Physics lesson needed?

[n6346m]

Ok guys and gals, here is my problem.
I drained all the fuel out of my L2 to refurbish the fuel valve. I put five to six gallons of fuel back into the right tank and it shows about half full. Since this is a six gallon tank that feeds directly into the header tank I assume that two gallons went there. Two days later, the right tank still shows half full and the left tank still shows empty. The two tanks are interconnected via a common fuel drain in the center at the bottom of the fuselage, they both feed the header tank and, in addition, both of the main tanks and the header tank are vented to each other. The fuel caps are also vented.
The airplane is on a level surface, shouldn't the tank quantities have equalized? Why is the left tank still empty?

[cvavon]

Re: Physics lesson needed?

Down here in the south we blame that kind of thing on the infamous mud dobbers

[Howard Wilson]

Re: Physics lesson needed?

Hi Bob, How are you measuring the levels? Gauges, stick?? Are there shut off valves for each individual tank? hmmmm h

[Hank Jarrett]

Re: Physics lesson needed?

OK little fluid mechanics and physics time.
If the line is small and there is no vibration the surface tension at the line may be high enough for a meniscus to form. The meniscus is the curve in the liquid when you fill a glass and the liquid actually rises HIGHER than the edge of the glass without running over. It is caused by surface tension in liquids and is also why when you see the astronauts playing with water it seems to be trying to form into spheres.
What happens in a large tank with a small line leading out of it is a meniscus can form at the end of the tube. The fluid will seek the minimum energy state (a sphere in zero G) and unless there is more energy in the pressure in the tank than in the surface tension (the tank is shallow in relation to the diameter of the tube), the fluid (fuel) won't flow through the line. Notice at the beginning I said in the absence of vibration? Vibration adds small amounts of energy to the fluid that can disrupt the meniscus. Once it is disrupted, the kinetic energy of the fluid moving will overcome the surface tension and prevent it from forming again. That's why you see pilots shake the wing a little to "get the flow started".
My 41 had the problem and it was made much worse by a kink in the fuel line between the wing tank and the nose. Prior to finding the kink, the solution was to ALWAYS fill the wing first if it was going to be used and listen for fuel flowing to the main. Once the line is filled with fuel, a meniscus can't form, and fuel transfer will start when the valve is re-opened (in the case of the 41 VERY SLOWLY because of the kink).
The solution, long term, is larger fuel lines, with NO RESTRICTIONS, and to add some vibration to get flow started. If you can't get fuel flow started there is a good chance you have a restriction that could be from a damaged lines, debris in the finger strainer or a plug in the line (don't need to go into detail on fixing those, it's a pain). First step is to fill teh highest tank full and then gently shake the plane. DON'T GRAB A STRUT OR WING AND SIMULATE "The BIG one", SMALL amplitude and high frequency is what you want to break the surface tension (I won't bore you any more than you already are with the math).
Once again, a little benefit from NASA research on fuel transfer. You think it is hard to break surface tension on an airplane? Imagine what we had to do to break it in a zero G fuel tank in orbit so the fuel would flow to the engines instead of forming balls of liquid in the tank! We actually worked BACKWARDS and put a screen in the end of the tank with lots of small holes to cause the fuel to form up at one end so we could push it down the lines. Got to think out of the box.
Hank

[flybikefarm]

Re: Physics lesson needed?

I thought a meniscus was what happened when the bartender put to much water in the glass with my wiskey??????

[Hank Jarrett]

Re: Physics lesson needed?

Water in with wiskey? Why in the world would anyone put water in wiskey? Don't you know what fish do in water?

Based on your spelling you must drink that American stuff. In Scotland (where REAL SCOTCH Whisky comes from, we spell it "Whisky"). My grandmothers family was from Ireland and they spelled it "Whiskey". Funny you can tell where your drink comes from by the spelling! Spell check must be from Ireland. It is the only one they seem to recognize. Of course if you drink enough, you don't care how it's spelled!
Hank

If you want the best, you want Single Malt Highlands! That lowlands stuff tastes like dirt.
And with that, this part of the conversation should move to the humor section. Scotch Whisky drinkers seem to be a bit opinionated! ;-)

[skyboltone]

Re: Physics lesson needed?

Lots of good Whiskeys out there. American ones too. Like Rip Van Winkle for instance, if you can find it.
So Hank, this is not an argument it's a question. When your liquid is gasoline isn't the surface tension low enough that the diameter must be extremely small in order for a meniscus to form? Whisky is like that too. It will crawl up the sides of the glass. I don't know if this is related to vaporization pressure or viscosity. You probably do though.

[Hank Jarrett]

Re: Physics lesson needed?

