Fuel gauges vs. fuel burn

[wombat]

I don't know how all of the rest of you are filling the tanks, but when I've filled the plane with long range tanks, (I've done it twice now total in my life!!  hahaha) I take both fuel caps off of one side, fill one of them, then immediately go and fill the other, then go back to the first.   I don't see how that could possibly leave room for 4.5 gallons per side more because of fuel moving between the tanks.     There is maybe 15 seconds between filling one and then the other.   

 

The second time I did this, after filling both tanks fully (going A->B->A) I then set the fuel nozzle down and gave the plane a good shake and was able to get roughly another gallon in, but I couldn't see the fuel meter from the plane so I don't know exactly how much more I got.   But when I shook the plane again after that, the fuel level didn't seem to change in either tank.

[Pinecone]

Wait a few minutes.

I take both caps off, then fill the inboard/main tank.  Put the cap on and fill the outboard/aux.   They do the other side.  Come back to the first side and cen get a couple more gallons in.   This can be repeated a number of times.  Or just wait, and then fill the outboards again.   

As I said, I did this recently for cheap fuel and after lunch, put in almost 9 gallons more.

The aux tanks are NOT THAT much higher than the mains.  As I said before, when I was calibrating dip sticks, even after putting in the rated 52 gallons for both, as I added more, the level would go down over time and I could hear bubbles.  I got 55 gallons in that wing.  And this was from running the tank to the engine sputtering.

[Will.iam]

On 5/13/2023 at 3:23 PM, Pinecone said:

Wait a few minutes.

I take both caps off, then fill the inboard/main tank.  Put the cap on and fill the outboard/aux.   They do the other side.  Come back to the first side and cen get a couple more gallons in.   This can be repeated a number of times.  Or just wait, and then fill the outboards again.   

As I said, I did this recently for cheap fuel and after lunch, put in almost 9 gallons more.

The aux tanks are NOT THAT much higher than the mains.  As I said before, when I was calibrating dip sticks, even after putting in the rated 52 gallons for both, as I added more, the level would go down over time and I could hear bubbles.  I got 55 gallons in that wing.  And this was from running the tank to the engine sputtering.

Yes what pinecone has experienced i have as well with the same numbers. The first time i tried to fill mine up full i thought i must have  a leak somewhere as i was well above my max claimed 104 gals but if i waited ling enough i could squeeze another .5 gal a side in. I stopped at 110 but as the time to wait vs the amount of gas i was getting in was diminishing returns and i would sure hate to have that cool fuel start heating up in the texas heat and expanding thus pouring out the vent line if i ever truly was able to completely top the tanks off.  

[wombat]

Well, my plane is getting the fuel tanks resealed right now, so after this I'll know exactly how much they hold total.   Won't know how high up the fuel drains are though.

And these issues that it sounds like we are discussing here don't really seem related to the slow draining of the fuel from one tank to the other, they are more about the isolated air pockets within each tank where it's hard to get the air out to let fuel fill that volume.

 

If it was just about the drain speed between the tanks, filling up one and then the other immediately would allow only a difference of the amount of fuel that could flow between them in the time it took to fill the other one up.

[Fly Boomer]

59 minutes ago, wombat said:

And these issues that it sounds like we are discussing here don't really seem related to the slow draining of the fuel from one tank to the other

Only has to be slightly faster than the 520 can suck it out.

[Pinecone]

On 6/1/2023 at 6:52 PM, wombat said:

Well, my plane is getting the fuel tanks resealed right now, so after this I'll know exactly how much they hold total.   Won't know how high up the fuel drains are though.

And these issues that it sounds like we are discussing here don't really seem related to the slow draining of the fuel from one tank to the other, they are more about the isolated air pockets within each tank where it's hard to get the air out to let fuel fill that volume.

 

If it was just about the drain speed between the tanks, filling up one and then the other immediately would allow only a difference of the amount of fuel that could flow between them in the time it took to fill the other one up.

 

Except that can be more than 10 gallons if you fill the main, then the Monroy, then do the other side, come back to the first side and fill the Monroy again, and back to the second side.  Then go to lunch.

As I mentioned, when I calibrated my tanks and dipsticks, even when I put in more than the rated amount of fuel, and just adding the last amounts to the Monroy, I could hear bubbling as the fuel still migrated from the Monroy to the main.

[wombat]

@Pinecone What I'm trying to say is that despite the fact that you might be able to get 10 extra gallons in after waiting, that isn't because of the slow draining between the two tanks, that's because of some other issue and you would get the same extra 10 gallons in if the tanks were connected with a 3" wide hose or not connected at all.

If I fill the main tank up so it's just over the flapper, and then immediately fill the aux up to the very top, the maximum amount of fuel that could have flowed from the main to the aux is however much gravity could flow through the connecting pipe in the amount of time it took to switch the nozzle between tanks and fill up the aux.  Maybe 1/2 gallon?

If you then then go to back and try to put more fuel into the main, the most you can put in due to the draining is that amount. (Guessing 1/2 gallon).   Actually less, because the aux will have drained back into the main a little in the meantime.   

 

The absolute worst case scenario would be a pinhole connection between the two, but you wait days between switching the main and aux. Also the worst would be if the tanks were actually at the same elevation, but we know the aux is slightly higher.   So let's run this worst-case scenario.

