Fuel draining at uneven rates

[n85ae]

Haha, I'm not wrong. But I have better things to do than argue about this.

Regards,
Jeff

Jeff,

Not to beat a dead horse, but with all due respect you are not correct. I'm know that Jim (jiott) can defend himself, but I would just like to repeat a couple of points...

First of all, Jim (jiott) explained that the different lengths of the feed lines on the left & right tanks and the ID size and flow rates that we are talking about, are a non-issue for all practical purposes - no measurable frictional difference. I believe that Jim is a retired engineer who dealt with hydraulic design for a living.

Secondly, Jim & I both explained that the vent line does not and can not feed any fuel to the header tank when the level in the wing tank is below the level where the vent line connects to the wing tank (which is up at the very top of the tank of course). The feed and vent lines are not a set of "double lines" that are both feeding the header tank.

[Danzer1]

The only way the header vent could have an impact - is when it is releasing air from the header to the tank - and even then it is only going to be the same volume as the air released to the tank - negligible!

The amount of friction loss difference between such a minor amount of tubing length is also negligible. Measure and drain both tanks through those lines and I'd bet you the difference might not even be measurable! 3/8" pipe friction loss for 1 foot of pipe at 5gph would be less than .002 feet of head.

The big difference would be the relative angle of the airflow into the tank vent inlets. My suspicion is; in flight with prop wash - the air angle on each tube is not dead ahead. Only way to check is to tuft both vent areas and observe/video from above in flight - static would only give an approximation.

Greg

[HighWing]

At some point the head
pressure (the weight of the fuel in the fuller tank) will balance against
this. Which is why when you approach zero, both tanks empty at the same time.
Jeff

Who knows why, but Jeff has it spot on in the above quote. Thinking, I could go down the list of my flying buddies over the years who have in some cases thousands of hours in their various Kitfoxes - None on this forum to my knowledge. In my mind's eye, I can see the look on their faces if they were to read this discussion. This has all been argued before - many times - and I guess it will be argued again, because there is no answer that will appeal to everyone. And perchance there was an answer, I seriously doubt it could ensure even fuel flow in every Kitfox built thereafter.

[n85ae]

5 feet of 3/8" tubing with a 3 ft. drop flows about 2.9 gpm, whereas a 2 ft.
shorter section flows 2.2 gpm. Take into consideration the pressure
differential between the tanks, and the impact the header vent line has
on this.

Yes there are VERY real reasons there's an imbalance. It's because it's a
dynamic system. It's not so simple as the plumbers among us would
believe.

The Earth is flat also, I have flown over to the edge and looked. I guess
not all of us can be engineers and do math. In any case, it's better to use
heresay, stake burning, and crucifixions.

Jeff

[Danzer1]

At some point the head
pressure (the weight of the fuel in the fuller tank) will balance against
this. Which is why when you approach zero, both tanks empty at the same time.

Yes it would, but the question is, why is one fuller in the 1st place and how to equalize it.

5 feet of 3/8" tubing with a 3 ft. drop flows about 2.9 gpm, whereas a 2 ft.
shorter section flows 2.2 gpm. Take into consideration the pressure
differential between the tanks, and the impact the header vent line has
on this.

I don't know where you get your numbers from, but they are way off. Could be you are reading Pd/100 feet of pipe and not correcting for that.

Also, these systems do not flow anywhere near 2 to 3 gpm - 2 to 3 gpH would be much closer. The Pd at 3 gph would be negligible. Pd is exponential to flow rate not linear = higher Pd with higher flow.

There should be no pressure difference in the tanks if they start out at an equal level.

Maybe the answer is - people making more left hand patterns instead of right hand?

It's because it's a
dynamic system. It's not so simple as the plumbers among us would
believe.

Hydraulics is hydraulics - doesn't matter the application, just the configuration of the system type. 20+ years owner of mechanical engineering firm specializing in hydraulic design. Thanks Jeff! BTW, plumbers work on toilets.

Greg

[n85ae]

Yes I calculated the rates incorrectly, but the principal is the same.

Actually, the REAL problem with this discussion, like many. Is it centers
around shooting down the other guys idea. ALL of the planes have the same
problem, and while I'm not an expert, I think my reasoning is sound as to the
reason.

I would like to see somebody propose an idea that isn't just shooting down
my ideas, or isn't that everybody has mounted there turn coordinator out of
level?

Clearly if somebody did 20 years worth of mechanical/hydraulic engineering,
they ought to be able to do the calculation that explains it Or maybe
it's more in the area of fluid dynamics (i.e. too hard)?

[Danzer1]

Yes I calculated the rates incorrectly, but the principal is the same.

So off by a factor of 60 (gpm - gph)? or factor of 50 5'-3'=2'/100? or both? plus the exponential difference of Pd between flow rates and length = negligible!

Greg

[n85ae]

Should we start exchanging flaming messages, until the list admin locks the
thread or bans us? That's the usual resolution to thread argument isn't it?

So off by a factor of 60 (gpm - gph)? or factor of 50 5'-3'=2'/100? or both? plus the exponential difference of Pd between flow rates and length = negligible!

Greg

[jiott]

I am really enjoying all this. We are all just putting forth our differing ideas on a very interesting subject, and no one here except n85ae seems to be taking any disagreement personally, certainly not me.

I still stand behind everything I said, and I did propose an idea to do some testing with the cap vent tubes at various angles into the wind.

N85ae's theory is correct (except for the "double feed line") in my view, but the problem is in the ASSUMPTION that frictional pressure drop effects are much larger than reality. I believe that ram pressure in the tanks and side forces due to uncoordinated flight or one wing being low have a much greater effect on this issue. Let's hear from some who have actually tried equalizing feed hose lengths-did it make any difference? I also believe there have been a few who have added a vent line to the left tank-did it make any difference? I propose another test idea: simply clamp off the header tank vent tube and fly for an hour or so to see if that vent tube has any effect on this issue (I don't believe there would be any problem in doing this other that maybe trapping a few air bubbles in the top of the header tank).

Great fun! Let's just try to be engineers and toss around technical ideas. There are no winners or losers, just possibly some solutions to a non-problem. At least it makes us all think and learn more about our airplanes.

[mr bill]

Amen, and to all--a good night