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Building Tip - Fuel Tank Vent Line

f1rocket

Well Known Member
Here's a tip for running your vent line on your fuel tanks that keeps the line out of the cockpit, reduces the number of fittings, and simplifies the installation.
Rigging164.jpg

This is a picture of my wing upside down. Inside the root, I ran a double loop of 1/4" aluminum tubing to a AN919-6D reducer fitting that I Prosealed to the skin. The 3/8" end has been sanded smooth using an electric drill and a bench grinder. The end has been cut at a 45 degree angle and rounded.

Many of the Rocket guys are using this set up and it works great. The double loop prevents most, if not all, fuel leakage from a full tank in the hot sun. As usual, all disclaimers apply. YMMV.
 
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Interesting - with the top of the fuel in the dihedral oriented tanks placing fuel above the top of the loop it does cause some concern. Do you extend a vent line on the inside of the tank from the fitting to the high point in the tank at the outboard end?
 
Yes, the tanks are built with the vent line running inside the tank all the way up to the top side of the farthest outboard end.

The only thing the line in your cockpit does is create a loop of line higher than the tank, thus preventing fuel expansion from pushing fuel out of the vent. The doule loop of line does the same thing effectively.
 
Is it really working?

Many of the Rocket guys are using this set up and it works great.

What did they use to determine this? Did they try flying in several different
attitudes with a chase plane to ensure no fuel was flowing out? I guess if
your tank was still off the wing you could move the thing around to see
if anything leaks.

This looks a lot simpler than the way I did it, for sure.
 
Let's play "devil's advocate" for a minute. What happens if you are forced to park with one wing low with full tanks putting the fuel level above the inside vent opening and also above the loop? Now with heat expansion fuel begins to flow through the vent line? Would this not creat a siphon effect?
Mel
 
For a siphon effect, air would have to be able to get into the tank to replace the fuel going out. I'd guess you'll lose whatever fuel the expansion pushes out of the tube and that's it. Same situation as if you ran the tubing through the cockpit.

PJ
 
rv8ch said:
What did they use to determine this? Did they try flying in several different
attitudes with a chase plane to ensure no fuel was flowing out? I guess if
your tank was still off the wing you could move the thing around to see
if anything leaks.

This looks a lot simpler than the way I did it, for sure.
Over 30 Rockets flying with this set up. The Rocket plans show this as the standard way to vent the tank. I thought the RV folks might be interested in using this. If not, that's okay by me.
 
Good tip

Over 30 Rockets flying with this set up. The Rocket plans
show this as the standard way to vent the tank. I thought the RV
folks might be interested in using this. If not, that's okay by me.

That's good enough for me. Unfortunately this tip will have to wait
for my next RV, since I've already done vents on my 8.

Thanks for sharing it with us, and please feel free to send over
any other rocket tips!
 
is this how they do it on RV-10? I thought I saw something like this on teh -10... but may be wrong..
 
That is tempting. I haven't even drilled the hole in the side of the fuse for the vent line yet. Hmmm...
 
This setup looks very tempting, thanx for the tip.:)

Now if we can just have more views from people Pro's & Con's and then one would be able to make up their own mind.
 
The only issue I can see with this setup is that when fuel flows into the tube, a certain amount of fuel will always be trapped in the line. Since the viscosity of fuel is quite thin, air equalization needed in the tank or out will gurgle past the trapped fuel indefinitely. The factory setup allows excess fuel that enters the line to either run back in the tank or out into the air or ground. Trapped fuel will either evaporate inside the line and could quite possibly gather dust or dirt.

Those are just my thoughts, and certainly not intended as unfriendly criticism or to start a "serious" discussion.
 
When you fly, (especially at "Rocket" speeds :) ) the air pressure pushes any fuel in the line back into the tank.
 
Ah...

f1rocket said:
I guess you haven't seen my posts on priming? :D
They are anodized at the factory along with all the internal parts of the wing.

Ah...nope, hadn't seen your priming posts. I'm used to seeing gold anodizing. You must have used clear or something like blue or gray just to mess with us. ;)

)_( Dan
 
OK, so I assume that you are using capacitance fuel senders. But where are the fuel pick-up tubes? What am I missing here? That is the inboard part of the wing, correct?
 
arffguy said:
OK, so I assume that you are using capacitance fuel senders. But where are the fuel pick-up tubes? What am I missing here? That is the inboard part of the wing, correct?
No, on this tank the float sender is placed in the back baffle of the tank so as to not interfere with the flop tube, which is in the nose of the tank.
 
