Scales Needed

My RV7 is going to the airport in the next couple weeks for final assembly at Oxford, CT KOXC. I am looking to beg, borrow or rent a set of scales for final weighing. Willing to travel a reasonable distance for pick up and return. Anyone in the northeast who might have a source please send a PM.

Regards,
Jake

Scales

If you know a law enforcement -pilot -enthusiast, contact them
That person may know a DOT officer with portable scales and they are happy to help you out----- Also if you have round track car crew around, most own their own for setting up their suspension

electronic scales are so cheap ,you only need one and it will get used in the future. when my plane got its cert. i bought one and it gets used at least once a year.

How do you use only one?

I thought you had to have the entire airplane on scales when doing the weight and balance

bobnoffs said:

electronic scales are so cheap ,you only need one and it will get used in the future. when my plane got its cert. i bought one and it gets used at least once a year.

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To the next post, Bob, (#4) what method do you use to settle the lateral loads when moving the scale? Or do you just roll off and back on?

I am considering the single scale purchase right now.

Sorry for being off topic . . .

dave4754 said:

I thought you had to have the entire airplane on scales when doing the weight and balance

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I just do one wheel at a time, and rest the other two on small platforms the same height as the scale. I made up plywood ramps to roll the plane up on the scale/platforms.



The scale is an Adam CPWplus 200 Platform Weighing Scale, 440lb/200kg Capacity, 0.1lb/50g Readability. $163 on Amazon.

i have platforms under the wheels that don't have a scale. for the lateral loads i have 5 or 6 pieces of condiut rolling on smooth plywood above and below.allows the gear to flex.

bobnoffs said:

i have platforms under the wheels that don't have a scale. for the lateral loads i have 5 or 6 pieces of condiut rolling on smooth plywood above and below.allows the gear to flex.

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I haven't thought till now how much a source of error this lateral loading from gear squat might be.

Next question: You went to the trouble to collect the weight data this way; did you measure your gear _stations_ while it was flexed on your conduit roller thingies so the moment arms are correct?

Check out EAA website

dave4754 said:

I thought you had to have the entire airplane on scales when doing the weight and balance

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Good article here from EAA on weighing homebuilts. It explains how to do this with only one scale - as others have indicated using platforms the same height as the scale.

Bill Boyd said:

I haven't thought till now how much a source of error this lateral loading from gear squat might be.

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This is kind of a trick question. Because the loading under question is lateral (sideways), it introduces no error in the calculation of cg (which we limit to a fore-aft number, not the up/down part, nor the left-right part) AS THE PLANE SITS. But when the plane sits with the lateral loading relieved, the gear geometry forces the wheels slightly aft. So, due to the weight of the wheels being aft, the cg will also be slightly aft of where it is during flight. I would argue that the correct measurement procedure is to NOT relieve the lateral loading. That will give you the wheels in a position closer to where they are in flight, which is what you really want to know. All this assumes that you measure the moment arms of the wheel axles as they actually are, on the scale. I haven't actually looked to see if the differences are measurable, but I suspect not.

From experience with a similar aircraft type (steel rod gear also made by Langair Machining, the source of standard RV gear), the gear flexes upward, aft and outward as weight goes on the wheels.

If one is using bathroom or similar "spring" scales, these non-vertical loads can create very significant errors in measured weight. I thought I would be smart and put the spring scales on 4-wheel carts. Even this solution didn't produce sufficiently little rolling friction to completely alleviate side loads on the scales. It wasn't until I went to full strain gauge (load cell) scales that I was able to overcome these errors.

With respect to measurement of gear stations in "weight on wheels" vs "flying" condition, the answer to the question posed above is actually pretty simple.

We don't care one whit where the wheels are when we're flying, nor does the airplane. What matters is that we calculate the balance point of the aircraft. We put scales under the wheels and measure both the weight and location of the scales. This data then is compared to the location of the Mean Aerodynamic Cord (MAC) of the wing.

If one thinks about this for a moment, we could, in a perfect world, place one scale under the very tip of the spinner and one scale under the very tip of the tail (assuming the wings stay level because we built an aircraft that's perfectly balanced laterally). We would then do the math on the position of these two scales and the weights they read in order to come up with an Empty Weight Centre of Gravity.

Because we relate everything back to MAC, where the scales are located is of no great importance, as long as we measure exactly the location of those scales.

To another of Bob's points, side loading on the scales can introduce very significant errors in calculation of C of G, simply because that side loading can cause some scales to read highly inaccurate weights.

Canadian_JOY said:

If one thinks about this for a moment, we could, in a perfect world, place one scale under the very tip of the spinner and one scale under the very tip of the tail (assuming the wings stay level because we built an aircraft that's perfectly balanced laterally). We would then do the math on the position of these two scales and the weights they read in order to come up with an Empty Weight Centre of Gravity.

To another of Bob's points, side loading on the scales can introduce very significant errors in calculation of C of G, simply because that side loading can cause some scales to read highly inaccurate weights.

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Mark, I respectfully disagree. Imagine your experiment above, with the two scales at the tip of the spinner and tail, but with a weightless string holding the wheels back, in the position they are in when on the ground. Now cut the string. The nose scale will get a bit heavier, the tail scale a bit lighter, as the wheels swing forward. As to "real" scales, I think you are correct. A very good scale will not react to side loads, but a less than very good one might.
What I think is most important is that we duplicate Vans' measurement methods. If they did it "wrong", we want to do it "wrong" the same way!

To eliminate the side load, just take one of those dark color large plastic HD trash bags, fold it over a few layers and place on the scale, lift, drag, roll, fly the plane onto the scales, just don't forget to zero the scale after the bag is placed on the scale. Ok, don't fly onto the scales

All the successful local circle track racing guys will have electronic scales, one for each wheel (you will have one left over) and their package will include a roller plate to accommodate the side loads on the main gears. The car guys have the same problem with their front suspensions ,assuming they are using a solid rear axle. I'm sure any of them would loan you their set up for a plane ride over their house or a gift card to Hooters

A local EAA club can purchase a set new for a grand and used for about five hundred bucks.

http://www.speedwaymotors.com/shop/...QGHYdytHaodD50Y0Mz87TESKbE0kFE2xoCqwsQAvD_BwE

BobTurner said:

What I think is most important is that we duplicate Vans' measurement methods. If they did it "wrong", we want to do it "wrong" the same way!

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I think we're chasing the same thought here, Bob, just using different words to describe it. The manufacturer says "weigh the airplane on its wheels" so that's how we do it. In doing so we are using roughly the same points of reference as the manufacturer and thus should produce relatively similar results. The magic multiplication of moment x arm ensures we get an accurate empty C of G as long as we accurately measure the wheel stations every time we weigh the airplane. If we change the weight of the airplane, move equipment around like the battery, and fail to re-measure the wheel stations then we're definitely not going to get a reliable Empty Weight C of G. This latter point is, I believe, where many people take shortcuts and end up with incorrect C of G calculations.

It would be much easier to get accurate C of G numbers if we weighed the aircraft using surveyed jacking points and load cell jacks. This technique is generally employed on larger airplanes. For our little airplanes the landing gear provide us with the easiest, cheapest and most accessible means of weighing the airplane. The inaccuracies of using this method are a good tradeoff against the added cost and complexity associated with other, more accurate means.