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skyyking

Active Member
Am running the fuel vents but the thought occurred, should they ice up (I will have a heated pitot) there is no alternate fuel vent like on a Cessna fuel cap.

Are my concerns valid?

Thank you for your interest!
 
Just a thought here - but if you're in icing conditions severe enough to ice over a fuel vent - and do it quickly enough that you could pull a vacuum in the tank sufficient to cause a power loss - then you've already got enough ice on the airframe that you're not worried about a power loss anymore. Power is no longer your primary problem.
 
There was a group of guys that drilled a small hole on the aft side of the vent tube for piece of mind. I'm not sure it's needed, but I remember a thread where several builders / owners did this mod.
 
Check Valve at top in cabin

I had the same worry and put a check valve at the high point of each vent tubing in the cabin. Now if the vent get clogged for any reason the check valve will open at about -2psi.
 
Not sure this is a good idea.

Geico266 said:
There was a group of guys that drilled a small hole on the aft side of the vent tube for piece of mind. I'm not sure it's needed, but I remember a thread where several builders / owners did this mod.
Click to expand...

The fuel vent inlet is cut so that it puts a small positive pressure on the tank. A hole in the aft side of it will reduce, if not eliminate this pressure.
 
Mel said:
The fuel vent inlet is cut so that it puts a small positive pressure on the tank. A hole in the aft side of it will reduce, if not eliminate this pressure.
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Mel,

I've always been somewhat puzzled by how the vent can need positive pressure, when its been often reported that a plugged vent can collapse a tank. It would seem that the fuel pump doesn't really need any extra help from the vent. Do you have any insight on this?

Not planning to mess with my fuel system, just curious.
 
You may not "need" positive pressure, per sey, but a little "positive" pressure will assure that you don't have "negative" pressure. It could also help in the event of a pump failure.
 
Not necessarily so,

airguy said:
Just a thought here - but if you're in icing conditions severe enough to ice over a fuel vent - and do it quickly enough that you could pull a vacuum in the tank sufficient to cause a power loss - then you've already got enough ice on the airframe that you're not worried about a power loss anymore. Power is no longer your primary problem.
Click to expand...

Small radii collect ice first. Thus temp probes are a good indication you are getting ice. Additionally, the horizontal stab with it's smaller radius leading edge ices up quicker than the larger wing leading to tail stall.

In my experience with ice, fuel vents are one of the first things to ice up. I am a proponant of the small hole in the back side of the vent, just above the angle on the front side so that the ice from the front doesn't also plug the backup hole in the back. There is no need to have positive pressure on the tank, i.e. Cessna vent right back of the strut. As was stated you want to assure that you don't have a negative pressure on the tank that have adverse effects.

Gary Specketer
 
Gary 40274 said:
k. There is no need to have positive pressure on the tank, i.e. Cessna vent right back of the strut. As was stated you want to assure that you don't have a negative pressure on the tank that have adverse effects.

Gary Specketer
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My understanding of the Cessna vents is that there is still positive pressure in the wake of the wing strut. Think of pictures of smoke or streamlines around airfoils in wind tunnels.

The advantage of the placement behind the strut is as a "poor man's inertial separator." Airborne water or ice is denser than the airstream and unable to "turn the corner" as quickly as the air that is carrying it. This is why leading edges of wings will ice, but the aft portion generally remains clear.

I've considered moving the vents behind the gear legs on my RV-8, but have only rarely operated the plane IFR, and never in anything remotely close to icing conditions.
 
skyyking said:
Am running the fuel vents but the thought occurred, should they ice up (I will have a heated pitot) there is no alternate fuel vent like on a Cessna fuel cap.

Are my concerns valid?

Thank you for your interest!
Click to expand...

Your concerns are certainly valid, so valid it is worth thinking about NEVER flying in icing conditions with these airplanes. Ice is way outside the envelope, don't do it or get near it.

A heated pitot and/or fuel vent line won't make it safe. This sad event just a little over a month ago (requiring one body bag) illustrates why what I say is true.

