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Fuel spillage through the fuel tank vent


dan tonner
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The Savannahaircraft Yahoo Group has an interesting thread going on the topic above and I would like to cast the net a little further for input and opinion by sharing the topic with Recreational Flying readers and contributors.

 

I've taken the liberty of posting my inquiry (below) on both this as well as the Yahoo site.

 

I am keen to see your reactions to the thread.

 

Fly safely,

 

Dan

 

I am a bit late jumping into this discussion but as my Savannah VG XL approaches flight readiness, the subject of fuel spillage is of great interest to me.

 

Anytime the secondary onboard tank is full and there is some fuel in each wing tank, gravity and/or centripetal force can move fuel from one tank to another as if they were directly connected across their outflow points.

 

Imagine a 6-foot long teeter-totter with a vented and filled tank mounted on each end. Now attach a hose along the plank connecting the two outlets. This, in effect, is what we have in our airplanes. While stationary, any tilt in either direction will pour fuel from the higher to the lower tank. Once the level reaches the top of the vent tube arc, fuel will spill out from the lower tank. Mental arithmetic suggests a 10 degree tilt with 6 feet of separation between the tank outlets would give a 6 to 8 inch height difference between the two tanks - given time, perhaps enough to empty the upper tank completely through the vent.

 

Uncoordinated turns would also result in fuel movement from one tank to the other due to centripetal or centrifugal forces acting on the liquid - imagine the filled and connected tanks mounted flat and side by side on the left and right side of truck bed while the driver veeres left or right on either a level or sloped roadway.

 

Stationary or moving, I suspect ram air pressure on the end of the vent tube would only marginally prevent fuel outflow. Of course, planes with tanks further outboard in each wing (dual tank situations) have greater tank separations resulting in larger height differences at any given degree of tilt.

 

Has anyone tried installing a fuel check valve in the fuel line between each wing tank and the onboard tank? (Aircraft Spruce P/N 05-00719 or P/N 10630). In the teeter-totter or truck bed analogy, the check valves would prevent fuel flow INTO the tank through each tank outlet.

 

In the airplane, although fuel could not flow from one wing tank into the other, it is still able to flow freely into the secondary tank as space permits.

 

 

 

I am eager to hear your views (or receive word on your experience) with this suggested setup.

 

 

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just install a valve on each tank... i have 3 tanks on my Savannah, fly firstly on the right inboard tank, as the engine return flows into here, and have the outboard right and left tank turned off at the valve. when the right inboard tank is nearly dry, i close it off, and run from the right outboard tank, then run it dry (until the red header tank light comes on), then close it, and then run off the full left hand tank, once this is nearly dry, i turn both left and right inboard tanks on. the return fuel from running on the right outboard and left tank will have refilled it to about half full. both left and right tanks then self level as you have described and usually have about 30% each full....

 

 

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i dont have fuel spill issues with my aircraft, only when i stuff up the tank management and the return fuel flows into a full tank, then overflows, i think the peuple that do have some issues seam to not have tap or valve accessible from in the cockpit to shut off each individual tank. and when flying with brimmed tanks, they have a crossfeed/overflow problem.

 

 

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You shouldn't HAVE to juggle fuel cocks . You should be able to select both tanks and they will feed to empty. The return fuel flow is off putting, and flying out of balance will affect the evenness of the usage but they won't vent if the plumbing is done right in accordance with what I have been informed. Nev

 

 

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Hello Rob and Facthunter;

 

Rob: I was considering the use of valves as you've described (I have only two tanks) with care to keep the right tank slightly below full when topping up. Beyond two more valves for my checklist, your solution seems quite sound. What are your thoughts on the check valves ?

 

Facthunter: I did draw out your suggestion of crossover vent lines - an interesting proposal. One thought that occurred to me was that it may set up an unintended siphon:

 

...if the plane were tied down on a 10 degree slope, it is possible that most the fuel in the higher wing's full tank would lie above the vent hole in the cap of the lower wing's full tank. If the crossover vent tubes were run through to the underside of the wing (as designed) and fuel flow got started through the arc of the vent line (expansion, sloshing ?) the upper tank would be siphoned to the level of the vent hole in the cap of the lower tank. Again, what are your thoughts on check valves for each tank?

 

Regards, fly safely,

 

Dan

 

 

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You don't have a vent hole in the tank cap. Your venting system serves that requirement.

 

Re one way valves . They make a fuel tank pump set up, free from allowing air to enter before exhaustion of the fuel in both tanks. If you are very low on fuel select BOTH and have both pumps on and working. Nev

 

 

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Hi Nev;

 

I think you lost me.

 

My Savannah 's wing tanks are each vented via a threaded fitting in the center of the filler cap. A plastic tube slips over the barbed end of the fitting, loops upwards 3-4 inches then turns down and passes through the wing structure. The open ends of the vent tubes protrude below each wing. To what other venting system are you referring?

