Weighing with electronic bathroom scales?

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karmarepair

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I will soon weigh my Sonex and I'm wondering if anybody has used the currently common electronic bathroom scales that have you tap the scale, so it zeros itself, then it tells you how fat you've gotten, then it shuts off. I THINK I can use them by lifting the wing, touching it back down, waiting till it zeroes, putting the wheel back down, reading it. Repeat on the other side.

Has anyone done this?

I do have ONE mechanical bathroom scale, but my EAA chapters scales have disappeared, and the local thrift stores only have the electronic scales, albeit at low, low prices.

My Sonex empty should only be about 650 lbs, so 3 bathroom scales should be enough.
 

Tiger Tim

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I’d want to calibrate the scale first, a jug with a measured amount of water close to the weight you expect would probably be a good idea. That or see if there’s a local A&P who can hire out their scales for a day.
 

addaon

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You can get analog bathroom scales for about $20 each, and 50 lb kettlebells or weight plates for about for $30 if you have a local source (shipping sucks); I would definitely calibrate around your measured value and not assume the scale is within 10%, but it should be pretty close if you can bracket it within 50 lbs (even 10% nonlinearity in a 50 lb window isn't that bad).
 

karmarepair

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Most bathroom scales don't go high enough.... Usually 350 odd lbs

Am I shopping at the wrong thrift store?
Sonex empty weight should be less than 700 lbs. Nose wheel might have 50#.
At the one Goodwill that had any scales, I saw 4 electronic scales, $5 each.
I got a DM from somebody who TRIED electronic bathroom scales and declared it a new low in not fun. So I'll keep looking. I haven't hit every thrift store in the county, yet.
No A&P on the field I'm at, so that's out.
 

Marc Zeitlin

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Has anyone done this?
Sure.

But that's not the right question. The right question is "is it acceptable to perform a safety critical aerodynamic procedure on my airplane, in which an error could kill me, with one or more uncalibrated devices that are not made for the purpose and have no known accuracy".

When you ask the right question, you can get the appropriate answer, which is "use calibrated aircraft or car scales".
 

Stolch

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My first flight was going to be from my property where I built the aircraft and I had no access to aircraft scales either and I was not about to partially disassemble the aircraft and transport it to an airport just to weigh it. So after a thorough search of the builders forums and NTSB database for my kit plane, I could not find a single issue, incident, or accident where the probable cause finding was the aircraft configuration as-built exceeded the kit manufacturer’s CG limits. If building something rare or never built before, or deviating significantly from the plans, I’d use aircraft scales. However, after completing my little Zenith 701 from factory kit number 1,001 without deviation, with a Rotax 912 hanging on the nose, so a configuration well documented with hundreds flying, I used three new 350 pound mechanical bathroom scales. I calibrated them to my weight, which has not deviated but by a pound or two over the last 20 years, is reasonably close to what I estimated the mains should weigh, and recently confirmed at the doctors office during my annual physical. For good measure I asked my wife to step on each one and verify if they were acccurate to her weight, and trust me, my wife knows exactly what she weighs. They all passed that check as well. Weighed the airplane three times, moving and leveling each scale each time so all three were used at each wheel once, then averaged the numbers, which did not vary but by a pound or two each weighment.
 
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thompsonbb

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I used a set of automotive wheel jacks and a livestock scales kit (basically, just the load cells and a readout unit) to build my own set of aircraft scales. I did end up having to modify them a bit to account for the much smaller tire diameter common on aircraft, but they work pretty well. I made 3 of them, so I can just roll up to the airplane and get a weight in under 10 minutes.
 

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crackle

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I will soon weigh my Sonex and I'm wondering if anybody has used the currently common electronic bathroom scales that have you tap the scale, so it zeros itself, then it tells you how fat you've gotten, then it shuts off. I THINK I can use them by lifting the wing, touching it back down, waiting till it zeroes, putting the wheel back down, reading it. Repeat on the other side.

Has anyone done this?

I do have ONE mechanical bathroom scale, but my EAA chapters scales have disappeared, and the local thrift stores only have the electronic scales, albeit at low, low prices.

My Sonex empty should only be about 650 lbs, so 3 bathroom scales should be enough.
For the purposes of monitoring changes, I bought a trio of digital bathroom scales on eBay that were enough to take the weight of my Champ and check calibrated them with water tanks. I jacked up the wheels and inserted the scales. I got very excited when the total weight was nice and low. However, when it was professionally weighed a little later the difference as about 100lbs. I'm told the error is caused by the lateral forces of the expanding undercarriage. So next time I will try making ramps to wheel the Champ onto the scales.
 

