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Weight and Balance


Fly Boomer
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Reading my AFM/POH weight and balance chapter (1989 M20K).  Specifically, the part that tells me how to weigh the airplane and establish a new weight and balance baseline.  It says to disconnect the fuel line at the boost pump outlet, and hook up a flexible line that will reach whatever container I am using to save the fuel.  So far, so good.  Now it says to use the boost pump to pump out all the usable fuel, leaving the unusable fuel.  Then you pour 1.5 gallons of "unusable fuel" back into each tank.  My question:  Did the boost pump suck out all the fuel, leaving the tanks bone dry, thereby requiring that 1.5 gallons per tank be poured back in, or are we going to count "unusable fuel" twice?  I thought the definition of "unusable fuel" was the fuel that remained after all possible fuel was sucked out by the fuel pump.  Seems like this procedure (without explanation) specifies two kinds of "unusable fuel":
1)  The kind that's left in the tank after the engine has sucked out all it can (engine quits)
2)  The kind that you pour back into the tank after draining everything that can be drained with the boost pump.
Mathematicians please check my work.

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2 hours ago, PT20J said:

Somewhere you missed a step. After you drain out the useable fuel with the pump, you need to completely drain the tanks at the sumps and then add back in the specified  unuseable fuel.

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I guess these instructions assume the reader knows that.

a. Disconnect fuel line at electric boost pump outlet fitting.
b. Connect to output fitting a flexible line that will reach fuel receptacie.
c. Turn fuel selector valve to the tank to be drained, and remove filler cap from fuel filler port.
d. Turn on boost pump until tank is empty.  Repeat steps c. and d. to drain the other tank.
e. Replace 1.5 gal. (5.7 liters, 1.25 Imp. Gal.) fuel @ 6.0 Ib./gal.(0.72 Kg/liter) * into each tank (unusable fuel).

* (USE 5.82 Ib/gal.{0.69 Kg/liter) for 100LL FUEL).

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1 hour ago, Fly Boomer said:

I guess these instructions assume the reader knows that.

a. Disconnect fuel line at electric boost pump outlet fitting.
b. Connect to output fitting a flexible line that will reach fuel receptacie.
c. Turn fuel selector valve to the tank to be drained, and remove filler cap from fuel filler port.
d. Turn on boost pump until tank is empty.  Repeat steps c. and d. to drain the other tank.
e. Replace 1.5 gal. (5.7 liters, 1.25 Imp. Gal.) fuel @ 6.0 Ib./gal.(0.72 Kg/liter) * into each tank (unusable fuel).

* (USE 5.82 Ib/gal.{0.69 Kg/liter) for 100LL FUEL).

I looked in the M20J POH and it has the same wording which is missing the drain the sumps step. I checked the Service and Maintenance Manual and the W&B procedure refers to another section for that particular step. The M20K manuals are probably the same. I suspect that whoever wrote the POH took the procedure from the Maintenance Manual and just left out that step since it is in a separate section of the manual.

The fuel outlet is maybe an inch above the bottom of the tank so that it doesn't pick up crud and water. That's why there is so much unusable fuel. The sump is the lowest point and will completely drain the tank if the plane is on level ground. The easiest way to do this is to remove the sump drain with a bucket underneath so you don't have fight the spring while draining out a gallon and a half of fuel.

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If the fuel makes it to the fuel sep…. It is usable….

un-usable fuel is found at the bottom of the tanks, below the fuel pickups…. And the bottom of the fuel sep itself…

the plane’s attitude will make a difference…

The fuel drains leave some unusable fuel behind as well… even when removed.

 

Soooo if measuring unusable fuel it takes drying out a few places that aren’t easily drained…

PP thoughts only…

-a-

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On some airplanes it may be easier to weigh it with full tanks and just back the calculations  out to unusable fuel.     I've done it both ways,  and if you're careful i don't think it makes much difference in the results.    It can save a lot of fuss and effort to weigh it full.

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On some airplanes it may be easier to weigh it with full tanks and just back the calculations  out to unusable fuel.     I've done it both ways,  and if you're careful i don't think it makes much difference in the results.    It can save a lot of fuss and effort to weigh it full.

As long as you don’t mind losing 18lbs of useful load.

The fuel tanks capacity aren’t exactly to spec, I can fill them with an extra 1.5 gallons each side.
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14 hours ago, EricJ said:

On some airplanes it may be easier to weigh it with full tanks and just back the calculations  out to unusable fuel.     I've done it both ways,  and if you're careful i don't think it makes much difference in the results.    It can save a lot of fuss and effort to weigh it full.

When I sent out inquiries last year to three weighing services, each one replied that their method calls for filling the tanks.

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If youre going to weigh it, id just do it with empty tanks. All you have to do is go out and fly your plane, run one tank dry and have 10 gallons in the other one. Makes it easy to empty the tank out.

At the end of the day its a lot more accurate that way. Less margin for error.

