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  #1  
Old 08-25-2015, 04:11 PM
runstrong92 runstrong92 is offline
 
Join Date: Jun 2011
Location: Carthage, Mississippi
Posts: 26
Default How to weigh the plane?

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?
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  #2  
Old 08-25-2015, 04:18 PM
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Kahuna Kahuna is offline
 
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Location: Gold Hill, NC25
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Quote:
Originally Posted by runstrong92 View Post
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?
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.
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  #3  
Old 08-25-2015, 04:35 PM
runstrong92 runstrong92 is offline
 
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Location: Carthage, Mississippi
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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?
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  #4  
Old 08-25-2015, 05:17 PM
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Kahuna Kahuna is offline
 
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Quote:
Originally Posted by runstrong92 View Post
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?
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.
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  #5  
Old 08-25-2015, 05:32 PM
esco esco is offline
 
Join Date: Jan 2011
Location: SoCal
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Default 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.
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  #6  
Old 08-25-2015, 05:34 PM
runstrong92 runstrong92 is offline
 
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Location: Carthage, Mississippi
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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!
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  #7  
Old 08-25-2015, 05:37 PM
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rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by Kahuna View Post
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.
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.
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  #8  
Old 08-25-2015, 05:54 PM
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Mel Mel is offline
 
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And don't forget the little things like, flaps up, canopy closed, etc.
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  #9  
Old 08-25-2015, 06:38 PM
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Toobuilder Toobuilder is offline
 
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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.
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  #10  
Old 08-25-2015, 06:58 PM
BillL BillL is offline
 
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Location: Central IL
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Avery tools rents a set of proper scales. $100

http://www.averytools.com/products.asp?dept=66
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cannot measure it, when you cannot express it in numbers, your knowledge
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