Mains: 37 gallons, aux: 15.

So after you first fill the main tank, you have 37 gallons out of 52 total per side, or 71.1%.  Since the time to drain is super slow, it's effectively 0 transfer.

But by the time you switch to the other tank, they are at equal levels, which would be 71.1% full (26.3 in the mains, 10.7 in the aux) 

So you then fill up the aux to full, by adding 4.3 gallons for a total of 41.3 of 52 gallons, or 79.4% full.  None transfers while you are filling but by the time you switch back to the mains, they are equalized again with 29.4 in the mains and 11.9 in the aux.

So that lets you add 7.6 gallons in the main, giving you a total of 48.9 gallons of a total of 52 possible,  or 94% full, with only 3.1 gallons of empty space.  

This is the absolute worst possible scenario, where no fuel transfers when you are filling, and it absolutely equalizes when you are not filling.  In real life, it does transfer when you are filling, and it doesn't totally equalize when you are switching between tanks.   

 

So worst case scenario for mains->aux->mains is that you are missing 3.1 gallons.

 

I don't dispute that you are adding 10 gallons per side after waiting a while.   The tops of the tanks have cavities (made by the ribs and supporting structure under the upper surface of the wing) that air can get trapped in and shaking the plane or heating and cooling the top of the wing might cause some of that air to flow to a vent line and then become at least partially filled with fuel. 

[wombat]

Back to the original question I made this post about....

The fuel gauges read 25 gallons in the tanks total (14 L, 11 R) but the fuel totalizer read 41 gallons.    I filled the tanks to the top, and was able to put 64 gallons in.

So the gauges were reading about 10 gallons low per side.

I double-checked this in flight by watching the fuel levels on the gauges, and then when I had 10 gallons left in one tank (according to the gauge) I started a timer and noted the 'EST FUEL' remaining on the engine monitor and then ran on that tank exclusively.  It took 1 hour exactly and 19.5 gallons to run the tank dry.  More than 30 minutes of that time the gauge read 0.0 gallons remaining.

[Pinecone]

20 hours ago, wombat said:

@Pinecone What I'm trying to say is that despite the fact that you might be able to get 10 extra gallons in after waiting, that isn't because of the slow draining between the two tanks, that's because of some other issue and you would get the same extra 10 gallons in if the tanks were connected with a 3" wide hose or not connected at all.

If I fill the main tank up so it's just over the flapper, and then immediately fill the aux up to the very top, the maximum amount of fuel that could have flowed from the main to the aux is however much gravity could flow through the connecting pipe in the amount of time it took to switch the nozzle between tanks and fill up the aux.  Maybe 1/2 gallon?

If you then then go to back and try to put more fuel into the main, the most you can put in due to the draining is that amount. (Guessing 1/2 gallon).   Actually less, because the aux will have drained back into the main a little in the meantime.   

 

The absolute worst case scenario would be a pinhole connection between the two, but you wait days between switching the main and aux. Also the worst would be if the tanks were actually at the same elevation, but we know the aux is slightly higher.   So let's run this worst-case scenario.

Mains: 37 gallons, aux: 15.

So after you first fill the main tank, you have 37 gallons out of 52 total per side, or 71.1%.  Since the time to drain is super slow, it's effectively 0 transfer.

But by the time you switch to the other tank, they are at equal levels, which would be 71.1% full (26.3 in the mains, 10.7 in the aux) 

So you then fill up the aux to full, by adding 4.3 gallons for a total of 41.3 of 52 gallons, or 79.4% full.  None transfers while you are filling but by the time you switch back to the mains, they are equalized again with 29.4 in the mains and 11.9 in the aux.

So that lets you add 7.6 gallons in the main, giving you a total of 48.9 gallons of a total of 52 possible,  or 94% full, with only 3.1 gallons of empty space.  

This is the absolute worst possible scenario, where no fuel transfers when you are filling, and it absolutely equalizes when you are not filling.  In real life, it does transfer when you are filling, and it doesn't totally equalize when you are switching between tanks.   

 

So worst case scenario for mains->aux->mains is that you are missing 3.1 gallons.

 

I don't dispute that you are adding 10 gallons per side after waiting a while.   The tops of the tanks have cavities (made by the ribs and supporting structure under the upper surface of the wing) that air can get trapped in and shaking the plane or heating and cooling the top of the wing might cause some of that air to flow to a vent line and then become at least partially filled with fuel. 

The part you are missing is, I can hear bubbles from the transfer for fuel to the main from the aux even after I have put in more fuel than the placarded usable fuel.

That is, if you fill the main, fill the aux, wait and fill the aux again.  And wait, the level in the aux will go down and you will be able to put in more fuel.  Even when I put 55 gallons into a 52 gallon tank system, I could hear bubbles coming from the transfer.

 

As for transfer rate, it is at least 20 GPH to keep up with fuel burn of a larger engine airplane.  So that is 3 minutes per gallon.  So, while you are filling the main, so of the fuel IS transferring to the aux.  And while you fill the aux, some that fuel is transferring back.

But try it.  Fill you main.  Fill your aux.  Go to the other side, fill the main and aux, then come back and fill the aux again.  And then wait and see what happens.  No shaking, just let the fuel levels seek their level.