Inverted landings

Another advantage of this setup is it appears that in the accidental flipping of the plane during a emergency landing, the vent exit will be above the tanks, preventing the vent line from turning into a drain line. I have read many stories of the plane flipping and subsequent fuel draining from the tanks. Is this accurate?
 
f1rocket said:
Yes, the tanks are built with the vent line running inside the tank all the way up to the top side of the farthest outboard end.

The only thing the line in your cockpit does is create a loop of line higher than the tank, thus preventing fuel expansion from pushing fuel out of the vent. The doule loop of line does the same thing effectively.
Randy,
I like this idea but I am wondering (for those of us building our tanks) why one could not simply place the three loops inside the outboard bay and place the vent hole out there also?

It would reduce fittings and tubing usage and I can't see where it would be hydraulically different relative to siphon etc. I suppose slips with full tanks might spill a little more but who knows?

-Mike

Oops! I do see a big difference in the wing down situation (parked or uncoordinated). With the vent line running inboard (as in the Rocket design), the fuel has to climb up the vent line and would equalize level with the fuel in the tank before beginning to spill.

Your idea and implementation is better!
 
I do have some more actual flying experience with this set up now that I'm in flight testing mode. When the tanks are full to the brim, the vents will spit some fuel on hot days. The last couple of weeks have seen ambient temps in the mid to high 90's. I've had the tanks topped off because I'm calibrating my fuel flow sensor on the GRT system. Every couple of hours, the vent would flow a couple of ounces of fuel. I don't think it's much of an issue, just a little nusance. With the fuel level below "brim" level, I've never seen it dribble fuel.

Just for a data point, I have the tanks full right up to the bottom of the neck of the fuel cap. Normally, I would not fill the tanks this full but have done so to get an accurate end point for comparing actual versus calculated fuel flows. Once I get the calibration done, I'll back off on the fill point a little to allow for some fuel expansion.
 
Randy,

How is the testing coming along?

I am wondering if another loop or two on the vent coil would mitigate that hot weather fuel belching issue? I see that you have about 2.5 loops there now.

Update please!

:) CJ
 
My flight testing is progressing well. I have updates posted on my web site. I don't know about adding additional loops. It wouldn't "hurt" anything.

After fixing some problems with toe-in and vertical stab alignment, I have the airplane flying totally hands off with the ball centered. It's perfect. The engine is broke in now and all temps are great. Performance is spot on the Team Rocket numbers. It will cruise at 199 KTAS at 23 squared buring about 15 GPH. If I crank it down to 2100 and 23", the speed drops to 195 KTAS and the fuel flow drops to 10.5 GPH.

I'm getting ready for paint right now. I've got some fiberglass work to do and then I'm going to paint it before the weather gets too cold.
 
Doesn't fit well on a -7

I've considered this vent design for my -7, but the root rib on the tank is set much closer to the edge of the tank skin than shown on the Rocket photo. Unfortunately, there isn't enough room for a vent fitting to be mounted through the bottom surface of the tank skin. :confused:

Dave Cole
RV-7 wings
 
Dave Cole said:
I've considered this vent design for my -7, but the root rib on the tank is set much closer to the edge of the tank skin than shown on the Rocket photo. Unfortunately, there isn't enough room for a vent fitting to be mounted through the bottom surface of the tank skin. :confused:

Dave Cole
RV-7 wings

What about this approach....used for an RV7 tip tank. Same principle with coil but venting out through front of wing. It was used by a builder I know...plane not flying yet so not sure how successful it is. But it looks OK although I think I'd go for at least one more loop.

Incidentally, Randy, I don't want to worry you but while your ribs certainly look like they're anodised.....that spar doesn't look anodised to me. Are you 100% certain it's been anodised.

[img=http://img89.imageshack.us/img89/962/tiptank57dz.jpg]
 
Ooops

OK, well that last attempt at sending the photo didn't work so well. Here's another try. Presumably you can enlarge the photo if you want to by clicking on it.

 
The RV-10 vent line comes off the very top of the tank 90 degrees down and exits the wing root fairing for a total length of 8 inches. I am not at the part of the build yet but I haven't heard from any RV-10 builders that the vents dripped.

