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vent lines again

bobnoffs

Well Known Member
a month ago i asked about running a vent line with a low point before it ran up to the siphon break. someone posted that it might be a point for ice and i thought that idea had merit. then someone posted about the vent lines in some wings that have one or several loops to prevent leaking fuel and what about ice in those loops. the question about the loops was never answered and i brought it up in another recent thread without any response. so the question still remains to me.........are low spots a danger for icing and what do the builders think of this that have loops in their vent lines. any takes on this would be appreciated.
 
It would seem pretty difficult to get any water in there. The vent tube starts out as high as it can get in the tank - water would never float up into that corner to get carried into the vent line.

All our vent lines have a low point, at the wing root, before either entering the fuselage for a single big loop or before starting the multi-loop "rocket-style" vent, because of the wing dihedral.

I suppose heavy rain could somehow get enough water into the drain end of the vent line and held in with pitot pressure that as the tank fuel is withdrawn, it could conceivably suck a blob of water in far enough to land at the low spot?

I don't really know, seems unlikely.
 
The RV-10 per plans vent line comes straight out of the tank a couple inches and then 90* bend down and out of the wing root fairing. No issues with icing there so I don't see why it would be any different in other models.
 
We have “rocket-style” vent loops on the RV-3, and aside from burping a little more fuel out if you park sideways on a steep slope, we haven’t had any issues. I was told by a DAR that he refused to license an RV because it had loops for the vent, and I asked what he thought about the Rockets - he was unaware that there are hundreds flying around that way. He said “but what about ice?!”, and I asked how water could get in there (see Steve Smith’s thoughts on that above...I agree), and his response was “well ..... it could!”

Not unless someone drove it there with a pressure line. Impossible? No. Improbable? Yes, in my studied opinion. Many more things that are probable to go wrong in our airplanes....

Paul
 
Vent lines.

I fully intend to go with the rocket style fuel vent plumbing on my 9 project. Seems like its well proven. Perhaps as continuing maintenance and to alleviate any concerns about water these vents could be blown out on a interval like annually or semiannually or any time plane flies in rain.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
Rocket-style vents here...500+ hours, no issues other than once or twice, with very full tanks, they "burped" a bit of fuel out when the hangar got hot during the summer.
 
Venting?

Pardon my ignorance hear and this is NOT to start a fight. Due to a one piece gear and using a wider gear web because it needs to be somewhat taller for the nonstandard engine I'm using , plan standard vent routings won't work for my build so the " Rocket coils " are my best option. So here's the question , what's the point of coils . The vent is in the highest point in the tank with an air gap unless the tank is overfilled the air gap in the tank will stop any siphon and even if overfilled the resulting vacuum of a sealed ( leek free) fule system will shortly stop the ground based siphon situation. In flight with the vent on the bottom of the wing in the area of high pressure siphoning should not be able to occur. So other than possibly preventing some sloshed venting of fuel in flight what purpose does making the coils do other than a setup like a " P-trap under the sink "?
 
I believe the risk with icing is at the opening of vent line that is hanging out in the open air. Anything that can get wet from moisture in the atmosphere has a risk of freezing. The fact that ice forms faster on small protrusions makes the small vent protrusions, common on our RV's, a high risk area. I have no concerns about ice in any of my vent tubing, but I have a great fear of ice forming at vent exit out in the free air. For this reason, I installed a check valve and interior vent exit that can only draw air in from the cabin if a vacuum exists in the tank and the main vent is clogged. It cannot let air from vent into the cabin, assuming the valve doesn't fail.

Larry
 
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Pardon my ignorance hear and this is NOT to start a fight. Due to a one piece gear and using a wider gear web because it needs to be somewhat taller for the nonstandard engine I'm using , plan standard vent routings won't work for my build so the " Rocket coils " are my best option. So here's the question , what's the point of coils . The vent is in the highest point in the tank with an air gap unless the tank is overfilled the air gap in the tank will stop any siphon and even if overfilled the resulting vacuum of a sealed ( leek free) fule system will shortly stop the ground based siphon situation. In flight with the vent on the bottom of the wing in the area of high pressure siphoning should not be able to occur. So other than possibly preventing some sloshed venting of fuel in flight what purpose does making the coils do other than a setup like a " P-trap under the sink "?