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45 minutes ago, Niko182 said:

If youre going to weigh it, id just do it with empty tanks. All you have to do is go out and fly your plane, run one tank dry and have 10 gallons in the other one. Makes it easy to empty the tank out.

At the end of the day its a lot more accurate that way. Less margin for error.

When mine was weighed I ran a tank dry in level cruise. Back at the hangar I removed the sump on that side and measured what drained out. I added that back to the tank, then removed the sump on the other side to drain it, then added back in the same amount I had measured from the first tank. At that point I had the amount of unusable fuel as measured in straight and level flight in each tank. 

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25 minutes ago, Skates97 said:

When mine was weighed I ran a tank dry in level cruise. Back at the hangar I removed the sump on that side and measured what drained out. I added that back to the tank, then removed the sump on the other side to drain it, then added back in the same amount I had measured from the first tank. At that point I had the amount of unusable fuel as measured in straight and level flight in each tank. 

Do you recall how much unusable fuel you had in each tank?

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28 minutes ago, Skates97 said:

When mine was weighed I ran a tank dry in level cruise. Back at the hangar I removed the sump on that side and measured what drained out. I added that back to the tank, then removed the sump on the other side to drain it, then added back in the same amount I had measured from the first tank. At that point I had the amount of unusable fuel as measured in straight and level flight in each tank. 

How much weight did you gain when you weighed the plane?

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51 minutes ago, Greg Ellis said:

Do you recall how much unusable fuel you had in each tank?

I don't remember, but it was very little. There is no listing in the POH for unusable fuel but the TCDS lists 3.4 lbs which comes to .57 gallons, or approximately .3 gallons per side. From Note1 in the TCDS: "The certificated empty weight and the corresponding center of gravity location must include unusable fuel (not included in fuel capacity) as follows: 4 lbs. (+47.6) for the M20 and M20A; 3.4 lbs. (+48.4) for the M20B, M20C, M20D, M20E, and M20G;"

The plane is in paint and afterwards I will weight it again so at that point can provide an amount if you are interested.

49 minutes ago, Niko182 said:

How much weight did you gain when you weighed the plane?

Lost 31.65lbs

The last time the plane was weighed was in 1987, in the 33 years since that time there were a lot of changes made to the plane. I bought it in 2016 and went back through the logs and created a spreadsheet for the W&B and found some errors in the logs along the way. There were also many of the changes didn't have anything corresponding to the W&B not to mention things that "disappeared" from the plane without corresponding logbook entries so hard to tell how accurate the W&B was. So, in 2020 I decided to have it weighed. I read a lot of comments along the lines of "Don't weigh your plane, you will always lose useful load." My take on that has always been that I don't care if I use theoretical useful load, I want to know what it really weighs. The laws of physics don't care about what is written in the log books.

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For the OP, the TCDS lists either 48 lbs or 18 lbs depending on the S/N.

"The certificated empty weight and the corresponding center of gravity location must include unusable fuel (not included in fuel capacity) as follows: 4 lbs. (+47.6) for the M20 and M20A; 3.4 lbs. (+48.4) for the M20B, M20C, M20D, M20E, and M20G; 15.0 lb. (+48.4) for the M20F and M20J; 48.0 lbs. (+48.59) for the M20K (S/N 25-0001 thru 25-0446); 18 lbs. (+48.59) for the M20K (S/N 25-0447 and ON); 36 lbs. (+48.43) for the M20L (S/N 26-0001 and ON); 36 lbs. (+49.23) for the M20M (S/N 27-0001 and ON) and M20R (S/N 29-0001 and ON) and M20S (S/N 30-0001 and ON); 36 lbs. (+49.23) for the M20TN (S/N 31-0001 and ON).;"

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8 minutes ago, Skates97 said:

I don't remember, but it was very little. There is no listing in the POH for unusable fuel but the TCDS lists 3.4 lbs which comes to .57 gallons, or approximately .3 gallons per side. From Note1 in the TCDS: "The certificated empty weight and the corresponding center of gravity location must include unusable fuel (not included in fuel capacity) as follows: 4 lbs. (+47.6) for the M20 and M20A; 3.4 lbs. (+48.4) for the M20B, M20C, M20D, M20E, and M20G;"

 

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.

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58 minutes ago, 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.

When my 1970 C tanks were resealed, it took 52.2 gallons to fill them to the caps on both sides. Rated capacity is 52 gallons. So I can easily believe 3.4 lb = 0.28 gallons per side unusable.

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When my plane was reweighed, the fuel pump was used to remove as much fuel as possible, then drained another cup to cup and a half from the sumps. Before weighing, the “unusable” fuel was added back it each tank.

Later, safely on the ground, Iran the engine on the unusable fuel for about ten minutes.  Most of that time the fuel pressure showed 0.  Probably it was sucking a lot of air but enough fuel to keep my carbed engine running.