Russ Daves
#40044 RV-10 working on canopy top
 
Rocket vents on -7,-8 wings

Unfortunately, there isn't enough room for a vent fitting to be mounted through the bottom surface of the tank skin.
Dave-
Had the same problem on my -8, as it has the same tank. I made the loops, put the vent fitting on to the end of the tubing, and leave it dangling. I then fasten it thru the wing gap fairing when I install it. Sounds like a pain, but it's really not any trouble. I'll try to get some pictures of the installation this weekend if you'd like.
Bill Waters
 
Tank vents were a major issue with my BD4 which I built in the late '70's. The design used a no dihedral wing...so the vents tended to spit fuel or sipon at any time. They used a top of the tank pickoff, routed the line to the rear spar of the wing and then down and out at the midwing flap end. The tubing (1/4") was bent forward for pressurization.

I would think (and plan to use on my 8 unless something else comes along better) that a vent pickup at the upper outboard tank location would do well with the dihedral of the wings. Then the vent line would be routed to a convenient location on the rear of the wing (or perhaps inboard to the wing root fairings as in the '10 as someone reported).

Another problem with wing tank vents is icing...I wonder if I couldn't somehow run the vent line out the wing just to the rear of the heated pitot and benefit from the heated pitot. (The BD4 solution was to drill a 1/8" hole in the vent tube on the upper side right after it left the the tank fitting). Any comments?

Deene.
 
Mild alteration

After reviewing a lot of input on the vent lines, I decided to do it per the attached image on my 7. It avoids the goofy bends around the rudder cables and still maintains most of the head height of the original system designed by Van. The loop at the top remains well clear of the engine mount bolts in the upper corner of the firewall.

Ignore the cardboard. Those are patterns for interior insulation panels.

14886479214a1c8f7782031.jpg
 
With the internal vent pickup and line located in the typical vans position next to the filler neck then in a vertical up attitude the vent loops ideally need to extend forward beyond the leading edge of the tank to prevent the fuel from draining.

I'm guessing that it is not likely to become an issue since there is only an inch or so of fuel above the vent pickup in normal climb.

For me though this is reason enough to stick with the original Vans design. Not everyone likes a shot of avgas, especially not while doing aerobatics.

Doug Gray
 
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works in a 7

This is exactly what I did on my RV7. Took about 20 minutes a wing to complete the coil and flares, and installation, and have had no issues with draining, dripping, leaking, spewing or anything else I can think of. Would definatly do it again.
 
Anyone just go out the outboard side, and use a simple checkvalve? Andair sells some good ones I believe.
 
Anyone just go out the outboard side, and use a simple checkvalve?

The first time you climb high enough for the differential pressure to pop your tank will cause you to re-think this strategy.

The vent not only allows air to replace the fuel removed from the tank, it also allows higher pressure vapor to escape to the atmosphere.

Pat
 
Not at all valid:

1. Using fuel faster than air can expand... even in RV's
2. Most check valves in this application (have to check Andair's) have a pinhole to allow small amounts of expansion

Even the lowly Cessna 177 I fly uses this setup, as does the 210 series, I retrofited a Glasair that had continual problems along these lines (Draining on an uneven ramp) with this setup and it worked excellent.

Really, pop a tank? :rolleyes:
 
Not at all valid:

1. Using fuel faster than air can expand... even in RV's
2. Most check valves in this application (have to check Andair's) have a pinhole to allow small amounts of expansion

Even the lowly Cessna 177 I fly uses this setup, as does the 210 series, I retrofited a Glasair that had continual problems along these lines (Draining on an uneven ramp) with this setup and it worked excellent.

Really, pop a tank? :rolleyes:

Dunno, go ahead and try it. Maybe you know something that Van doesn't and can improve upon the design.
 
I went with the same setup, used 3 loops and installed a doubler on the lower wing root fairing. With the JF air screened fuel vents. Very simple, and keeps vent line out of the cockpit. In the event of a rollover fuel would drain out the vent wing root area not behind the firewall in the vicinity of the engine compartment and hot exhaust.
Jake



 
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Yes, the tanks are built with the vent line running inside the tank all the way up to the top side of the farthest outboard end.

The only thing the line in your cockpit does is create a loop of line higher than the tank, thus preventing fuel expansion from pushing fuel out of the vent. The doule loop of line does the same thing effectively.

