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How to weigh the plane?

R

runstrong92

Member
Trying to figure out how to accurately weigh the plane and I'm not quite sure how to do it. I tried using a board across two bathroom scales but I question the accuracy of the numbers that gave me. It's very likely I'm overthinking it. What is the best method for getting a good reading?
 
runstrong92 said:
Trying to figure out how to accurately weigh the plane and I'm not quite sure how to do it. I tried using a board across two bathroom scales but I question the accuracy of the numbers that gave me. It's very likely I'm overthinking it. What is the best method for getting a good reading?
Click to expand...

Your process should work just fine and will be accurate enough for what you are trying to accomplish. Just make sure that the plane is 'leveled' per instructions and weighing one wheel at a time is just fine. Yes you are over thinking it. Toughest thing is getting a mechanism that is safe to hold the wheel, and of course subtracting the weight of what ever device you make to keep the plane on the scales from moving, or worst, falling, off the gismo.

A poke around your local EAA chapter may reveal some scales that will better sever your purpose.
 
Is it acceptable to jack the plane - place the scales - then lower the plane back down onto the scales. This method, as easy as it is, appears to give a bit of side load before settling the weight onto the scales. Any thoughts?

Also, in a level attitude, the tail is light on our stand. (Afraid of it tipping forward). Any thoughts on placing the scales for the tail measurement?
 
runstrong92 said:
Is it acceptable to jack the plane - place the scales - then lower the plane back down onto the scales. This method, as easy as it is, appears to give a bit of side load before settling the weight onto the scales. Any thoughts?

Also, in a level attitude, the tail is light on our stand. (Afraid of it tipping forward). Any thoughts on placing the scales for the tail measurement?
Click to expand...

Yes jacking is the best most stable way. There will be no side load if you level the plane. Light tails are normal depending on the model. Set the scale up on a stool or some mechanism building up with blocks to set the scale on. Again level attitude based on the plans. It will feel light as it approaches level.
 
quick, easy

With two people, one can take a slight squat position with the spine aligned along the spar, at a point where the dihedral makes this accessible.

Straighten the legs to easily raise the wing (from a "shallow" squat, it's easy to move silly amounts of weight.) Person two places a scale beneath the lifted tire. One squats a bit less to lower the tire to the scale, and exits from beneath the wing, smiling.

Repeat for the opposite side, and reverse to remove scales.
 
The plane is an RV-4. When I was jacking the plane up to align the wheel pants I noticed that the gear would change position quite a bit. This led me to suspect there might be a side load on the scales. I'll give it a try again tomorrow with the suggestions. Thanks for the help!
 
Kahuna said:
Yes jacking is the best most stable way. There will be no side load if you level the plane. Light tails are normal depending on the model. Set the scale up on a stool or some mechanism building up with blocks to set the scale on. Again level attitude based on the plans. It will feel light as it approaches level.
Click to expand...

Actually, if you lower an airplane onto scales there will be side loads on the main gear....
When you lift the airplane and unweight the gear, it returns to its undeflected / no weight state.
When you lower it onto scales, the lateral friction of the tire on the scale prevents it from deflecting outward. This can cause scale errors and maybe even damage depending on the scales being used
One way to get around it is to use grease plates on the scales (two pieces of aluminum with grease between them). They will allow the tires to slide laterally on the scales as the gear legs deflect.
If you roll the airplane onto the scales the legs are already deflected so it doesn't matter.
 
And don't forget the little things like, flaps up, canopy closed, etc.
 
I've used bathroom scales in a pinch, but they are notoriously inaccurate. Yes, they might get you in the ballpark, but the initial weight of the airplane should be done as close to perfect as you can get it. After all, all future modification are probably going to be done by calculation, and those are generally pretty inaccurate. You don't want to compound mistakes. Also, when weighing the tail, a bathroom scale error can be a significant percentage of the actual weight, which in turn can move your calculated CG quite a bit.

Personally, I borrow a set of race car scales, confirm each pad's accuracy with a known weight, and weigh the airplane 3 times, moving each pad to a new position, then average the results. I do this with the tail up (for CG) and with the tail down (to confirm empty weight). Yes, I roll the airplane on and off the scales 6 times for each session.