I think perhaps to try to avoid the "burping" due to fuel expansion by creating more space for the fuel to expand into? As noted, even with 2 or 3 coils, some people have this occasionally happen, so it might be much more common with fewer coils. Dunno for sure, though.
 
thanks for all the replys guys. after reading i am comfortable with a low point in my vent line.
in the original post where a concern was expressed maybe that concern was related to a fuel line. low spots in fuel lines [at least in cars] are a spot for ice. entirely different for vent lines. thanks again.
 
So here's the question, what's the point of coils.

It's a good question. There are enough contributing variables that I suspect there is no absolute answer. The conventional thinking is that they serve as a reservoir...if some small amount of fuel gets pushed out into the vent line, it may trap there and get pushed back in later.

Sometimes it is reasonable to try a new thing by first assessing the possibility of harm. If the experiment can't hurt (i.e. unquestionably satisfies the primary requirement), a trial installation to see if a secondary factor is better or worse becomes a reasonable choice. So, if you're sure a no-loop vent will in fact vent the tank, give it a try. If it dumps too much fuel on the ramp when parked in the sun with a full tank, it's easy to drop the root fairing and retrofit as many loops as desired.

Be aware that standard RV-8 vents, even with a high point all the way to the top of the gear towers, will barf fuel if the tanks are filled in the sun. A no-loop vent would just be a much shorter version.

I'm told some states and/or airports have become militant about fuel spills, so I may make a simple addition which should eliminate the problem. It's an 8oz header tank added to the vent line in the very top of each tower. It won't change venting, but it should temporarily hold expansion fuel, while allowing air bubbles to pass through and out the vent.

sawgzq.jpg
 
good idea dan, will be very easy for me to put this into my venting on the bulkhead.
my tank is a custom behind the seat of a 12. vent line is at front pilot side of tank top side. the top right side has a 1/4npt flange in it also. painless to put a small valve in that flange. like a valve you find on the bottom of an air compressor tank. never a problem to have the vent not doing its job in flight...........if you could keep it together enough to remember to open the valve!
 
Couple of solutions

Theres an old thread around here (unfort couldnt find it just now) where a check valve is T'ed into the vent line. I did this - T'ed into the top of the vent line before it turns down and overboard. The check valve allows air in, but not vapor out and is hidden inside a wing bay so its protected. A mesh screen over the end of the check valve prevents trash from getting into the tube. I used a check from McCarr Master that has a very low opening vacuum (I think its .5 psi) and is speced for vapor as well as liquid fuel.

Also, when I went to punch the skin hole for the vent, I didnt like just the tube sticking out, so I went with a JDAir streamlined vent, which also incorporates a secondary ice hole. http://www.jdair.com/fuel-vents-for-all-vans-rv-aircraft/
 
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I'm told some states and/or airports have become militant about fuel spills, so I may make a simple addition which should eliminate the problem. It's an 8oz header tank added to the vent line in the very top of each tower. It won't change venting, but it should temporarily hold expansion fuel, while allowing air bubbles to pass through and out the vent.

sawgzq.jpg

I like This !!! I am thinking the tank would need to have slightly more volume than the length of vent line leading to it. I am tired of linemen telling me I have a leak. I once came back to find two buckets under my vents...
Have you identified a supplier for the header tank ?
 
Have any of you guys who are talking about vent check valves and/or shutoff valves done the math on the air pressure change from, say, a 30 degree night temperature to a 80+ degree daytime temp, in a partially full tank?

I haven't, but I'd want to know before trapping that air in the tank.

Charlie
 
''downside to everything''.........if you forget the conditions are right for your ''header tank'' to be full and you open your gascap.............
 
jd vents on an 8?

I'd really like to have the nice JD vents, but I don't see a reasonable way to use them on the RV-8. I have the fuselage mostly done and the vents per plans pass through the landing gear cover/inspection plates. There is no way to attach and tighten the vent line to the fitting on the JD product without a lot of severe bending of the tubing? Anyone do this? JDair did not sell these for the RV-8 the last time I looked, but now they are for "all RVs", but no specific guidance on how to.

I also would not want a check or release valve anywhere in the cabin, or a check valve in the line inside the tank - would not want to tear apart the tank to fix a stuck valve, and do not want any potential source of fuel vapor in the cabin.



Theres an old thread around here (unfort couldnt find it just now) where a check valve is T'ed into the vent line. I did this - T'ed into the top of the vent line before it turns down and overboard. The check valve allows air in, but not vapor out and is hidden inside a wing bay so its protected. A mesh screen over the end of the check valve prevents trash from getting into the tube. I used a check from McCarr Master that has a very low opening vacuum (I think its .5 psi) and is speced for vapor as well as liquid fuel.