Actually it doesn't prevent fuel from escaping due to expansion. It only prevents fuel from being forced out due to atmospheric pressure acting at the fuel's surface level. When pumping cold fuel into a warm, sunlite tank, expansion forces (expanding fuel displacing available space is different than the effect that atmospheric pressure creates) easily push the fuel up 10's of feet in the right circumstances.

Larry
 
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Fuel vent design and paint blisters

I know this, I believe the Van's design is part of the elusive fuel tank paint blister problem. On my RV, as long as I leave the tank caps cocked open on a hot day while on the ground or hangar, no blisters. If I leave the tanks closed up, blisters. When I pop the fill caps on a hot day, I can hear the pressure release from the tanks. That small amount of pressure seems to induce fuel vapor migration through the sealant. With a good impermeable paint paint layer on top of that, one that cannot pass this vapor fast enough, and shizam, a paint blister! A venting system that does not allow liquid fuel to enter it and create head pressure when the tank heats up and fuel expands and even vaporizes and tries to escape through that already fuel filled vent tube.
 
I didn't love the intricate routing of the fuel vent line through the fuselage, and saw this thread and built my vent for an RV-7A according to this thread.

The DAR looked at it today and wouldn't sign off on it. He pointed out that there is a low point in the loop where sludge or water could accumulate. If it then froze at altitude, the vent line would be blocked and it would cut off fuel flow from that tank. I'll have to go back and rebuild according to plans.

Just a heads up for others considering this tip.

David
 
I didn't love the intricate routing of the fuel vent line through the fuselage, and saw this thread and built my vent for an RV-7A according to this thread.

The DAR looked at it today and wouldn't sign off on it. He pointed out that there is a low point in the loop where sludge or water could accumulate. If it then froze at altitude, the vent line would be blocked and it would cut off fuel flow from that tank. I'll have to go back and rebuild according to plans.

Just a heads up for others considering this tip.

David

I love the random arbitrariness of DAR inspections. Mine saw it, said it looked good and that a lot of Rocket guys do that, and moved on.

Sludge or water? Uh, yeah, so? It's the VENT line, how's it supposed to get in there in the first place? Osmosis? One end is below the tank, the other is at the high point in the tank itself. Hard to see ho water would get in there in the first place.

I swear...

BTW, isn't there a low point in ANY vent line that doesn't have a continuous downhill run? You know, like the plans-shown line that goes into the cockpit, UP and then back down? Wouldn't that have a low point prior to the upward portion, between the tank and the top of the uppermost section? Couldn't water or some other mysterious substance magically appear there, as well?
 
When pumping cold fuel into a warm, sunlit tank, expansion forces (expanding fuel displacing available space is different than the effect that atmospheric pressure creates) easily push the fuel up 10's of feet in the right circumstances.

Yes. In hard numbers, gasoline has a coefficient of thermal expansion around 0.0005 per degree F. The change in volume for a temperature rise is dV = V * 0.0005 * dT. So, 21 gallons in an RV-7-8 tank filled to the brim will expand roughly 1/10 of a gallon for every 10 degree F rise in temperature. Although the vent line itself provides some expansion volume, it's not much, thus some fuel is pushed out of the full tank when it warms in the sun.

I am sure the coiled line works fine as a tank vent. Given the numbers don't lie, I'm also pretty sure the coiled line spits some fuel on the ground if the owner tops off to the neck with cool fuel, then leaves the airplane in the sun.

Is it better to run the vent line up high in the fuselage per plans? Again, the numbers say it will spit fuel if topped, then warmed.

There is variation between airplanes. Least fuel on the ground will be a case in which the builder got the outboard end of the vent line hard up against the top tank skin, in the small airspace higher than the bottom edge of the T-406 filler neck flange.
 
Seems better practice.

I know this is an old article. However! After recently purchasing a used RV6A and finding fuel vent lines inside the cabin (passenger side) that have been compromised likely from a nervous passenger in the past, this seems an excellent retrofit to eliminate the additional source of fuel even potentially entering the cockpit. Of course all engineered considerations above i.e. parking slope, lateral vent line to tip slope etc. Looks to me to be a great option.
 
Big thanks!

Thanks Randy for sharing, and a big thanks to Steve Riffe for helping me with the RV-8 aspects.
 

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