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20100104X51148&key=1
 
Check Valve

I ask to those of you who installed the check valve, where did you purchase it, or better yet, could you send a pic.

Yes, don't fly in icing, but the world or weather, is not that perfect. I am sure some of us have flown in the clear, of clouds and picked up icing and as was mentioned on the earlier reply to this thread, it is the small protrusions that pick it (ice) up first.

Once again, I really appreciate the interest and help in finding a solution.
 
Interconnected vents.

Ice is one thing you want to avoid anyway, but a fuel vent could get blocked easily by a bug. That's why I interconnected the two ventlines, at the top, under the instrument panel. It is not a lot of additional weight: 2 tees and about 3 feet of 1/4 tube.

If one vent gets blocked, the other one will still service both tanks. I gues if one ices up the other one will follow within seconds, so this will not help in icing conditions.

I did drill the holes in the back of the vents, but I will probably close them off again, because more possitive pressure on the tanks will help prevent vapor lock between the tank and the boost pump. I will be running on Mogas, so I want to use any opportunity to avoid vapor lock.

If you look on Rene Bubbermans Website (Netherlands), you will see that the engine will not quit imediately if your vent lines are blocked, it will take a while and you will notice there is something wrong when the tanks start to get smaller! It happened to him on his first flight.

Regards, Tonny.
 
airguy said:
Just a thought here - but if you're in icing conditions severe enough to ice over a fuel vent - and do it quickly enough that you could pull a vacuum in the tank sufficient to cause a power loss - then you've already got enough ice on the airframe that you're not worried about a power loss anymore. Power is no longer your primary problem.
Click to expand...

Absolutely not true.
What is required for a vent to ice up is moisture and freezing temps. No air frame ice is required I assure you. No one should think they are safe because they have no airframe ice.
Been discussed.
 
Heated fuel vent

I was concerned about fuel vent blockage too, whether from ice, or insects (a plague of locusts?) or any other unlikely cause. I think the concern about never flying in icing conditions misses the point, which is to design and build the safest, most foolproof systems possible in our aircraft. For my RV-9A I considered relocating the fuel vents to a warmer location (aft of the cowling exit duct, risk of fire) or heating them (too much alternator draw) or placing an inertial separator (strut) in front of them (hard to know if they will still be in an area of positive pressure). I finally decided to put a tee at the highest point in both vent lines inside the fuselage and install a check valve in each line. Vent lines will not be interconnected. Two possible choices for a check valve are ACS 10630 from Aircraft Spruce or Andair CK375M. Latter is available with a bulkhead fitting on one end which I may use to secure the valve to a bracket or bulkhead. In addition, I will be using an AN818 coupling nut and AN819 sleeve with a screen prosealed on the end to prevent critters from getting into the valve.
 
Heated fuel vent

Hi Tonny,
The check valves are spring loaded to prevent flow of either vent line pressure or fuel into the cabin, but should the external fuel vents become blocked for any reason, will open and allow air to vent into the tanks.
 
Check Valves

Folks, the Andair CK375M and the Aircraft Spruce ACS10630 are check valves only, they won't relieve to atmosphere. Please confirm that it is McMaster-Carr Part No. 4320T13 (Page 460 in catalog) 1/4" tube fittings, that you want to purchase and install. There are different specs that you need to specify when ordering, please confirm also that you want it at 3" water vacuum not as a pressure relief, .5 psi to 10 psi (ref. catalog description)?

Thank you!
 
Ah... I see, so your are venting inside the cabin? I guess that is another way of dong it. I just interconnected the lines.

Regards, Tonny.
 
