to weigh your plane cause its not done.
     
  16. Apr 19, 2017 #16

    Winginit

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    The scales I recommended above were $65. And actually I have used different scales that I own to weigh many components and make decisions/choices on how to proceed with my airplane. I would think that most builders would find it beneficial to weigh many things other than just the final weight and balance of their airplane. Also, when doing weight and balance, multiple checks may need to be done if some components need relocation to achieve the best condition. making those modifications may require days and even weeks to complete properly.

    Don't really know why you wanted to take a cheap shot like that. Hopefully that made you feel better somehow.
     
  17. Apr 19, 2017 #17

    Arthur Brown

    Arthur Brown

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    I'd expect a tail wheel load to be 10 - 15% of gross weight, or a nose wheel load to be about 20 - 30% of gross weight. I wouldn't assume three wheels taking equal loads.

    Three bathroom scales is the cheapest, use a lever arm made from a scaf plank or the like in a divide by three (or four) arrangement. 1200lbs split between two main wheels is 600lbs divide by 3 gives a scale reading of 200lbs a common (UK) 20 stone scale would take and read 280 lbs, so all seems OK

    Always it's cost vs availability
     
  18. Apr 20, 2017 #18

    deskpilot

    deskpilot

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    Probably a dumb question but why the need for 3 scales. Surly, the weight on each wheel will be the same whether its sitting on the ground or a couple of inches higher on a scale. Add all 3 weights together and you've got the weight of the aircraft...No?
     
  19. Apr 20, 2017 #19

    Little Scrapper

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    He wasn't taking a cheap shot, come on now.

    I don't trust $65 scales, but that's me. I know people do use them and they have good luck I hear. That's cool.

    It's very common to group buy some scales, well, at one time it was. If he spent $70,000 on a RV I'm not sure an extra $100 matters. Most EAA members know people with scales.

    I drive 45 minutes to movie theatre, which incidentally is more expensive than the scales. Haha.
     
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  20. Apr 20, 2017 #20

    lr27

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    I think with cheap scales there's a real need for calibration. Just to be sure of the results. If the results are repeatable, one could always develop a table to look up the real weight.

    BTW, as I recall, various places on earth have variation in g on the order of 1 percent. Even the rotation of the earth at the equator will change the results by 1/3 of a percent. At 1,500 lbs gross, that difference is about 5 lbs. There are also local effects due to the earth's shape, local mass concentrations, altitude, etc. Apparent gravity in Anchorage is about 1/2 percent more than in Kuala Lumpur. I suppose typical density altitudes may make up for that. So balances will be slightly more accurate than spring scales. OTOH, a spring movement clock will likely keep the same time in either place, but a pendulum clock would have to be reset. Not that we'd have either in our aircraft these days.
     

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