Weight question came up today while weighing my plane

[hubcap]

Finishing up the upgrade on Myrtle, so today while putting her on the scales it was discovered that she weighs more on the right side by about 60lbs. Does this seem in the “normal” range?

 

[PT20J]

Seems high. Mine was only 12 lbs heavier on the right.

Left 788

Right 800

Nose 639

The engineer in me always suspects instrumentation and would swap the scales and repeat the measurement.

Skip

[Ragsf15e]

Sure you got all the fuel out? 10 gallons hiding?

[Fly Boomer]

Just based on what I read, I think what @PT20J (Skip) says is pretty likely.  In fact, I would be surprised if half the airplanes that are weighed, come back with an accurate number.

[EricJ]

Is it level laterally?   Were the scales zeroed prior to weighing?

There's no guidance in the SMM for the K model on lateral imbalance.    It's not usually an issue whether there's imbalance or not.   I've seen numbers on some airplanes in that region, but it is higher than typical (I've weighed about a half-dozen, Mooneys and otherwise).    Double check the fuel level in the tanks.

If you really want to get it level if it isn't level laterally, you can level it by letting a little air out of the high side tire, but then recheck the longitudinal levelling.    Might take an iteration or two to get it level both ways.   Otherwise make sure the scales are zeroed before your start, and that they haven't deviated much when you're done (i.e., zero before putting the airplane on the scales, check the weights after the airplane has been removed from the scales).    If you're using jacks that's pretty easy, if you're rolling on and off it's a little more work (and a little more sketchy).

Personally I wouldn't worry about it too much, especially if the scales are zero or close to it after you take the airplane off.

[Fly Boomer]

12 minutes ago, EricJ said:

Were the scales zeroed prior to weighing?

And recently certified?  I don't know how much difference it makes, but I read about airplanes that mysteriously gain hundreds of pounds.

[1980Mooney]

1 hour ago, PT20J said:

Seems high. Mine was only 12 lbs heavier on the right.

Left 788

Right 800

Nose 639

The engineer in me always suspects instrumentation and would swap the scales and repeat the measurement.

Skip

 2,227 lbs?  Is that supposed to be the empty weight of your J or was there fuel in the tanks?

[1980Mooney]

50 minutes ago, Fly Boomer said:

And recently certified?  I don't know how much difference it makes, but I read about airplanes that mysteriously gain hundreds of pounds.

I would think with the advances in today's digital load cells (with wireless connectivity, load cells on jack points, pads, etc,) that issues with accuracy are largely a non-issue today.  I bet those "mysteriously gaining pounds" stories are more idle shop talk and denial by owners.  It is like going to the doctor - telling them your weight on the appointment forms - then getting on the scale and acting surprised. "Gosh - I wonder where that weight came from..."

[N201MKTurbo]

The scales are a bitch! You always want to slap that bitch. But she is righteous and you can’t argue with her.  You could have passed her by and ran the numbers, but you wanted to know for sure, and then we’re sorry you asked.   

[PT20J]

1 hour ago, 1980Mooney said:

 2,227 lbs?  Is that supposed to be the empty weight of your J or was there fuel in the tanks?

Those were the scale weights, but there were some adds and subtracts as it was weighed with full tanks and interior removed. Final empty weight was 1895 lb. Sorry, should have mentioned that. 

[ArtVandelay]

Those were the scale weights, but there were some adds and subtracts as it was weighed with full tanks and interior removed. Final empty weight was 1895 lb. Sorry, should have mentioned that. 

Those late model Js are heavy…my 78 is 1728…167 lb difference.

My fuel capacity is 3 gallons more than spec, so the full tanks method would increase my empty weight by 18 lbs. And this is why I always require them to do the math, which I usually have to correct.

[Shadrach]

4 hours ago, ArtVandelay said:

Those late model Js are heavy…my 78 is 1728…167 lb difference.

My fuel capacity is 3 gallons more than spec, so the full tanks method would increase my empty weight by 18 lbs. And this is why I always require them to do the math, which I usually have to correct.

I don’t understand where that much extra weight comes from. That’s 215lbs heavier than my F. Perhaps some Internal Ordinance that we can’t see?

[PT20J]

42 minutes ago, Shadrach said:

I don’t understand where that much extra weight comes from. That’s 215lbs heavier than my F. Perhaps some Internal Ordinance that we can’t see?

Probably a bunch of little things like cowl flap motors, split rear seats with headrests, articulated front seats, overhead wemacs, a tone generator and second overhead speaker for gear and stall warnings instead of sonalerts. But, my gross weight is 2900 so I still have 1000 lb. useful load.

I’m pretty sure my 1978 J was a few knots faster, too. 

Skip

[PT20J]

5 hours ago, ArtVandelay said:

Those late model Js are heavy…my 78 is 1728…167 lb difference.

My fuel capacity is 3 gallons more than spec, so the full tanks method would increase my empty weight by 18 lbs. And this is why I always require them to do the math, which I usually have to correct.

Mine was right on accounting for the unuseable. Then there is the fact that Mooney’s weighing procedure calls for using a fuel density of 5.82 lb/gal.

I was more interested in the CG than the weight. Because of the trailing link landing gear design which causes axel positions to vary, you have to make a lot of measurements to do it correctly. 

Skip

[Shadrach]

37 minutes ago, PT20J said:

Probably a bunch of little things like cowl flap motors, split rear seats with headrests, articulated front seats, overhead wemacs, a tone generator and second overhead speaker for gear and stall warnings instead of sonalerts. But, my gross weight is 2900 so I still have 1000 lb. useful load.

I’m pretty sure my 1978 J was a few knots faster, too. 