Toobuilder

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One can also place the wheel on a beam of known length between two points of contact, one point being the earth and one point being the scale, then calculate from there. If the beam is 2 feet long and the 500 pound wheel load is located at the 1 foot mark, the scale will see 250 pounds.

The trouble with this is that you have to calculate tare, the setup is a *****, and bathroom scales are horribly inaccurate.
 

BJC

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Bought a digital bathroom scale with great reviews on the www. Repeatability was about +/- 15 pounds out of 225. Returned it. Got a different brand. Repeatability is about +/- 0.05 pounds. It matches the balance scales and the digital scales at doctors’ offices.


BJC
 

Marc Zeitlin

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My first flight was going to be from my property where I built the aircraft and I had no access to aircraft scales either and I was not about to partially disassemble the aircraft and transport it to an airport just to weigh it. So after a thorough search of the builders forums and NTSB database for my kit plane, I could not find a single issue, incident, or accident where the probable cause finding was the aircraft configuration as-built exceeded the kit manufacturer’s CG limits...
So the fact that something is inconvenient (scales can be rented or borrowed, and they're easily moved) is not a reason not to do the right thing.

And I know (off the top of my head - might be able to come up with more with a bit of thought) of at least two fatal accidents and one non-fatal of very common aircraft that were caused by CG being out of range. Maybe for a Zenith, it's never happened. And maybe for a Sonex, it's never happened. But every plane is different - folks make modifications, changes, and do things that can cause weight/CG issues. I would not trust any W&B performed on bathroom scales.
 

FinnFlyer

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Bought two 400 pound low profile glass bathroom scales. less than 1.5 pound difference between them. Yes I used them (ganged them up and weighed one wheel at a time).
A real pain to use. Roll plane off them, lift and set down and roll plane back on and hurry to read before turned off.
Repeated multiple time and confident within a few pounds.
Weighing an RV-4 the tail weight is much much more critical.

Finn
 

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karmarepair

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Thank you Marc for pointing out the risks.

I'm looking into having a "circuit riding" A&P weigh the plane.

And I've been pleasantly surprised at the price of the 400 kg postal scales on Amazon/Ebay. It looks like 3 scales, suitable for commerce, and of the proper capacity for my plane can be had for less than 2 Benjamins.

And the "Group Mind" has confirmed that using the digital bathroom scales is do-able, but not desirable.
 

Stolch

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Marc,
No problem, trust is important. If you don’t want to ride in my bathroom scale airplane, it won’t offend me. With regard to your memory citing aircraft accidents caused by an out of CG condition (these were first flight, post build EAB kit planes right? That would be in the context of my post. Anything else would be disingenuous), show me the data. I just couldn’t find anything and I‘m partial to weighing risks (pun intended) based on data. Can you post a link to the NTSB report or Kathryn’s Report or Aviation Safety Network or a Builders Forum? I just want to do the right thing.
 

rv7charlie

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The “designer” / builder / pilot rejected constructive input on several occasions.
I suspect that this data point is far more important than the issue of uncertified scales.
I suspect that it would be a safe bet that over the last 70 years or so, more homebuilts have been weighed on non-cert scales than cert scales. IMO, the bigger issue is whether you do your due diligence. As others have said, calibration checks matter, and understanding *how* to weigh an a/c is critical too (reference the errors due to sideload mentioned above). Then there's the issue of leveling the plane; it wouldn't surprise me if planes have been weighed tail-down or otherwise unlevel, in which case the most accurate scales in the world couldn't help.
 

ClaudeR

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I bought a shipping scale off of Amazon and then was able to take it to my local county weights and measures office where the gentleman whom tests the accuracy of business scales was nice enough to check the calibration of my scale with different weights. We found that the error was between 1 to 2/3 percent at different weights from 25 pounds to 300 pounds (330 pound capacity scale). I made up a chart showing scale weight reading and the actual weight that I use when doing a W&B on my Hornet airplane.

My main points are that I chose to have the scale tested and validated rather than just assume it was okay, and it may be possible to have an expert test and validate your scale too.

But, each to his own.
 

rv7charlie

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I understood that, but my point was that with less arrogance and more conscientious attention to detail, the out of of cg issue would have been detected before flight. Scales didn't cause the crash; mental and attitude inadequacies did.
 
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