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Fuel Tank Vents

ghatch

Active Member
Has anyone either put a 90deg bend in the fuel tank vent with the vent hole facing forward or added a 45deg forward facing cut in the fuel tank vent?
The reason being to the negate a lower pressure in the tanks caused by the vents being on the bottom (low pressure area) of the wing and again to stop the Venturi effect on caused by the vent pointing straight down with the end being 90deg to the airflow? I found that I need to do the above to allow the engine driven pump to suck fuel from the tanks in either high altitude, high temp or both situations. Has anyone heard of doing this ?
Thank you in advance for your input.
Gerry
 
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Yep, my plans called out a 45 degree bevel facing forward- assuming this is still the case….
P44-7 on my 2004 era plans

Phil
Rv-10
Bountiful, Utah
KBTF
 
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Has anyone either put a 90deg bend in the fuel tank vent with the vent hole facing forward or added a 45deg forward facing cut in the fuel tank vent?
The reason being to the negate a lower pressure in the tanks caused by the vents being on the bottom (low pressure area) of the wing and again to stop the Venturi effect on caused by the vent pointing straight down with the end being 90deg to the airflow? I found that I need to do the above to allow the engine driven pump to suck fuel from the tanks in either high altitude, high temp or both situations. Has anyone heard of doing this ?
Thank you in advance for your input.
Gerry
The bottom of the wing isn’t “low pressure” area, it’s high pressure.
 
The bottom of the wing isn’t “low pressure” area, it’s high pressure.

And, Van's have said previously that ram effect is not needed for proper operation of the fuel vents:

... I have always been concerned about inadvertent ice accumulation on the RV's. ... The main thing bothering me was the fuel vents, so I decided to try something. I asked at Van's if we really needed the ram air into the tanks, and since the answer was no, I decided to put one of the cable fairlead covers like we use where the rudder cables exit the fuselage ahead of the fuel tank vent. The vent line does not protrude below the depth of the cover. I have made many flights now, at all pwer settings and altitudes, and I have not noticed any difference in fuel flows or pressures.

If you're not getting adequate fuel flow in some conditions, maybe you need to look at any restrictions in your vent lines or fuel lines.
 
I didn't go the plans route and also don't believe ram pressure is necessary. I made a smooth round disc with a hole for the vent (think static port) and mounted it on the bottom of the wing. No issues getting fuel to the engine.
 
I left mine cut straight with no ram effect. I’ve considered this a builders/my error but have never had a problem with it so decided to just leave it. If it isn’t broke don’t fix it. I’m hangered but have still added sticking a wire up the ports in my preflight to make sure nothing has nested up there. Made some port plugs for when I’m tied down outside.

Edit - Like Mel suggested I’ll angle file my vents toward the front. Good point.
 
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Refuelling the tanks to full has two consequences, firstly making a mess and spilling a small amount of fuel and secondly a full stream flushes the vent lines and ensures they are clear.
 
I recommend cutting the vents at an angle towards the front as per the plans. Even though ram air is not necessary, a cut that is square and/or slightly angled to the rear, can have a venturi effect causing a small negative pressure in the tank.
 
Reposting what I do.

I worry about two things for tank vents:
- Mud daubers
- Ice
Either one will block a vent which could lead to tank collapse.

Here is a mod I did for the RV-10. Note:
- The piece out in the breeze is 3/8” tubing, cut on a bevel with a piece of aluminum screen JB welded across the bevel. This is for the mud dauber risk. The 3/8” tubing bevel cut provides more surface area for the JB Weld.
- The black nylon thing is a vacuum breaker suitable for fuel use. It only takes a few inches of vacuum to open the valve. Here if ice forms on the outside piece you suck in are from the wing root area. I never had a problem with fuel leaking past the valve.

The valve is very light and available from McMasters.

Carl
4-F29705-E-2224-4456-942-C-202455-BAA251.jpg
 
Reposting what I do.

I worry about two things for tank vents:
- Mud daubers
- Ice
Either one will block a vent which could lead to tank collapse.

Here is a mod I did for the RV-10. Note:
- The piece out in the breeze is 3/8” tubing, cut on a bevel with a piece of aluminum screen JB welded across the bevel. This is for the mud dauber risk. The 3/8” tubing bevel cut provides more surface area for the JB Weld.
- The black nylon thing is a vacuum breaker suitable for fuel use. It only takes a few inches of vacuum to open the valve. Here if ice forms on the outside piece you suck in are from the wing root area. I never had a problem with fuel leaking past the valve.

The valve is very light and available from McMasters.

Carl
4-F29705-E-2224-4456-942-C-202455-BAA251.jpg

Would you have a part # or a link from McMasters?
 
Searched McM every which way:
"The valve is very light and available from McMasters." > NO JOY.
Where it be?
 

The webpage says these are check valves for oil and fuel. But here we're using it for air. Does that make a difference? I.e., it's spec'd to flow at .33 PSI fluid pressure, but what air pressure is required? Sorry, sounds like a dumb question.
Also when the check valve is open it's flowing air from inside the wing. I know the wing isn't airtight so it should work... but if this works then why not have the vent itself stay inside the wing? wouldn't it be less likely to freeze? Is it because we don't ever want back flow to splash fuel inside the wings?
There you go... a series of dumb questions... but I'm confused.
-c
 
The webpage says these are check valves for oil and fuel. But here we're using it for air. Does that make a difference? I.e., it's spec'd to flow at .33 PSI fluid pressure, but what air pressure is required? Sorry, sounds like a dumb question.
Also when the check valve is open it's flowing air from inside the wing. I know the wing isn't airtight so it should work... but if this works then why not have the vent itself stay inside the wing? wouldn't it be less likely to freeze? Is it because we don't ever want back flow to splash fuel inside the wings?
There you go... a series of dumb questions... but I'm confused.
-c

The valve opens on a slight vacuum (as in a breath).
You want a valve rated for fuel. If the tank is full and burps on a hot day, you want the vent to pass the fuel outside the plane, not into the wing root.

