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Question about Rocket style fuel vents

blueflyer

Well Known Member
Probably rehashing old stuff here, but I didn't see the answer to my question in the archives. I understand the rocket style fuel vents are just coiled a couple of times and then out the bottom of the wing. Seems elegantly simple and right up my alley.

I want to use this coiled vent system in my 7, so I recently installed the coiled vent system in the wing tanks on another, currently flying experimental for a test run. The fuel system is a little different that an RV. On this one, the engine is fed by a main header tank behind the panel. It does have auxiliary wing tanks. Both wing tanks feed into the main tank. Previously, these wing tanks have not been used to transfer fuel to the main tank. I just flew using the main header tank. I had always burned about 9gallons per hour.

I made 2 test flights where I transferred fuel to the main tank. On both flights, the fuel burn was about 12 gallons per hour. There was no fuel loss in the wing tanks due to expansion or siphoning prior to flight. All power settings were the same.

That leaves me to guess that turning and/or turbulence during flight caused the 3 gallons of fuel to escape through the coiled fuel vents. If that is the case, I don't want that system on my 7.

Am I missing something, because that seems to be the only solution to me. My mind has "locked on" to this one explanation and I am not be able to think about other logical explanations for the fuel loss.

Anyone have any other ideas?
 
Was the taper on the bottom of the coil coming out the wing tapered forward. We always use 4-5 large coils. With topped tanks you will loose a little sitting on a warm day.
 
Was the taper on the bottom of the coil coming out the wing tapered forward. We always use 4-5 large coils. With topped tanks you will loose a little sitting on a warm day.

I only used 2 coils (about 6" in diameter) and the taper was facing forward. Is somewhere around 3 gallons of fuel loss to be expected when using the coiled system?

I don't think the loss was from expansion. Outside temps were in the 50s and I filled the tanks and flew right after that.

I am just surprised that I am losing about 3 gallons. That is more than I would have expected.
 
what is the size of the header tank and the other tanks and were they all topped off at the same time?

They were all topped off at the same time.
main tank=29 gallons
pilot side aux tank=8gallons
pax side aux tank=8 gallons

I flew for long enough to burn enough fuel from the main tanks prior to pumping fuel from the aux tanks, so that I did not just pump fuel out of the main tank vent.
 
I am just surprised that I am losing about 3 gallons. That is more than I would have expected.

I'd be surprised and dismayed as well - and we haven't lost a measurable amount of fuel from our RV-3 with coiled vents in over 300 hours of flying - much of it aerobatic.

I could see siphoning if you were flying uncoordinated, but you'd really have to work at it.

Paul
 
I have the coiled vents using custom built submerged ducts on the bottom of the gap fairing as my vents. Have not had any fuel loss, in the air, or on the ramp. I could not see you losing fuel, in flight, unless something was seriously wrong with the execution as the vent is supposed to be pressurized.
 
any chance the feed to the main from the aux is feeding back and fuel is going out the aux vent when you're starting off burning fuel from the main? 3 gallons is to much fuel. Trying to figure out your plumbing route.
 
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I have the coiled vents using custom built submerged ducts on the bottom of the gap fairing as my vents. Have not had any fuel loss, in the air, or on the ramp. I could not see you losing fuel, in flight, unless something was seriously wrong with the execution as the vent is supposed to be pressurized.

I'm glad to hear this, and that is the result I was expecting to have. Could you post a picture of your "custom built submerged ducts on the bottom of the gap fairing..." I am not following that.

I'd be surprised and dismayed as well - and we haven't lost a measurable amount of fuel from our RV-3 with coiled vents in over 300 hours of flying - much of it aerobatic.

I could see siphoning if you were flying uncoordinated, but you'd really have to work at it.

Paul

I hadn't considered coordination, but the ball was centered on these "straight and level" test flights.

any chance the feed to the main from the aux is feeding back and fuel is going out the aux vent? 3 gallons is to much fuel.

This is interesting. I will test this. I hadn't considered that.

OK, it seems I need to look at my coiling/aux system because something isn't right.
 
that's the only thing that would make sense to me, but then again I have no clue of your plumbing design in this aircraft. Good Luck! :)
 
WE have a F1 Rocket and a NEW RV8 (YEAH!!) flying with this vernt line. NO issues at all, and way easier to plumb.
Tom
 
Does the vent line inside the tank go all the way to the furthest rib in the tank? If it only goes through the first rib the you'll definitely lose several gallons.

Best,
 
Does the vent line inside the tank go all the way to the furthest rib in the tank? If it only goes through the first rib the you'll definitely lose several gallons.

Best,

Pierre,
I have to admit, I don't understand what you are saying. Are you referring to the Rocket fuel system, the standard RV fuel system (they may be the same, I'm not sure), or speaking generally? You got my attention with your post because I am losing several gallons just like you say.

Maybe the Rocket fuel system is different than what I am trying here, therefore the coiled vent system in inadequate in my application. There are no ribs or baffles in these wing tanks. The wing tank vents are situated as shown, at the top outboardmost position. But, the coils are no higher than the top of the fuel tank. Could this prevent the coils from working as expected? This is the highest part of the tanks while sitting straight and level. The wings tanks are slightly canted inboard so the fuel will flow towards the fuel line exiting the wing tanks. Below is a diagram of the fuel system. The two wing tanks feed up to the main tank which is behind the instrument panel.

