Weight and Balance

[PT20J]

8 minutes ago, Will.iam said:

So if the TCDS for my plane is 18 lbs, why not just drain the tanks and weight the plane then add 18lbs to the total since that is what is required anyway? You could add back the 1.5 gallons each side for CG measurement but how much would 3 gallons change the cg since the 3 gallons is essentially over the cg anyway?

I don't see why you couldn't do that because the TCDS gives the weight and arm of the unusable fuel. So you would weigh it dry and then add the weight and moment for the unusable fuel as specified to get the legal empty weight and CG. This is most likely how they did it at the factory.

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[802flyer]

Either way it’s measuring with a micrometer, marking with chalk and cutting with an axe
 

I will be stealing this awesome saying, thank you


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[EricJ]

3 hours ago, Will.iam said:

So if the TCDS for my plane is 18 lbs, why not just drain the tanks and weight the plane then add 18lbs to the total since that is what is required anyway? You could add back the 1.5 gallons each side for CG measurement but how much would 3 gallons change the cg since the 3 gallons is essentially over the cg anyway?

+1 that that is totally feasible.   You can also fill the tanks and mathematically back out the usable fuel.   Draining them will likely be the more accurate of those two methods, but often the burden of draining the tanks is having some place to put the drained fuel and potentially a safe place to drain it.   If those aren't issues, then it's a very reasonable way to do it.

[carusoam]

29 minutes ago, 802flyer said:

 

I will be stealing this awesome saying, thank you


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It is an oft used statement by engineers in the know….

probably goes back to the days of construction, centuries ago…

Use it as required, it has been in the public domain for a really long time…

Go MS!

Best regards,

-a-

[802flyer]

It is an oft used statement by engineers in the know….
probably goes back to the days of construction, centuries ago…
Use it as required, it has been in the public domain for a really long time…

Go MS!
Best regards,
-a-

I’m an engineer and have somehow never heard it, but will be sharing it widely from here forward.


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[carusoam]

1 minute ago, 802flyer said:

I’m an engineer and have somehow never heard it, but will be sharing it widely from here forward.


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I learned it from my mechanical engineer friends… building industrial machinery… they like to measure in mils…. 

What type of engineer are you?

Best regards,

-a-

[PT20J]

45 minutes ago, EricJ said:

+1 that that is totally feasible.   You can also fill the tanks and mathematically back out the usable fuel.   Draining them will likely be the more accurate of those two methods, but often the burden of draining the tanks is having some place to put the drained fuel and potentially a safe place to drain it.   If those aren't issues, then it's a very reasonable way to do it.

Not to open a can of worms, but what the heck 

If you mathematically adjust for fuel weight, you have to know how much a gallon of gas weighs. The commonly used 6 lb/gal (actually it is 6.01) is weight at 15 deg C (standard day). According to the weight and balance section in my M20J POH, the fuel weight to be used is 5.82 lb/gal which is the density of 100LL at a warm 39 deg C (102 deg F). So, maybe the Mooney procedure cheats me out of 11.2 lb of useful load. 

Skip

[Hank]

1 hour ago, carusoam said:

It is an oft used statement by engineers in the know….

probably goes back to the days of construction, centuries ago…

Use it as required, it has been in the public domain for a really long time…

Yep! Even those of us not in construction use it. Then again, I've been measuring in hundredths of a millimeter or thousandths of an inch forever, it seems like. 

55 minutes ago, 802flyer said:

I’m an engineer and have somehow never heard it, but will be sharing it widely from here forward.

I'm a mechanical engineer in manufacturing, making and inspecting parts. What kind of engineering do you do, @802flyer?

[EricJ]

1 hour ago, PT20J said:

Not to open a can of worms, but what the heck 

If you mathematically adjust for fuel weight, you have to know how much a gallon of gas weighs. The commonly used 6 lb/gal (actually it is 6.01) is weight at 15 deg C (standard day). According to the weight and balance section in my M20J POH, the fuel weight to be used is 5.82 lb/gal which is the density of 100LL at a warm 39 deg C (102 deg F). So, maybe the Mooney procedure cheats me out of 11.2 lb of useful load. 