 

The check valve arrangement I am considering would permit fuel to flow out of each wing tank to the header tank behind the passenger seat - a tank vent is required to allow air to enter as the fuel flows to the header tank.

 

"...both pumps on..." ??? Are you suggesting pressurizing unvented wing tanks using the mechanical and electric fuel pumps?

 

I believe I've misunderstood something in your reply.

 

Fly safely,

 

Dan

 

 

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I've never intended pressurising a tank, but with a non return valve you can't transfer fuel unintentionally, no matter how you select and operate your pumps (where fitted). Your fuel return will create an imbalance, with some engine installations. or could cause fuel to vent overboard if you selected the incorrect tank with fuel full or nearly so, in the other (If you put more fuel into a tank already full, it will vent . I don't like vents in the fuel caps with more than one tank fitted. It's OK in something like a Tiger moth.

 

A properly designed overflow system wouldn't use tank cap vents. You CAN do a lot better than that . Nev

 

 

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I have a 4 tank 4 vented fuel cap Savannah S.

 

I only experienced once venting of fuel from on overfull tank - caused by not flying level over an extended period (and exacerbated by fuel return into that tank).

 

In my experience, the movement of fuel between tanks is gradual and slow and not instantaneous so while in theory the problems you pose are there and real, in practice they should not materialise unless as you say you park on uneven ground or you fly one wing down for a long period.

 

I don't have experience with check valves but they sound like a good idea. I have seen a Savannah in flight where the right outboard fuel tank cap came off in flight. The ensuing vacuum was so great, fuel would erupt from the tank every 30 seconds and was sufficiently strong to drain dry all four tanks!

 

 

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...an unintended siphon:...if the plane were tied down on a 10 degree slope, it is possible that most the fuel in the higher wing's full tank would lie above the vent hole in the cap of the lower wing's full tank.

...

All this has me a bit confused. Are people adding too much complication to their fuel systems? If each wing tank is vented from its inboard end to the nearest wingtip it can't syphon. If that wing is down, the fuel sloshes to the end of the tank, leaving the vent above fuel. How can it syphon?

 

 

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i would not be a fan of check valvesin the savannah system, because if running on both tanks, the fuel cant cross feed and find its own level, even trying your best to fly balanced, you will find one tank will drain faster than the other, then it will swap around as the levels drop, no idea why, but fuel does flow both ways between the 2 tanks when your flying...

 

if you have 2 tanks, you might only need to install a tap on one tank, to prevent cross feeding if you dont park on level ground...

 

i have not heard of losing fuel overboard in savannahs with 2, 3 or 4 tanks to be a real issue in flight... only if one tank is brimmed, then the returing fuel going into that tank might cause an overflow.

 

 

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Most of the uneven fuel levels I have noticed, and many of them are significant as to the variation, could be put down to flying unbalanced, (Ball off centre) and I do believe that was the cause most times. picked up when fuelling the plane. Pilots who fly ball off centre will have more troubles than uneven fuel. You are guaranteeing a wing drop at stall which is most likely in a turn when you are distracted.

 

OK if the fuel can flow from tank to tank when sitting the lower tank will become overfilled and overflow no matter where the vent is in the top of the tank. IF you take the vent back over the other side, it won't. The overflow will only hold fuel up to the same level as in the highest tank fuel level.. Nev

 

 

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...OK if the fuel can flow from tank to tank when sitting the lower tank will become overfilled and overflow no matter where the vent is in the top of the tank. IF you take the vent back over the other side, it won't. The overflow will only hold fuel up to the same level as in the highest tank fuel level.. Nev

Agreed, Nev. This would only happen in very out-of-balance flight, or when parked with fuel taps left open.

My vents are in my Jodel's bent-up wingtips, so not really a problem.

 

 

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But the Jodel is French, Vive La difference. Nev

Mine's not. It's dinky-di Aussie: built from Frank Rogers' plans out of Alaskan spruce, Queensland hoop pine, Howard Hughes' wheels, Jabiru engine... and mobs of home-made stuff.

 

 

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  • 5 months later...
All this has me a bit confused. Are people adding too much complication to their fuel systems? If each wing tank is vented from its inboard end to the nearest wingtip it can't syphon. If that wing is down, the fuel sloshes to the end of the tank, leaving the vent above fuel. How can it syphon?

The Sav tanks are vented through the filler cap, which is at the outboard (wingtip) end. So if that wing is down and the fuel sloshes to the end of the tank, it'll be up against the filler cap.

 

This makes sense as you want your filler cap on the highest point of the tank; dihedral means that point is outboard.

 

 

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thanks. have you got a web address for them?

http://fly-buylsa.com/Savannah.html

Scroll down to options and accessories to see them.

 

I have been using motorcycle fuel tank vents on my Sav. They have a little valve in them to let air in but but stop fuel from spilling out. They seem good so far but I've only used them about fifty hours so I reserve my judgement on them. Available from most motorcycle shops or eBay.

 

image.jpg.b68f8140904c7cd83241dcb20339a50d.jpg

 

Cheers

 

Rick

 

 

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