Weight/Balance measurement...done!

Peter Ha

Peter Ha

Pre-takeoff checklist
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Nov 18, 2019
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Been looking for someone with scale to get empty weight on my C150. Found a pilot in my airport to do it for buying lunch. I love the aviation community; such helpful folks!
Not sure why my right-side is 25 lbs heavier? :rolleyes:
Weight of plane (with 5 1/4 oil) and all other items out and avgas removed.
https://imgur.com/WeBK2pS
https://imgur.com/N4GWIBD
 
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Flap motor and associated hardware.

Just a guess.
 
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Curious, did you purposely remove all the fuel just for this W&B or are you having work done that requires you to empty it.

Also curious what were all three weights? Nevermind just saw the second image. In the picture is left the pilot side?

I could see things like if the starter and battery are not mounted centerline you would start seeing numbers like 25lbs. Or maybe you didn't get all the fuel out.
 
Peter Ha said:
Found a pilot in my airport to do it for buying lunch.
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FYI: you do realize that if you plan to use this new empty weight and balance record for your Part 91 flight calculations it needs a sign off in the logbook by an A&P with all the appropriate data? The EW Is actually part of the aircraft TC and is considered a minor alteration when changed.
 
did you do a calibration prior to taking readings?
 
Tantalum said:
Genuine curiosity, what effect do parking brakes have?
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It causes erroneous scale values when the brakes are set due to side loads created. Wheel chocks are recommended if needed, but with subtracting their weight from each scale.

Also 100% of the plane must be in doors. The slightest winds will affect the load on the scales.
 
NordicDave said:
It causes erroneous scale values when the brakes are set due to side loads created. Wheel chocks are recommended if needed, but with subtracting their weight from each scale.

Also 100% of the plane must be in doors. The slightest winds will affect the load on the scales.
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most scales are not that sensitive.
 
Tom-D said:
most scales are not that sensitive.
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1 pound on the end of a 18 foot wing is how many pounds on the gear that’s 5 feet from centerline?
.001 psi of a draft on a wing that is 7000 square inches is how many pounds?

scale sensitivity is irrelevant.
 
Sinistar said:
I could see things like if the starter and battery are not mounted centerline you would start seeing numbers like 25lbs. Or maybe you didn't get all the fuel out.
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Battery is on the right side. ELT is on the right. Flap motor on the right, as already noted. Fire extinguisher is often behind the right seat in a 150. Starter and generator/alternator are in the center.
 
All force transducers have 6 axes of sensitivity - three linear (Fx, Fy, and Fz) and three moments (Mx, My, and Mz). Ideally we'd like 5 of those to be zero but it takes a lot of fussing and checking to make sure they indeed are zero, or at least acceptable small so that the Fz calibration is accurate.

You can partially check your weighing system by stepping on the four corners of each scale and see if you weigh the same. That will check each transducer for moment sensitivity about the two horizontal axes, Mx and My. I don't know an easy way to check for Mz and Fx and Fy sensitivity.

Making force transducers that are accurate is not simple. I've seen years of structural testing that had to be junked because of cross axis sensitivity that was demonstrated to an ashened-faced lab manager in a couple of minutes.
 
Dan Thomas said:
Battery is on the right side. ELT is on the right. Flap motor on the right, as already noted. Fire extinguisher is often behind the right seat in a 150. Starter and generator/alternator are in the center.
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Thanks for posting that! the battery alone puts a good dent in the 25lbs, throw in a fire extinguisher and a old ELT full of d cell batteries and it's getting close!
 
Salty said:
1 pound on the end of a 18 foot wing is how many pounds on the gear that’s 5 feet from centerline?
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Ah ... one pound?
Are you thinking the outboard 1 foot of wing weighs more than the inboard 1 foot of wing?
 
Domenick said:
Ah ... one pound?
Are you thinking the outboard 1 foot of wing weighs more than the inboard 1 foot of wing?
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Are you saying if I put a pound on the end of the left wing, the left wheel will be 1 pound heavier and the right wheel exactly the same? Uh. No.

You're moving the CG and the left wheel gets more than 1 pound heavier, and the right loses some weight.
 
Since we don’t calculate left/right CG placement and the weight increase (1.25 pounds left /-0.25 pounds right for example) on the left wheel will exactly equal the weight decrease on the right wing, does it matter? The total weight is the same (other than the hypothetical 1 pound weight) and since the Fore/Aft CG is generally close to the wing location, adding weight to the wing would not change that CG calculation more than a tiny fraction.
 
Matthew Rogers said:
Since we don’t calculate left/right CG placement and the weight increase (1.25 pounds left /-0.25 pounds right for example) on the left wheel will exactly equal the weight decrease on the right wing, does it matter? The total weight is the same (other than the hypothetical 1 pound weight) and since the Fore/Aft CG is generally close to the wing location, adding weight to the wing would not change that CG calculation more than a tiny fraction.
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That’s a good point, but most of the wing is behind the cg so a breeze can also effect that more than you might think.
 
Salty said:
Are you saying if I put a pound on the end of the left wing, the left wheel will be 1 pound heavier and the right wheel exactly the same? Uh. No.
You're moving the CG and the left wheel gets more than 1 pound heavier, and the right loses some weight.
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Thank you for the clarification on your thinking.
 
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