This sounds like a bunch of work, but if you have platforms set up the same height as the pads, it only takes a few seconds to move the airplane on and off.

This is important stuff and you are likely to only do it once... My advice is to get it as right as you can.
 
I rented a set from Avery. Cost was reasonable but shipping was expensive as I had ship them back. The shipping to and fro was as much as the rental...But, I'm confident about the accuracy and when I called Avery and ordered them, they showed up the next day! Very easy to use!
 
Hi John,

I think I visited your project a few years ago. Didn't realize it was you until I looked at your profile.

The Jackson EAA chapter has a set of proper a/c scales. Email me or give me a call tomorrow afternoon & I'll try to put you in touch with the right people.

Charlie
Slobovia Outernational

[email protected]
601-879-9596
 
Race car guys oftentimes have or know people who have scales, too. Talk to your local speed shop and see if they can help.
 
For the side load thing, take a HD plastic trash bag, fold it a few times to get 8-10 layers, and set the tire down on it, the CF of the plastic sliding on its self will allow the gear to flex outward when lowering, I do this trick for taking the pre load off the steering components on big trucks doing a field front end alignment check.
 
I used the bathroom scales, made a bridge between 2 of them (tare weight subtracted) then made a plywood ramp same height as bridge. I rolled mine up onto the scales and put tail wheel on stand to get level. after completeing mains, I used a single scale for tail. Do the roll-on 3 times, and take average of the 3 readings. We do the same thing with airliners using roll-ons at day job...except the tail wheel of course!
 
Modern bathroom scales are far more accurate than the spring-and-gears versions of yore. I like this one, and it's only $26.

http://www.amazon.com/EatSmart-Precision-Bathroom-Capacity-Technology/dp/B001KXZ808

The only potential problem is that max capacity is 400 lb. My RV was light enough that the mains came in at about 385 and the nose wheel far less, but if you've built a porker, the mains may exceed scale capacity.

It's also nice to have this scale around the hangar to weigh baggage and passengers not willing to divulge their weight. :eek::D:eek:

It's very low profile, so some 3/4-in. plywood with a 1x1 strip beneath on the scale end makes an excellent ramp. I weighed each wheel about 5 times and averaged the values...the variance was only about 0.5-0.6 lb. Race scales would be better, but this is an inexpensive and accurate-enough method.
 
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So just a question..... Why have to level the plane?

I get that it is because it is how the numbers have always been generated... But why can't you just develop numbers for leaving the tail on the ground?

I mean you could get WB info from a three point attitude. Overall weight should not be significantly different if you just add all three up.

Am I missing something?
 
ssmdive said:
So just a question..... Why have to level the plane?

I get that it is because it is how the numbers have always been generated... But why can't you just develop numbers for leaving the tail on the ground?

I mean you could get WB info from a three point attitude. Overall weight should not be significantly different if you just add all three up.

Am I missing something?
Click to expand...

The total weight should be the same but the center of gravity will be aft of the proper location. You want to know the CG when the aircraft is in level flight. The CG is the point around which the airplane will pitch in flight and knowing it exactly is critical to keeping the airplane within the flight envelope. This is very important stuff and should be performed very carefully and according to accepted methods.
 
ssmdive said:
So just a question..... Why have to level the plane?

I get that it is because it is how the numbers have always been generated... But why can't you just develop numbers for leaving the tail on the ground?

I mean you could get WB info from a three point attitude. Overall weight should not be significantly different if you just add all three up.

Am I missing something?
Click to expand...

Total weight will be close but your CG calculations will be off. CG numbers are the ones that can hurt you.

:cool:
 
bret said:
For the side load thing, take a HD plastic trash bag, fold it a few times to get 8-10 layers, and set the tire down on it, the CF of the plastic sliding on its self will allow the gear to flex outward when lowering, I do this trick for taking the pre load off the steering components on big trucks doing a field front end alignment check.
Click to expand...

Brilliant!
 