Since we fly in Cape Cod area, IFR is routine, so I have been concerned about watching my tank start to collapse in case of a vent closure.
One thought I had was a couple of deflectors or tubes from each exhaust pipe towards the fuel vents. This is rather easy to do with my dual exhaust pipes.
The other thought is similar to posted suggestions, but rather than a spring loaded relief valve, just a manual valve in the cockpit that will open up the T'd fuel vent lines to cabin pressure. So when pitot heat goes on, open backup fuel vent.
Yes, it's easy to say do not fly in ice, but there are times that you will have to fly through or down in some clouds, so be prepared if you fly IFR.
Regards
Jack
 
HMMM, my vents are heated. The vent line travels three or four feet through the heated cabin, then exits the cabin just behind my warm engine with a 1.5" standpipe. I doubt any ice will collect on the vents. I imagine the interior routing was planned on purpose to warm the tube and prevent icing. At least, that's what I am convincing myself of, so don't confuse me with any facts. Someone needs to ask Van's this question.
 
Heated fuel vent

skyyking said:
Folks, the Andair CK375M and the Aircraft Spruce ACS10630 are check valves only, they won't relieve to atmosphere. Please confirm that it is McMaster-Carr Part No. 4320T13 (Page 460 in catalog) 1/4" tube fittings, that you want to purchase and install. There are different specs that you need to specify when ordering, please confirm also that you want it at 3" water vacuum not as a pressure relief, .5 psi to 10 psi (ref. catalog description)?

Thank you!
Click to expand...

From the Andair website the CK375M valve:
The internal shuttle and spring are extremely light giving a cracking pressure of between 0.4-0.7 PSI.

Please note that I intend to mount this check valve in the cabin on a tee in the vent line. In normal operation the external vent is connected to the fuel tank through the tee. The spring in the check valve keeps the valve closed to any internal pressure in the vent line. If the external vent becomes blocked the check valve will open on the other leg of the tee and connect to the fuel tank preventing collapse of the tank.
 
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Where Commander Aircraft puts them is...

on the back of the engine baffel with a 90 degree bulkhead fitting.
Yep they run a -4 hose to the back of the engine baffels right above where the cylinder base bolts to the case. At that location they have a downward facing elbow/bulkhead fitting. I thought that was an excellent place and will probably do that next time.
 
Rick of Austin said:
on the back of the engine baffel with a 90 degree bulkhead fitting.
Yep they run a -4 hose to the back of the engine baffels right above where the cylinder base bolts to the case. At that location they have a downward facing elbow/bulkhead fitting. I thought that was an excellent place and will probably do that next time.
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Do you know any other details of the Aero Commander vent system? Do you have a schematic? Is there a check valve between the engine compartment and the tank(s)? Interesting location for a vent, but I wonder is it susceptible to fuel sloshing during braking on landing or flight manoevers?
 
apkp777 said:
HMMM, my vents are heated. The vent line travels three or four feet through the heated cabin, then exits the cabin just behind my warm engine with a 1.5" standpipe. I doubt any ice will collect on the vents. I imagine the interior routing was planned on purpose to warm the tube and prevent icing. At least, that's what I am convincing myself of, so don't confuse me with any facts. Someone needs to ask Van's this question.
Click to expand...

Follow your vent line from start to finish and consider the possibility that it is because some of it is heated that you have a potential problem, and because some of it is NOT heated that you have a problem. Consider the design, low points, loops and curls. The possibilities for various temperatures and moisture contents. Consider what may already be in your vent line when you launch, when you switch tanks, when you climb to a freezing OAT etc etc etc. There are a hundred combinations that will put ALL RV's in trouble that are flying VFR.

Your heated pressure entry and cabin may in fact contribute to a failure mode you have not considered.
 
The vent lines and vents on an RV -9 are heated by the cabin air temperature. I think it is important that you get in contact with Van's on this one before you start to modify the vent system. There is little danger of the stock vents freezing over. Icing only occurs in a relatively small OAT range and the temperature of the collecting surface (vent) must be below freezing. Well, if I can't get my cabin up to at least 60 degrees I doubt it would be a useful cold weather machine. I am sure someone out in RV land has a background in thermodynamics and can speak to this more accurately.

And of course, the plane must be piloted into icing conditions, uh, I am not sure that's a good idea.
 
Mel said:
The fuel vent inlet is cut so that it puts a small positive pressure on the tank. A hole in the aft side of it will reduce, if not eliminate this pressure.
Click to expand...

