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  #11  
Old 08-14-2013, 11:22 AM
Bob Kuykendall's Avatar
Bob Kuykendall Bob Kuykendall is offline
 
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Location: Douglas Flat, CA
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Quote:
Originally Posted by Finley View Post
No Gas.
Might have an extra bucking bar in there! (just kidding Stephen)
My magnetometer is in the rt wingtip, along with the extra wire and OAT probe. Hardly think that would be more than a pound.
This is an interesting problem, and it made me curious. So I sketched out a lateral schematic and did a quickie moment balance analysis:



The following table shows an lateral moment equilibrium. The neat thing about working problems like this is that you can choose any reference point, and as you can see from the diagram I chose the left wheel.

Just as an example, I assumed that the magnetometer weighs 1/2 lb, and that the wire that serves it also weighs 1/2 lb. Of course, those weights are estimates open to debate, but let them serve for this example.

---------------mass-----arm-----moment
Wire-----------0.5------107-----53.5
Magnetometer---0.5------166-----83
Totals---------1---------------136.5


As you can see, the total moment of the magnetometer and wire, measured from the left wheel, is 136.5 in-lbs. To find the force required at the right wheel to keep the system in balance, we divide that moment by the arm of the right wheel, 74" (again, not exact, but it serves this example):

137.5 in-lbs / 74" = 1.84 lbs

To finish off, we assume that all the forces acting on the magnetometer and wire are in some sort of balance, that those items aren't rotating around their axes or levitating or sinking. Since the parts weigh 1 lb, and the force applied at the left wheel is 1.84 lbs, we know that the force at the right wheel must be 1 lb - 1.84 lb = -.84 lb, or else the parts would be accelerating upward and maybe end up going through the roof. The difference between the forces at the right and left wheels is 1.84 - -.84, which simplifies to 1.84 + .84 = 2.68 lbs.

This example shows that adding about 1 lb of stuff relatively far out in the right wing results in about a 2.7 lbs difference between the two scales. That doesn't account for all the difference measured in the case at hand, but it likely accounts for a lot of it.

(Note: No 0.8 correction factors were harmed in the production of this moment balance analysis.)

Thanks, Bob K.
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  #12  
Old 08-14-2013, 11:33 AM
nauga nauga is offline
 
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Quote:
Originally Posted by Bob Kuykendall View Post
(Note: No 0.8 correction factors were harmed in the production of this moment balance analysis.)
You're still my hero. Bob.
(I have a ~2 lb difference between left and right mainmounts, right is heavier. Trim servo in rt aileron, Duckworks landing light in left outboard LE).
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Last edited by nauga : 08-14-2013 at 11:35 AM.
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  #13  
Old 08-16-2013, 03:49 PM
Finley Finley is offline
 
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Location: Sharon Grove, KY
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Default Lateral schematic!

Lateral schematic! Yes, that's what I needed all along!!
Thanks Bob, your great!
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  #14  
Old 08-17-2013, 08:02 PM
Dmadd Dmadd is offline
 
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Location: Camas, WA
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Was the canopy open when you weighed it? I know a guy who... ahhh... did that...
:-)
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  #15  
Old 08-17-2013, 10:31 PM
jimgreen jimgreen is offline
 
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Location: Vancouver island, BC Canada
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An obvious point, but if it's not completely level laterally you will see quite a difference in weights. Done that.
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  #16  
Old 08-19-2013, 05:04 PM
CATPart CATPart is offline
 
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I took a guess at the wheel spacing, but at 4 foot wheel spacing, one wheel would have to be .5 inches inboard to make a 10 lb difference. At 6 foot wheel spacing you would need a wheel .8 inches inboard to cause a 10 lb difference.

edit....and good job keeping it light! 916 is great.

Last edited by CATPart : 08-19-2013 at 05:20 PM.
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  #17  
Old 08-20-2013, 08:39 PM
Finley Finley is offline
 
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DMADD- yep the canopy was closed. Still got your 4?

Jim- We worked hard to get it level in all the right places!

CATPart- Thanks, I was surprised at 916-
I'll ck the wheel spacing from center, Right main arm = 61.5, Lt = 61.25.

At this point I'm not to worried about the difference. Should I be?
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  #18  
Old 08-21-2013, 07:44 AM
dealfair dealfair is offline
 
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Thanks all for a most educational, interesting and entertaining thread!!

Cheers & happy flying,
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