 
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Pierre,
I have to admit, I don't understand what you are saying. Are you referring to the Rocket fuel system, the standard RV fuel system (they may be the same, I'm not sure), or speaking generally? You got my attention with your post because I am losing several gallons just like you say.

Maybe the Rocket fuel system is different than what I am trying here, therefore the coiled vent system in inadequate in my application. There are no ribs or baffles in these wing tanks. The wing tank vents are situated as shown, at the top outboardmost position. But, the coils are no higher than the top of the fuel tank. Could this prevent the coils from working as expected? This is the highest part of the tanks while sitting straight and level. The wings tanks are slightly canted inboard so the fuel will flow towards the fuel line exiting the wing tanks. Below is a diagram of the fuel system. The two wing tanks feed up to the main tank which is behind the instrument panel.


I think you may have misunderstood the principal of fuel tank venting. Your two wing tanks should be constructed with independent venting systems. The internal vent lines should start at the extreme top of the tank (usually the outboard tip if you have positive dihedral), run the length of the tank internally and exit at the root where the rocket vents would sit, venting downwards. This prevents fuel from siphoning out in any attitude. A little bit will be lost to expansion with full tanks, determined by the number of coils in the rocket vent. This is also true with the standard Van's vent.

You would then have your pump and header tank with it's own vent. This vent line would start at the internal top of the tank, curl underneath the tank and be open to the atmosphere (or through a ram air fitting). It, too can be a Rocket vent (coil) to provide a reservoir for expansion.

I hope this is clear. You can always mock up tanks and vents with a milk jug and some tubing. By moving it around in all attitudes you will come up with an understanding of how properly placed vents work.

Cheers,
 
Vern
I think I'm following you. Thanks for the explanation. Unfortunately for me, this plane was bought flying. Its not a huge issue for me, so I'll likely just make some compromises with the current aux tanks and the venting system rather than try to pull the tanks. Thanks for all the input.
 
Vent outlet

I know this should seem obvious but since it wasn't mentioned and no one has asked.... Assuming the vents are just the end of the tubing, are the vent opening pointed forward like a pitot tube to put positive pressure on them?
 
I know this should seem obvious but since it wasn't mentioned and no one has asked.... Assuming the vents are just the end of the tubing, are the vent opening pointed forward like a pitot tube to put positive pressure on them?

Hey Wade, yep, they do face forward. I think after discussing it here, the reason I lose fuel during flight is because the aux tank fuel vents do not have an internal line that run to the inboard (opposite) side of the tank. So, when I turn, the low wing aux tank just dumps fuel like a tea pot. It has nothing to do with the coiled vents I installed.

That sounds logical to me anyway. Would you agree?
 
My Solution

Hey Wade, yep, they do face forward. I think after discussing it here, the reason I lose fuel during flight is because the aux tank fuel vents do not have an internal line that run to the inboard (opposite) side of the tank. So, when I turn, the low wing aux tank just dumps fuel like a tea pot. It has nothing to do with the coiled vents I installed.

That sounds logical to me anyway. Would you agree?

Heres my solution: My aux tanks hold 8 gallons apiece. So, I will only fill each aux tank to 7.5 gallons to allow for a generous 6% fuel expansion in the tank. I will put a one way check valve (pointing towards the tank) on the fuel vent line. That will allow the tank to vent but no fuel will escape.

This way, both expansion and venting are addressed. This seems to be the easiest solution. After all, I have 3 hours of flight time (plus 30 min reserve) on the main tank, so the two aux tanks at 7.5 gallons each will give me another 1.5 hours of flight. That's a long time in the seat.
 
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...So, when I turn, the low wing aux tank just dumps fuel like a tea pot. It has nothing to do with the coiled vents I installed.

That sounds logical to me anyway. Would you agree?

Not if you are coordinated in a turn. With the ball centered, you could have an open bucket of fuel on the cabin floor filled to the brim and not spill a drop even in a steep bank.
 
The vent should be the only unsealed portion of the tank and it. So even if the plane was on a rotisserie and you put one wing down it should not spill fuel because there should be no way for air to get in to takes its place (long narrow vent line used). Vents spit because with expansion the tank is at a higher pressure than the atmosphere outside. I assume you are fairly level when you are climbing. :)

If the internal opening of the vent is at the highest part of the tank and the fuel vents are seeing ram pressure then it should highly unlikely to loose fuel from them. If not and you also have loose caps that easily allow air for displacement, it may be possible. Question, could you be flowing fuel out of the header tank, through it's vent when you transfer fuel from your aux tanks? What prevents that from happening?
 
The vent should be the only unsealed portion of the tank and it. So even if the plane was on a rotisserie and you put one wing down it should not spill fuel because there should be no way for air to get in to takes its place (long narrow vent line used). Vents spit because with expansion the tank is at a higher pressure than the atmosphere outside. I assume you are fairly level when you are climbing. :)

If the internal opening of the vent is at the highest part of the tank and the fuel vents are seeing ram pressure then it should highly unlikely to loose fuel from them. If not and you also have loose caps that easily allow air for displacement, it may be possible. Question, could you be flowing fuel out of the header tank, through it's vent when you transfer fuel from your aux tanks? What prevents that from happening?

It is physically possible that fuel can escape from the main tank through the vent, but I use about an hours worth of fuel from the main tank prior to pumping fuel into it. Your question does make me ask myself if my pump is pumping fuel into the main tank at a faster rate than what I believe. I need to re-verify my flow rate.
 
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