If the ratio of hole sizes is large, the reduction of the pressurization will be negligible. I think I drilled a .062" hole as Gary described in an earlier post in this thread. The area ratio is probably more than 10 to 1.
 
apkp777 said:
The vent lines and vents on an RV -9 are heated by the cabin air temperature. I think it is important that you get in contact with Van's on this one before you start to modify the vent system. There is little danger of the stock vents freezing over. Icing only occurs in a relatively small OAT range and the temperature of the collecting surface (vent) must be below freezing. Well, if I can't get my cabin up to at least 60 degrees I doubt it would be a useful cold weather machine. I am sure someone out in RV land has a background in thermodynamics and can speak to this more accurately.

And of course, the plane must be piloted into icing conditions, uh, I am not sure that's a good idea.
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Tony Im afraid you are completely wrong on so many fronts.
The Vans fuel vent system is fraut is potential faiure modes due to moisture content freezing that you are not considering.
Suggest reading post 26.
 
The subject may be moot.

I don't think Tony will fly in cold weather because his cabin temp will not be 60F. For sure when it is the OAT is 10F.

Did you guys know that the fighters involved in the Battle of Britain and even later, the P-51 had no cabin heat. The Hurricane had heat ported to the guns to keep them from freezing up but none to the cockpit area. It kind of makes us look like wimps, expecting to fly around all comfy in the dead of winter. :)

I have heat but it sure is not working well with all the built in summer ventilation around the canopy.
 
Kahuna said:
Tony Im afraid you are completely wrong on so many fronts.
The Vans fuel vent system is fraut is potential faiure modes due to moisture content freezing that you are not considering.
Suggest reading post 26.
Click to expand...

I really don't think so. How is the vent going to freeze? Has anyone actually experienced this. I doubt it. Again I would love to see the question posed to Van's or an expert in thermodynamics. Really, if it's a true problem, I think it is serious enough to mandate a design change from the mother ship. The vent system is not "fraut is [with] potential faiure". I saw the original post and wondered? My conclusion, it's not likely. Probably more likely that 2 bumble bees hit each vent at the same time. I am concerned that someone starts to modify the vent system and reduce the slight "head" pressure, that I think is required. As always, I could be wrong. Certainly would stake my reputation on it. I just trust the design on this one.

Remember you will get as much fuel vapor/liquid in the line as you will water. The fuel vapor/liquid will work to prevent icing.
 
Kahuna said:
Tony Im afraid you are completely wrong on so many fronts.
The Vans fuel vent system is fraut is potential faiure modes due to moisture content freezing that you are not considering.
Suggest reading post 26.
Click to expand...

So far, I'm more inclined to go with Tony's thoughts. But then my 6A doesn't have curls in the line somewhere. I may have one low spot in the cabin area, but not sure. And I've been an outstanding & "brilliant" heating/air conditioning guy for close to 40 years.:)

What I'd really like to know is the temp of the heavy floor skin during cold temps in the area of the inlet fittings. This is considering cabin temp, & effect from the exhaust pipes.

L.Adamson --- RV6A
 
apkp777 said:
You won't get icing when its 10F. And yes I will fly when its below 60F inside the cabin, but I'll complain a lot. Hey I hate the cold, don't ask me why I live in Wisconsin.
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I've been -6 on occasion in a friends 9A. It get's cold over some of these Utah mountain ranges...

L.Adamson --- RV6A
 
Kahuna said:
There are a hundred combinations that will put ALL RV's in trouble that are flying VFR.
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LOL. Please don't take this personal. I am just bored and really am not a jerk, but that quote is reaaaalllllllyyyyy stretching it. Hey and that's an awesome Super8!
 
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Kahuna said:
Yes.........
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Tell us more please. Which aircraft type? Was the vent system built as per Van's plans, or did it use a coil of tubing in the wing root area? What were the conditions that created the problem? What were the symptoms of the problem? What was the outcome of the flight?
 
Vent icing has been discussed before.
I had a post #25 in one here.