Skip

As alluded in the recent comments, the error budget in this process is pretty generous.   The tolerance in the scales and repeatability aren't as good as many might think.    Even so it is true that weighing with full tanks has a couple of additional error sources, including how much fuel is actually there, I think it's still within the general normal error tolerance on this stuff.   Weighing empty has some potential error sources as well, they just might run on average a bit less than for full tanks.   
 

Edit:   A driving reason to weigh an airplane full is that some airports (like mine) don't allow fueling/de-fueling in a hangar, and it's often difficult to store the amount of fuel that might need to be drained.    Defueling or draining is definitely likely to provide a more accurate result, but sometimes it's just not practical.

[PT20J]

34 minutes ago, EricJ said:

As alluded in the recent comments, the error budget in this process is pretty generous.   The tolerance in the scales and repeatability aren't as good as many might think.    Even so it is true that weighing with full tanks has a couple of additional error sources, including how much fuel is actually there, I think it's still within the general normal error tolerance on this stuff.   Weighing empty has some potential error sources as well, they just might run on average a bit less than for full tanks.   
 

Edit:   A driving reason to weigh an airplane full is that some airports (like mine) don't allow fueling/de-fueling in a hangar, and it's often difficult to store the amount of fuel that might need to be drained.    Defueling or draining is definitely likely to provide a more accurate result, but sometimes it's just not practical.

And that’s the point: It’s no use being overly precise about something which is fundamentally inaccurate.

[Shadrach]

On 11/8/2022 at 1:34 PM, Greg Ellis said:

I was curious because that is what I have come to realize with my 63 C model.  When I drain a tank dry in flight, and land, if I take a flashlight and look inside the tank I cannot see any fuel at all, none.  If I move the wing to try to slosh some fuel around to see if there is anything in there, nothing sloshes around.  When my CIES fuel senders tell my EDM900 that my tanks are empty, they are really empty.  Doesn't seem to be any unusable fuel.  I am sure there is, but like you and the TCDS, it ain't much.

Where is the filler on a 63C? I’m betting there is a rib assembly between the filler and the area where the unusable fuel is.  I would not expect to be able to see a few cups of fuel pooling in the aft part of the tank closest to the fuselage. The pick up in your plane is not on the floor of the tank, be glad that it isn’t.

[802flyer]

I learned it from my mechanical engineer friends… building industrial machinery… they like to measure in mils…. 
What type of engineer are you?
Best regards,
-a-

Also mechanical


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[DonMuncy]

12 hours ago, EricJ said:

As alluded in the recent comments, the error budget in this process is pretty generous.   The tolerance in the scales and repeatability aren't as good as many might think.    Even so it is true that weighing with full tanks has a couple of additional error sources, including how much fuel is actually there, I think it's still within the general normal error tolerance on this stuff.   Weighing empty has some potential error sources as well, they just might run on average a bit less than for full tanks.   
 

Edit:   A driving reason to weigh an airplane full is that some airports (like mine) don't allow fueling/de-fueling in a hangar, and it's often difficult to store the amount of fuel that might need to be drained.    Defueling or draining is definitely likely to provide a more accurate result, but sometimes it's just not practical.

Putting aside the rules relating to your hangar, it is quite easy to fly a tank empty, then go back to the hangar and have the other tank filled. Then you can drain, syphon or pump from the full tank and pour into the empty one, one or two gallons at a time to calibrate a stick. I would recommend grounding well to prevent a static spark. This is not without risk, but I think it is not much more than having the truck fill a tank.

[Hank]

20 minutes ago, DonMuncy said:

Putting aside the rules relating to your hangar, it is quite easy to fly a tank empty, then go back to the hangar and have the other tank filled. Then you can drain, syphon or pump from the full tank and pour into the empty one, one or two gallons at a time to calibrate a stick. I would recommend grounding well to prevent a static spark. This is not without risk, but I think it is not much more than having the truck fill a tank.

I'm at my fourth airport in fifteen years. only one of them [I was there just over one year] had a fuel truck. Everyone else has a pump that I taxied over to.

[DonMuncy]

I keep forgetting how lucky you guys are. My home drome did away with their self service operation, and if I want fuel at home, I have to call the truck.