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I've had a fair bit to do with weighing in industrial settings, both static and dynamic. In a perfect world, aircraft would be weighed using calibrated and certified scales under each wheel. However, most of us don't have access to that sort of gear. And there are also practical considerations to 'under each wheel'. When it came to weighing my Savannah for it's initial Weight and Balance, here is how I went about it. I thought this may be of use to others.

1. Choose your weighing device and check the accuracy of it. Most electronic scales nowadays are surprisingly accurate, but there's no point in weighing with unproven scales.

I used our bathroom scales, and checked them against the calibrated scales at a large local bakery. Most bulk food processors have calibrated scales. We used test weights, 50, 75 and 100kg, weighing them first on the calibrated scales, then on my scales. Where there are no test weights, this could also be done by weighing different sized people. Test at several weights to ensure the scales are accurate across the scale (and not at just one point on the scale). Make sure the scales are on a flat level surface when doing this.

And as well as accuracy, check for repeatability: weigh the same person or weight several times, you should see the same result each time.

2. Cut/make a couple of packers the same thickness as the scales. These will be placed under whichever wheels are not being weighed.

3. Place aircraft on flat surface. Place packers under main gear and weigh under the nose wheel. Take at least 3 weighings by lifting the nosewheel then lowering it on the centre of the scales. If each weighing gives the same result, then you have an accurate weight. If you are getting different results then either the scales are not on a flat surface, or the wheel is not coming down cleanly on the scales. Note that the wheel does not have to be in the exact centre of the scales: the loadcell setup in the scales compensates, provided the wheel is more or less central. But it is important to persevere until you are getting multiple weighings all the same.

4. With packer under the nose and one main wheel, weigh the remaining main wheel. I did this by jacking then lowering that mainwheel, but was getting variations in the weight due to the wheel pushing sideways on the scales as the undercarriage flexed. I tried putting multiple layers of polythene under the wheel to allow it to slide sideways, but this was little better. Finally, I ran a strop between the main wheels, pulled fairly tight to prevent the undercarriage from spreading. I then got multiple identical weighings.

5. Repeat 4. for the other main wheel.

6. (Subtract weight of strop!)

 

In summary:

You must get multiple identical weighings to have confidence in the result.

I learnt that the aircraft wheel has to come down on the scales without any sideways movement to get an accurate weighing.

 

I have heard of others rolling the aircraft on and off the scales. I have not tried this.

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I got mine done on a set of interconnected load cells so didn't have a problem. If you have access to 3 bathroom scales that are the same height & you check the accuracy of each that makes it simpler & pretty accurate.

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Yes, kgwilson, I'm told the professional way to have this done is with a calibarted purpose-built 3 point loadcell setup.

And it may be that rolling the aircraft onto such an arrangement (rather than lowering it) would reduce the problems where the undercarriage spreads laterally under load.

I was intrigued by that: I've designed and commissioned weighing setups from packhouse fruit-graders to live-sheep-on-conveyors to industrial- flour-silo-contents, but none of these had a strong sideways component, which does seem to upset the accuracy of the conventional loadcell setup in bathroom scales.

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An accurate scale ( Balance or spring) suspended by a vertical rope/chain will eliminate any error due sideways displacement or forces. Nev

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Facthunter, yes. Provided you could be sure it was centered exactly over the lift point, and that the attitude of the aircraft remained the same as each point was weighed. In practise I think it could be rather demanding. Which is to say I think you would have exchanged one problem for another.

 

And the same applies to the 3 bathroom scales approach: now you have 3 sets of scales to check for calibration which, done properly, will take longer in practise than moving one set round the three wheels.

 

My point was that the job could be done, and accurately too, with minimal basic equipment (1 scale, 2 packing blocks, 1 strop, 1 hydraulic jack), provided that the scale calibration was checked, and some means was available to prevent the undercarriage from spreading: in the case of the Savannah, a simple strop between the main wheels.

Given those items, the procedure itself is quick and easy.

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I have weighed both my aircraft. The Corby with bathroom scales and the RV with calibrated scales.

In both cases I made a ramp to roll the wheels on to either the scales or a packer to get the height correct.

With a taildragger you need to have the tail up at just about flying attitude.

One thing not well known is the effect wind can have on your weighing. I tried to weigh the RV with the hangar door only 1 of the four sections open, I could not get a steady reading, close the door and reading was good. I use at least 3 weighings to ensure I get consistent results.