There are lots of factors that change surface tension. A drop of dish washer soap will REALLY break the tension (but isn't a good idea as a fuel additive!) I have seen Gas not flow trough a tube, but have to admit the fluid column was fairly short and there was NO vibration. Shorter than the depth of my wing tank. A prior owner of my 41 had put a fuel cap with a big tube/scoop on it and had problems with fuel transfer too, and that was with the vibration of an engine to help. I suspect that since the big tube was in the low pressure area above the wing it may have made things worse. That was why I would make sure the transfer tube was always full. With the tube full it never failed to transfer, but it was still slow. I was not surprised at all to find the kinked tube, but it was REALLY well hidden! Now fuel should flow like Niagara Falls, but I would STILL make sure the line was filled before filling the main. Costs nothing and just might be needed.
As for good drink, I am of Scottish heritage and love the Highlands Single Malt, but being Scottish background I have a real conflict. I only drink the good stuff, but am too thrifty (Debbie would say CHEAP) to drink much of it. That's why I will never be an alcoholic. I'm thrifty and picky. Can't be both and get drunk very often!
Hank

[swoeric]

Re: Physics lesson needed?

on the american side just a dash of water will bring out more of the flavors also if you let it sit in a glass for just a bit the flavors wil also have a better chance of coming out similar to wine......I enjoy the woodford reserve just a pinch of water if im in a hurry or let it sit with some ice a couple minutes....but lately i have really been on a Jameson and ice only kick

[Larry Lyons]

Re: Physics lesson needed?

I would add to Hanks explanation that due to the very low difference of static height, long lines, and small diameter lines, there just may not be enough energy to over come the pressure loss due to friction in the fuel line. We deal with this problem all the time in water systems, however I would follow all this up with the "plane shake" to make sure everything is kosher. L

[maximumvmo]

Re: Physics lesson needed?

Very interesting thread. I'll add Bulleit Frontier Whisky to the list.

[n6346m]

Re: Physics lesson needed?

I just want to update those who may not be familiar with the L2 fuel system. The L2 has no main tank as such. It has two six gallon wing tanks that feed into a two gallon header tank. The only valve in the entire system is the main fuel on/off valve at the bottom of the header tank. The header tank has a vent at the top and is "teed" to the two wing tanks with a 1/8 vent line. In addition, both wing tank fuel caps are vented. The tanks have a front and rear 3/8 fuel lines. The rear lines of the tanks join together at a common fuel drain at the bottom in the center of the fuselage. The rear lines then join the front lines and with a "tee" go to the header tank. Drawing it out helps. The bottom line is that the two wing tanks are connected together via two 3/8 fuel lines and one 1/8 vent line. I tried "shaking" the airplane with no success. This is day three now and the right tank still shows half and the left one shows empty on the "coffee urn" type sight gauge so it doesn't lie. One more factor that I forgot to mention is that in order to avoid hitting my head on the trailing edge of the wing when the airplane is in the hangar for a while, I pick up the tail and set it on top of a 55 gallon barrel. This makes the airplane pretty much level instead of the normal tail down attitude.

I agree that surface tension may make the fuel uneven if only the 1/8 vent line connected the tanks together. Could it also happen with two additional 3/8 lines connecting the tanks together?

[n6346m]

Re: Physics lesson needed?

I just want to update those who may not be familiar with the L2 fuel system. The L2 has no main tank as such. It has two six gallon wing tanks that feed into a two gallon header tank. The only valve in the entire system is the main fuel on/off valve at the bottom of the header tank. The header tank has a vent at the top and is "teed" to the two wing tanks with a 1/8 vent line. In addition, both wing tank fuel caps are vented. The tanks have a front and rear 3/8 fuel lines. The rear lines of the tanks join together at a common fuel drain at the bottom in the center of the fuselage. The rear lines then join the front lines and with a "tee" go to the header tank. Drawing it out helps. The bottom line is that the two wing tanks are connected together via two 3/8 fuel lines and one 1/8 vent line. I tried "shaking" the airplane with no success. This is day three now and the right tank still shows half and the left one shows empty. One more factor that I forgot to mention is that in order to avoid hitting my head on the trailing edge of the wing when the airplane is in the hangar for a while, I pick up the tail and set it on top of a 55 gallon barrel. This makes the airplane pretty much level instead of the normal tail down attitude.

I agree that surface tension may make the fuel uneven if only the 1/8 vent line connected the tanks together. Could it also happen with two additional 3/8 lines connecting the tanks together?

[Hank Jarrett]

Re: Physics lesson needed?

If the flow is being restricted by surface tension (starting to sound like that is NOT the problem) I would drain some fuel from the header tank and see if one of the wings drains to it and not the other. That would tell you that the one not draining has some obstruction. Have you let all of the fuel out and inspected the finger strainer in each tank? If your tanks were sloshed sometime in the far past or some debris got into the tank, it may have partially plugged the strainers. I do NOT think it is the vents. Even a restriction that prevents normal transfer wouldn't have prevented transfer with vented caps on the wings while sitting.
If there is something in the lines you could try to blow it out but that can REALLY damage a tank if you don't have some way to protect it from over-pressure (it does NOT take much pressure to mess up a tank!).
I used a condom on my tank filler neck and pressure tested my tanks by filling them through the drain. The condom will get HUGE but will burst before the tank is damaged (for your test you could probably use a balloon since I needed the condom to go over the filler neck and the balloon wouldn't stretch enough). In your case you would need a cap that will seal with a T where the vent is and a condom on one leg and a tube to blow into on the other. ALWAYS have the condom where the air is being added so the blocked line will not cause the air being added to damage the tank before it gets to the condom.
This is an interesting problem so keep us up on the solution you find. Wish I was close. I spent a lot of time troubleshooting fuel systems on Navy planes. You found some REALLY strange things in those tanks.
Hank

[3Dreaming]

Re: Physics lesson needed?

I work on the flight design CT series aircraft. If the fuel system is completely dry and you put 5 gallons in each tank it will not start flow on its own. You have to help it get started with a little pressure on the vent system. There might be a little air trap in the lines some where. tom