Carl
 
The valve opens on a slight vacuum (as in a breath).
You want a valve rated for fuel. If the tank is full and burps on a hot day, you want the vent to pass the fuel outside the plane, not into the wing root.

Carl

OK. Still not quite with you... sorry.
You didn't say taking vent air from inside the wing is bad. Assuming that's less likely to freeze then why not use it for vent air, rely on the valve as pictured to prevent fuel spill inside the wing, then have another valve reversed and venting fuel to the outside?
I.e., same picture as you already have, but another valve on your current vent flowing out only.
 
i think this is the gist of the interior vent

. . . then have another valve reversed and venting fuel to the outside?
I.e., same picture as you already have, but another valve on your current vent flowing out only.

The intent of the vacuum vent check valve is to allow the tank to vent from the inside of the wing (within the wing root fairing) if the exterior vent gets blocked by anything (e.g., ice, mud dauber, insect strike); that is - to relieve a vacuum inside the tank.

The check valve isn't intended to allow fuel to vent from the tank which it won't unless you pulled up behind a tanker for an in-flight top-off and the tank over filled. Then it would overflow via the outboard vent port.

Right?
 
OK. Still not quite with you... sorry.
You didn't say taking vent air from inside the wing is bad. Assuming that's less likely to freeze then why not use it for vent air, rely on the valve as pictured to prevent fuel spill inside the wing, then have another valve reversed and venting fuel to the outside?
I.e., same picture as you already have, but another valve on your current vent flowing out only.

The check check valve is supposed to be a safety device, providing an alternate vent in case the main vent is blocked. Your proposal turns the check valve into the only vent (since the second check valve now blocks the primary). Probably less safe as now a stuck check valve can cause fuel starvation.
 
Thank you everyone for your input. I have tried everything that has been suggested including replacing the mechanical fuel pump. As a last resort, on advice for (2) separate builders and USAF Colonels, that I try attaching a 90deg into the airstream addition to the tank vent. Result... Instant correction to the problem. I have not had a chance to try the 45deg scarf on the other tank but I will. Apparently I missed that part on p44-7. I started this thread to see if anyone else has had the problem and if the solution I found had been tried by others. I really appreciate the quick and thorough responses. Thank you. If anyone has additional ideas or comments, I like to hear them.
Gerry
 
I filed a forward facing scarf on my tank vents that had been working ok with a straight cut. So - with my straight cut vents I sometimes got a whiff of fuel smell in a climb when my tanks were full to the brim. Now I get a more pronounced smell in a climb even with tanks that are only half full.

I replaced the o-rings in my Usher fuel caps & lubricated with Krytox. The left one had leaked around the stem when I filled the tanks to the top and the fuel expanded sitting in the hanger. I filled up high after todays flight so I'll see how the caps seal now. In flight there's no evidence of leakage.

Bottom line - anyone else had a similar issue? Should I be looking for another leak. (There is none in the in cabin stuff - valve, filter, fuel pump etc). I'm thinking of going back to the straight cut vent outlets.
 
To the OP - I am thinking you might have a different problem here. How hot does your tunnel get in flight? RV10s are known for tunnels getting very hot and I have mod that I do to solve that.

I had a case where fuel pressure started dropping with the occasional engine surge with it. Ended up to be a hot tunnel and fuel vaporizing. I have used Mogas at the time.
 
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To the OP - I am thinking you might have a different problem here. How hot does your tunnel get in flight? RV10s are known for tunnels getting very hot and I have mod that I do to solve that.

I had a case where fuel pressure started dropping with the occasional engine surge with it. Ended up to be a hot tunnel and fuel vaporizing. I have used Mogas at the time.

+1

If you need ram air pressure in the vents, it is masking another problem in your fuel delivery system. I use a rivet head shaped termination. No ram pressure and likely a bit of venturi induced vacuum. No issues feeding fuel to the engine in 200 hours.

Ever seen a picture of a fuel tank after the vent became clogged. The force is strong enough to dent the tank skins inward. A clear indication that pressure in the vent is NOT required.

Larry
 
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Vent physics questions ?

Good info above about vent line ends, re: squared off/angled/ and rivet heads. Apparently the pump suction can overcome realistic negative air pressure.

I employed the “rocket style” double ring of tubing in my RV3 and worked fine. However, the loops were limited on capacity and if they had fuel in them, the pump suction would be required to bubble air through the trapped fuel. The same would be true of the Vans design inside the cabin. There will be a liquid fuel head somewhere for the pump suction to overcome. Apparently it can if a mud dauber plug can collapse a tank skin.

Here is my question: If you made steep (1 G,ball centered) turn putting the “Off” or “On” tank down at vertical, does fuel go into the vent line when you level the wings ? You could assume the compressed air in the “off’ line or “forced “air from a chamfered line in the airstream would push the fuel back in the tank? However, I in-vision the air passing over or through the liquid such that some fuel is left to drain into the rocket vent or the Vans vent. If this is true, whether you filled the vents from expanding fuel in the sun, or steep maneuvers, you are always pulling air through a fuel head that could be as high as 3 or 4 inches depending on plumbing design. Your thoughts ?
 
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