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Yenn is right about the pitch attitude which should approximate the flying attitude . You need frictionless platforms. Ha Ha. We always do each main wheel usually with bathroom scales or something not much better and the tailwheel with attitude constant.

Why not try a single lift moved around above it till you get the actual Cof G balance point by a plumbob and the total weight at the same time. Easy to lift off the average Hangar roof girder. If that's not adequate prop the attach point on both sides .Nev

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It’s probably worth adding that the SAAA have sets of scales available for hire to members around the country. I can’t remember the cost but from memory it wasn’t a lot and they allow you to weigh all wheels at once. The single set of scales method can work but it is a lot more work and a lot easier to get it wrong.

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  • 4 months later...

After many years of hiring the gliding ( certified) scales I have finally bitten the bullet and bought enough load cells to make my own set of 3 scales.

I ordered these before reading Ibob's ideas which I do like. One problem I have had with bathroom scales though is that they turn off before I want them to. This will not be a problem with home-built scales.

The chinese load cells were cheap and I will be able to make up 3 scales for less than one bathroom scale and they will be good for up to 200kg, so they will suit both the Jabiru and the Lancair.

I'll provide details when the scales are built.

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In summary:

You must get multiple identical weighings to have confidence in the result.

I learnt that the aircraft wheel has to come down on the scales without any sideways movement to get an accurate weighing.

I have heard of others rolling the aircraft on and off the scales. I have not tried this.

 

I had to set up a race car accurately with each wheel taking a different weight per cent. If I adjusted the suspension for the right rear to be heavier, I could get oversteer, provided I checked where that weight had come from.

 

I found that the centre of each tyre footprint was the critical down factor and it gave different results from the centre to the outer area of the scales because the top face depressed slighly, so fitted 12 mm T&G hardwood wheel pads to the top of the scales. That fixed that.

 

I always centralised the steering straight ahead because that's when a steer wheel is protruding down the most.

 

I found, as you did that the tyre footprint centrelines move around with deflection of the suspension due to the suspension geometry and this thrwos the weights off substantially.

 

I didn't weigh 1 wheel because there is a leverage/weight transfer between it and the other jacked wheels.

 

I mounted the scales on ballrace castors which allowed each suspension to travel through its own arcs and the tyre footprints to move where they needed to be, and I had a cross reference to all up weight and where the weight was going wheel by wheel.

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Like some others I used 3 readily/cheaply available electronic bathroom scales. Found the scales turned off after a few seconds. Solution, raise aircraft onto three small platforms (being careful that they dont contact scales) get three people (wife & two sons) to take readings, as I pulled/ pushed aircraft on/off scales several times for multiple reading.

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I used the SAAA scales and they are free to members. You have to pay freight. I got a bill for over $300, before I returned the scales. I queried as to ether that was for the return as well and was told No. I sent them back by a different carrier for less than $100 and complained bitterly that someone was ripping us off. SAAA gave me a discount on my next renewal. I think someone was getting backhanders from Toll who were the initial carrier.

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The trouble with using digital bathroom scales is that they only display for a short period (30 secs?) then revert to zero. Not enough time to read all three.

 

So when I weighed my MiniMax, I bought 3 old digital scales from the tip shop - $2 or $3 each. I calibrated them with my own weight, which was about the same value as 1/3 the weight of the Max.

 

Then I set everything up and recorded each reading at my leisure.

 

I suppose if the microlight you're weighing is 6-700 kg you could use 6 scales, with a plank across each pair. But it wouldn't be a microlight. . .

 

Bruce

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Hi Soleair....as described I just used the one set of scales and took them to each wheel in turn (with the other 2 wheels packed up to the same height). Yes, they turn off after a few seconds, but I found this no problem:

For the nosewheel I had someone else raise and lower it by pressing down on the tail.

For the main wheels I raised and lowered them with a hydraulic jack under the point where the gear meets the fuselage, and which I found ideal for raising and lowering the wheels vertically and cleanly.

In each case I was right by the scales and easily able to take the reading before they turned off.

Also as described, I took multiple weighings at each wheel in order to be confident that I was getting good consistent weighing.

 

It's actually very quick and easy once you get the gear together: having weighed mine, I then weighed a friend's, it took no more than 10mins.

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