Also, when I went to punch the skin hole for the vent, I didnt like just the tube sticking out, so I went with a JDAir streamlined vent, which also incorporates a secondary ice hole. http://www.jdair.com/fuel-vents-for-all-vans-rv-aircraft/
 
The vent must always vent

Have any of you guys who are talking about vent check valves and/or shutoff valves done the math on the air pressure change from, say, a 30 degree night temperature to a 80+ degree daytime temp, in a partially full tank?

I haven't, but I'd want to know before trapping that air in the tank.

Charlie

Never trap air in the tank. The fuel vent must always vent.

I view the two major risk that will lead to a collapsed tank and/or fuel starvation are:
- Vent clog by insects (mud dauber)
- Vent line block from ice forming on the vent overboard

I?ve done two things for this:
- The last few inches of the vent line transition to 3/8? line. The 3/8? line is out in the breeze but the line is terminated on a angle cut, the cut itself has a small piece of aluminum screen epoxied on. This is the mud dauber mitigation.
- The vent line in the wing root (RV-10) or in the line running up the tower leg (RV-8) has a vacuum breaker check valve. Air can go in as the tank sucks, but fuel cannot come out. This is the ice mitigation. The vacuum breaker I use is: https://www.mcmaster.com/#catalog/124/528/=1bjw7gf (Part 5492K51)
Add AN fittings as needed.

To keep fuel from venting out the tank:
- Fill on a level surface
- Fill as the last step before departure.

Carl
 
Rocket-style vents here...500+ hours, no issues other than once or twice, with very full tanks, they "burped" a bit of fuel out when the hangar got hot during the summer.

Same here, just over 200 hours with several flights in rain and I regularly cruise in freezing temps. So far I'm stubbornly refusing to die. In this world there are plenty of "solutions in search of a problem", perhaps it's more effective to concentrate our time and effort on actual problems with a higher likelihood of occurrence. If you're flying in icing conditions in an RV, you've got higher priority problems than worrying about a fuel vent getting blocked. Your iced-up tailplane is going to kill you before the blocked vent does.
 
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While I agree that the risk is minimal, I wouldn't think the risk is in-flight icing, but of getting enough moisture in the coils to later freeze on the ground. If that *did* happen (again, very low probability), you probably wouldn't know until long after departure on the next flight.
 
moisture

I am not sure how you would get moisture in there. water cant drip up from the vent hole, so it has to be from condensation. Not sure there could be enough condensation to form enough ice in the loop. Any "pumping" action from heating and cooling the fuel could force moist air in there, but would also bring the fuel vapors out, pushing the condensable moisture out on the next temp cycle.

My fear would be icing from flight in freezing moisture leading to fuel starvation.

I would be inclined to put the angle cut on the backside not the front; besides I cant see there being any noticeable "ram" air pressure from having the angle cut on the front anyway. The positive pressure should come from the wing doing its job, not the angle cut.
 
Have any of you guys who are talking about vent check valves and/or shutoff valves done the math on the air pressure change from, say, a 30 degree night temperature to a 80+ degree daytime temp, in a partially full tank?

I haven't, but I'd want to know before trapping that air in the tank.

Charlie

The check valve doesn't trap air in the tank. You tee off the main vent for the check valve, so that tank always has direct access to the main vent line. The check valve allows air into the vent line, under vacuum, if the main vent line is blocked. The primary purpose of the check valve is to prevent vapor from entering the cabin when you have a secondary vent ingress location.

Larry
 
Same here, just over 200 hours with several flights in rain and I regularly cruise in freezing temps. So far I'm stubbornly refusing to die. In this world there are plenty of "solutions in search of a problem", perhaps it's more effective to concentrate our time and effort on actual problems with a higher likelihood of occurrence. If you're flying in icing conditions in an RV, you've got higher priority problems than worrying about a fuel vent getting blocked. Your iced-up tailplane is going to kill you before the blocked vent does.

All of the icing literature that I have read indicate that the first things to ice on a plane are small protrusions into the air stream. This is why they recommend looking at a temp probe or similar object for signs of ice. Small protrusions will ice over well before the tailplane does. Further, it takes very little ice to block the opening on the vent port. Cutting an angle and facing it into the oncoming air makes it even easier for ice to plug the the 1/4" opening.