It seems that usable fuel is what can be reliably used in level, coordinated flight.

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1 hour ago, Hank said:

When my 1970 C tanks were resealed, it took 52.2 gallons to fill them to the caps on both sides. Rated capacity is 52 gallons. So I can easily believe 3.4 lb = 0.28 gallons per side unusable.

but did you jiggle the wings to get rid of the bubble? :D  Next time after you finish filling to the top, put the cap on and then walk out to the wing tip and give it a shake up and down. Reopen the cap and I think you'll be surprised at the new fuel level.

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3 hours ago, Shadrach said:

but did you jiggle the wings to get rid of the bubble? :D  Next time after you finish filling to the top, put the cap on and then walk out to the wing tip and give it a shake up and down. Reopen the cap and I think you'll be surprised at the new fuel level.

The FBO filled it and it sat for several days before I arrived to pick it up. It was still nicely full, a pleasant change for me. Afterwards, my weekly breakfast run and refill went from 9 gallons to just six.  :)

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There can be a lot more than just unusable fuel that can’t be seen looking into the fill port. The wing has dihedral with the port near the outside limit while the fuel pickup and remaining unusable is at the wing root next to the cabin with multiple ribs between it and the filler port.

it’s great information to “measure” your useable fuel in flight. But unuseable fuel is an FAA approved and defined in our TCDS. We don’t get to change it when weighing an aircraft.

Just one of the many challenges in weighing the plane with tanks full is that seldom do owners find that their tanks hold exactly the defined capacity. Mine hold less, which is common, and others such as Hank says a bit more. With our hand made planes there is considerable variation. If accuracy is a goal, weight empty with unusable added back. Or really know how many gallons are in it.

Sent from my iPhone using Tapatalk

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The airplane I built had 114 gl fuel tanks each side, if one didn’t hold 114 gls then we had to do an Engineering change for that one aircraft as it didn’t meet TCDS. That would mean of course a by serial number POH etc., because you can’t sell an airplane that doesn’t hold the stated in the POH fuel quantity.

All of them held about 115 gls, in the I guess 14 years I worked there we never had one not hold the required 114 gls per side. They were wet tanks and of course hand made and I’d bet money Mooney wings were built to a higher precision being laminar flow and all.

All Military aircraft I ever weighed, we weighed full of fuel, if you’re after no kidding accuracy and want to know your exact useful load etc., then weigh with the tanks full. That’s an actual, not calculated weight, weighing empty your calculating based on what the tanks should hold, not what they do hold.

Either way it’s measuring with a micrometer, marking with chalk and cutting with an axe, meaning it doesn’t need to be that level of precision, not really.

Military wise we even got down to floating a calibrated bulb in fuel to determine the exact specific gravity of that fuel at that temp, never was enough to matter really.

It’s not unheard of for an average 40 yr old or more GA airplane to be several lbs off calculated weight, I’ve seen some gain 20 lbs. Then when you do weigh the airplane, read the weight kits level of accuracy, every one I’ve seen will state a plus or minus in lbs or percent whichever is greater, the scales may not be as precise as we would want.

 

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Weighing while empty is best…. Empty, is pretty well defined in Mooneys…

Weighing while full can be a bit challenging…. some Mooneys got a full mark at the factory… a small hole drilled into the fuel neck… at the calibrated height…

Others don’t have a hint of where full actually is…

For the M20R…

The POH claims 89gal useable…  careful filling nets 100+

careful filling includes allowing air to escape the tiny vent holes in the ribs at the top of the tank….

careful emptying includes making sure the tiny drain holes in the ribs are still open and not blocked with sealant…

 

 

Sooooo…. Since weight is important…. And fuel capacity is important…

You may have to start empty and fill her up one gallon at a time…. While building your fuel stick….  :)

Then there is the whole level ground thing…

Our tanks easily change capacity due to listing one direction or the other….

 

PP thoughts only,

-a-

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On 11/8/2022 at 12:29 PM, Skates97 said:

For the OP, the TCDS lists either 48 lbs or 18 lbs depending on the S/N.

"The certificated empty weight and the corresponding center of gravity location must include unusable fuel (not included in fuel capacity) as follows: 4 lbs. (+47.6) for the M20 and M20A; 3.4 lbs. (+48.4) for the M20B, M20C, M20D, M20E, and M20G; 15.0 lb. (+48.4) for the M20F and M20J; 48.0 lbs. (+48.59) for the M20K (S/N 25-0001 thru 25-0446); 18 lbs. (+48.59) for the M20K (S/N 25-0447 and ON); 36 lbs. (+48.43) for the M20L (S/N 26-0001 and ON); 36 lbs. (+49.23) for the M20M (S/N 27-0001 and ON) and M20R (S/N 29-0001 and ON) and M20S (S/N 30-0001 and ON); 36 lbs. (+49.23) for the M20TN (S/N 31-0001 and ON).;"

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?

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