I weighed mine with U beaut scales borrowed from SAAA in Australia. Freight was a rip off but otherwise good.
I tried to weigh in the hangar with one door open, that is about 3 metres wide. I couldn't get a stable weight because of wind creating lift.
Closed the door and I got 3 identical weighings.
 
f1rocket said:
The total weight should be the same but the center of gravity will be aft of the proper location. You want to know the CG when the aircraft is in level flight. The CG is the point around which the airplane will pitch in flight and knowing it exactly is critical to keeping the airplane within the flight envelope. This is very important stuff and should be performed very carefully and according to accepted methods.
Click to expand...

I know the importance of CG, was a loadmaster years ago.

But maybe I didn't explain my question well enough... I'll try again.

So Arm X Weight=Moment and CG envelope is based on these calculations.
So let's just use the tailwheel weight since it has the most arm (~240).

I level the plane and the TW weight is ~50#. And the mains are about 430# each giving us 910# total weight and a TW moment of 12,000.

So we weigh it in a three point attitude and we get:
Tail 70# and 420 each for the mains.... The TW moment would READ 16,800 but the plane is STILL in CG.... Dead center CG as a matter of fact. The total AC weight would still be 910, we would just need to shift the CG numbers on paper. So dead center is now 16,800 instead of 12,000.

I think I know the answer... And that it is gear differences could change the numbers.
 
In the 3 point attitude the arms are not the same as when level. You've changed the geometry so the distance from the datum to each wheel contact point is shorter.
 
ssmdive said:
So just a question..... Why have to level the plane?
Click to expand...
The short answer is that you need to turn the measurement into a linear measurement parallel to the canopy rail... To do that, you'd be doing a lot of trigonometry to work out you CG location, and every time you make one of those calculations you risk making an error. You also need to know your exact mechanical dimensions more accurately, as small deviations from the "design ideal" could be magnified by each calculation.

My procedure for measuring tailwheels, which I have used on a large number of tailwheel aircraft including my own:

1. Weigh the entire plane in 3-point attitude. This is your empty weight, and is independent of attitude.
2. Raise the tail until the plane is level, and read the weights on the two main gear. Don't bother trying to weigh the tail.
3. Add the two main gear weights from (2), and subtract that total from the empty weight (from 1). This gives you the weight on the tail when level.

You now have the weights on the mains, the weight on the tail, and the empty weight. My personal opinion is that using the "book values" for the locations of your main gear and tailwheel will be just fine, provided you have built the plane nominally to the plans. Even if you've used different-sized wheels, the axles will still be in the same places when the plane is level.
 
Snowflake said:
The short answer is that you need to turn the measurement into a linear measurement parallel to the canopy rail... To do that, you'd be doing a lot of trigonometry to work out you CG location, and every time you make one of those calculations you risk making an error. You also need to know your exact mechanical dimensions more accurately, as small deviations from the "design ideal" could be magnified by each calculation.

My procedure for measuring tailwheels, which I have used on a large number of tailwheel aircraft including my own:

1. Weigh the entire plane in 3-point attitude. This is your empty weight, and is independent of attitude.
2. Raise the tail until the plane is level, and read the weights on the two main gear. Don't bother trying to weigh the tail.
3. Add the two main gear weights from (2), and subtract that total from the empty weight (from 1). This gives you the weight on the tail when level.

You now have the weights on the mains, the weight on the tail, and the empty weight. My personal opinion is that using the "book values" for the locations of your main gear and tailwheel will be just fine, provided you have built the plane nominally to the plans. Even if you've used different-sized wheels, the axles will still be in the same places when the plane is level.
Click to expand...

Great answer.

Thanks.
 
Vertical CG?

You would need to know to Vertical CG to make the trig work. We normally work the other way, three swings to get the vertical CG. Nose up, level (for the CG) then nose down. We needed the vertical CG to put an off center JATO bottle on a target drone.

For a tail dragger:
1. find the CG at the surface of a level airplane (this is what we normally do) and draw a line through the CG, perpendicular to the ground.

2. Draw a line from the mains to the tail for a new reference line. Now determine the CG along the new line with the tail on the ground. Draw a line through the CG, perpendicular to the (new) ground reference line.

The two perpendicular lines will intersect at the airplanes vertical CG.
 
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