Larry
 
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Vents

FWIW: Many Mooney’s and Cherokee’s have the same 1/4” aluminum vent tube that we do on our RVs. It sticks down and out of the bottom wing skin about 3/8” and is angle cut on the front side just like ours. Only real difference is these vents usually tap into the outboard rib of the wing tank. I don’t have any numbers but i’m certain the certified fleet with similar vent arrangement to RVs have amassed many many 1000s of hrs without icing trouble. Gives me some confidence that the stock vents or the rocket style loops will work just fine.


Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
I like This !!! I am thinking the tank would need to have slightly more volume than the length of vent line leading to it. I am tired of linemen telling me I have a leak. I once came back to find two buckets under my vents...
Have you identified a supplier for the header tank ?

Yeah, my junk bin.

Seriously, it is the sort of thing I would just make...two lengths of 6061 tube and weld on some end caps. If you want to buy nice little tanks I'd look at standard Vans brake reservoirs.
 
Venting

RV7A Flyer & DanH thanks for the comments. I think I'll give it a try with out the coils after all this is EXPERIMENTAL AVATION it will definitely vent and hopefully not burp too much, if it does easy enough to revise to the proven coils. It's still going to be a little while before first flight so further insights and comments are welcome & appreciated.

Dan you've just got all kinds of neat ideas please keep sharing.
 
Here is an example of why the tank vent lines drip fuel when "full". This is the per-plans method of venting the RV-9 tanks and probably the same for other models. You can see the vent lines have a straight termination after they attach to the filler flange. I was surprised when I checked my quick build tanks that I could almost fit the tip of my finger between the top of the vent tube and the tank skin. So there is about a 1/2" gap total that the fuel can siphon out through the vent line until the fuel gets below the lowest point of the vent line termination opening. Even if you fill the tanks to just below that point, as the fuel warms and expands a bit, it will rise and start dripping. When I built the RV-10 tanks, I turned up the vent line after the filler flange attach point leaving only 3/32" gap flush to the tank top skin. Never had a problem after filling unless I overfilled. When I open up my RV-9A quick build tanks to go over all the sealant areas and increase the capacity, I will re-work the vent lines also.
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[/url]DSC02010 by David C, on Flickr[/IMG]
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[/url]DSC02011 by David C, on Flickr[/IMG]
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[/url]DSC02012 by David C, on Flickr[/IMG]
 
my take

When I built my 9A tanks, I left the vent tube a little longer so that it goes into the end as much as possible, I also turned the end away from the proseal so the tube wont get clogged when installing the back baffle. During filling the fuel will spill out the filler before the vent is covered by the fuel level, and during climb, the vent is at the highest point in the tank so that the air expanding in the tank I am not using wont push the fuel out. If the vent is towards the rear baffle, I figured it will be under the fuel level during climb out, pushing out raw gas.
 
FWIW: Many Mooney?s and Cherokee?s have the same 1/4? aluminum vent tube that we do on our RVs. It sticks down and out of the bottom wing skin about 3/8? and is angle cut on the front side just like ours. Only real difference is these vents usually tap into the outboard rib of the wing tank. I don?t have any numbers but i?m certain the certified fleet with similar vent arrangement to RVs have amassed many many 1000s of hrs without icing trouble. Gives me some confidence that the stock vents or the rocket style loops will work just fine.


Don Broussard
RV9 Rebuild in Progress
57 Pacer

Be careful! I?m pretty sure Cessnas, and probably others too, have a backup vacuum relief valve built into their tank caps. So their good field experience does not necessarily prove their main vent lines never ice over.
 
Be careful! I?m pretty sure Cessnas, and probably others too, have a backup vacuum relief valve built into their tank caps. So their good field experience does not necessarily prove their main vent lines never ice over.

Bob: Great point. You are correct, most of the planes with caps that stick up above the wing have a vent mechanism in the cap. Lots of planes however have flush caps like RVs and very few of those have any kind of vent in the cap. My concern is adding any risk by going with rocket type loops vs stock vent location. Either way looks equally susceptible to vent icing just like any other RV.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
Wouldn't a Vans brake fluid tank make a good catch tank?

8.5 GPH is 33 cubic inches a minute - What happens in a rain?
 
fwiw, it is an extreme example of how much your fuel will expand but i ran the numbers and from 50 deg f to 90 deg f my 27 gallons will expand .5 gal.
i am liking the idea of a small emergency vent valve right behind passenger seat more and more as this thread goes on.
 
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