Skip

I suppose.  I have some of those thing as well. Mine came with split rear seatbacks with individual recline from the factory. I’m sure the weight of the motors adds up. I don’t doubt it’s true it’s just hard to get my head around it adding up to 215lbs 

[Shadrach]

1 hour ago, PT20J said:

Probably a bunch of little things like cowl flap motors, split rear seats with headrests, articulated front seats, overhead wemacs, a tone generator and second overhead speaker for gear and stall warnings instead of sonalerts. But, my gross weight is 2900 so I still have 1000 lb. useful load.

I’m pretty sure my 1978 J was a few knots faster, too. 

Skip

Is your plane limited to 120lbs in the baggage compartment and 10lbs in the hat rack?

[philiplane]

15 hours ago, hubcap said:

Finishing up the upgrade on Myrtle, so today while putting her on the scales it was discovered that she weighs more on the right side by about 60lbs. Does this seem in the “normal” range?

 

Was it weighed correctly, with fuel drained, and leveled laterally and longitudinally? Weighing with full fuel guarantees errors. Out of level does too.

[A64Pilot]

1 hour ago, Shadrach said:

I suppose.  I have some of those thing as well. Mine came with split rear seatbacks with individual recline from the factory. I’m sure the weight of the motors adds up. I don’t doubt it’s true it’s just hard to get my head around it adding up to 215lbs 

Maybe it’s been repainted, paint can add more weight than you would think.

Scales being digital and maybe wireless etc aren’t any more accurate, it’s the cell that determines accuracy and I don’t think they have changed.

‘Every scale I’ve seen used under wheels was actually originally made for weighing trucks etc. “Aircraft” scales use cells that mount on top of jacks, or the ones I’ve used do.

These are the scales we used at the aircraft factory where I worked, sold as aircraft scales, but with the name road runner and the fact they look exactly like truck scales you have to wonder. 

http://www.scalesinc.com/road_runner_aircraft_scale.htm

Accuracy was I believe .2% of rated capacity of each cell

We weighed an aircraft three times, taking it off of the scales and re-leveling it each time, it was surprising how much variation there was between each weighing, we averaged the weighings.

[Aerodon]

17 hours ago, hubcap said:

Finishing up the upgrade on Myrtle, so today while putting her on the scales it was discovered that she weighs more on the right side by about 60lbs. Does this seem in the “normal” range?

 

Swap the left and right scales around.  Something is wrong, 60lbs is a bag of concrete and them some.  Or 10G fuel remaining in the tank?

Aileron servo is mounted on one side, 2lbs out there might make 6-8 lbs difference at the wheel.

Just for fun I weighed a C172 with and without levelling, fore and aft.  There was an 80lb change in weight on the nose wheel, so it really does help reduce wear and tear by holding the control wheel back while taxying.

A 60lb imbalance would bug me.

 

Aerodon

 

 

 

[MikeOH]

If the following statement is wrong, explain why:

If I want to know ONLY the weight of my aircraft, NOT CG, I don’t care if the plane is level, either longitudinally or laterally; just add up the weights on the individual wheels.

[wombat]

If the individual scales are not level, they might be measuring wrong.   But the overall statement you made is correct.

[1980Mooney]

4 minutes ago, wombat said:

If the individual scales are not level, they might be measuring wrong.   But the overall statement you made is correct.

I think you mean that the individual scales are measuring right - they are accurately measuring the weight on the individual load cell - but if the plane is tilted at an angle, more weight at that particular moment will be on one side of the plane than the other.  

[EricJ]

14 minutes ago, MikeOH said:

If the following statement is wrong, explain why:

If I want to know ONLY the weight of my aircraft, NOT CG, I don’t care if the plane is level, either longitudinally or laterally; just add up the weights on the individual wheels.

If you're weighing an airplane after modifications that may affect CG, you're also trying to determine the new CG from the scale weights.    For-aft weight matters here, but not lateral, so the MMs typically specify to level the aircraft longitudinally.   It will make a difference in the accurate determination of CG, but not overall weight.

[EricJ]

1 hour ago, 1980Mooney said:

I think you mean that the individual scales are measuring right - they are accurately measuring the weight on the individual load cell - but if the plane is tilted at an angle, more weight at that particular moment will be on one side of the plane than the other.  

The scale needs to be reasonably level as well so that the normal force on the scale is aligned with the measuring axis of the load cell.   That said, very small angle changes from level won't affect it much.   The amplitude of sin(theta) doesn't change much for very small values of theta.

Edit:   Derp, cos(theta) doesn't change much for small angles around zero.    

[1980Mooney]

12 minutes ago, MikeOH said:

If the following statement is wrong, explain why:

If I want to know ONLY the weight of my aircraft, NOT CG, I don’t care if the plane is level, either longitudinally or laterally; just add up the weights on the individual wheels.

I think you are exactly right.  

I agree that you need to level the plane to drain the fuel tanks to the same point between different weighings in time to insure the same amount of "unusable fuel" on board.  But if one does properly remove the usable fuel (leaving only "unusable"), at the moment the plane is weighed, the total weight of the plane will not change regardless if the plane is level or not.

If one measures the weight simultaneously from 3 points, using accurate load cells, the total weight will be the same regardless of the level, laterally or longitudinally.  A plane does not change in mass x the coefficient of gravity at that particular point on earth (i.e. "weight") regardless of if it is in a 15-degree climb, a 45-degree banked turn or completely nose down in a dive.  Physics are physics and they don't lie.

Perhaps the "level" talk is a vestige of the old days when they had to use a single massive mechanical scale which required jacking wheels up one at a time.