The standard RV vent system install per plans has high and low points. Opportunity to trap moisture. Moisture contamination into the vent system can happen in a variety of ways. The large temperature variation of the vent pathway does not help the situation.

In my specific documented case (I wrote an article on this somewhere), I was flying in high moisture content air. Hard blowing snow IMC goo. Engine failure from fuel starvation. Took ~7 minutes for the engine to lean to the point that the mixture was full rich and the engine finally quit. All airports around me were below minumums. It was not a pleasant time.

Once on the ground and parked, I popped the fuel cap and the tank vented measured only by my ear and the suction sound. Next I blew in the fuel cap into the tank, no air out the vent. The tank held my lung pressure for about 5 seconds. Next I pushed the plane into my warm hanger. 10 minutes later I blew again into the fuel cap and my partner caught the liquid coming out of the vent in hand. Was just a couple drops. Enough to display moisture in the palm of the hand. Tried to lite it with a lighter, it was not flamable at that moment. I did not test it in a lab to see if it was water I can only make an educated guess based on other factors.

While I did not see the snow/moisture enter the vent system, my personal investigation led me to the following conclusion.

1. Moisture entered the vent system through the pickup.
2. The moisture made it way through the system to a location where it froze. OAT was ~25f as I recall at the time of engine failure but I had decended out of a temperature that was probably mid teens.
3. The rest is self explanitory

There are quite a few methods for moisture to contaminate the fuel vent system I can think of. Rain, snow, plane washing, condensation, etc. Visible airframe icing is not required. The fact that your vent runs along a section that is 100F changes nothing. If that moisture makes its way to a place where it can freeze, like up into your cold tank area, then freeze it will.

Anyway, that is the short version. I have not had the oportunity to validate my theory or this particular set of circumstances, but it would not be very hard this time of year to do it.

I would argue this.
1. Getting moisture into the vent system is easy
2. Getting that moisture to freeze in the vent systems in an RV is also easy
3. Neither #1 or #2 requires you to be IMC in the goo.
 
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Yikes - Well, I'll eat my words. It does appear that you had icing/blockage of your vents. Now the question is what's the solution? Is there a vented cap?
 
Yes there are vented caps. Those create their own set of failure modes that need to be considered as well as design requirements needed to support the platform.

Frankly I dont see that there is an inherent problem with the design given the balance of risk, design change, and system requirements. While this failure did result in an engine stoppage, other system designs (separate tank and fuel vent), supported flight continuation to a safe conclusion. Everything worked as it should have, as it was designed.

It would be interesting to do some further investigation on capillary action inside this tube diameter (alum vent line) to see how it behaves as it moves through a system like this. Surface tension, capillary action, air pressure relieve, etc, would probably prove interesting.

Im envisioning setting up a simple system with clear plastic tube that emulates our vent lines. Clear so I could watch it. Add a drop of water and some pressure and suction to see how it behaves in the lines. Does it(water droplet) come out? Does it move some then drop back down to the low point as air flows by it? These types of questions will help view it in action. Would be interesting to watch and change some of the variables.

I have 4 temperature probes scattered about my 8 (4 independent OAT readings in the cockpit). I have done temperature tests just for my own edification in various places like the engine compartment, cowling inlets and exit, wing woots, NACA vents on fuse, naca vents out in the wings and so forth. Temperature rise w/i 1" of the surface aft of the cowl 2' or less is upwards of +10f degrees ambient. Wing roots are +8 if fuse entry points are blocked... just to give you some data points that are also interesting.

In the end, blanket statements are not helpful. Actual study of the dynamics of the issues involved make for good discussions and interesting reading for those who care enough to spend time dealing with it.

BTW, did I mention both my vent systems had the same failure that day? Not surprising since they were both exposed to the same environment. One made the engine quit, the other lasted long enough to get my butt on the ground.
Both had vacuum on the tanks, both had an undetermined nonflammable vapor exit the vent after tank warming and after tank pressure was applied.

It was an excellent learning experience for me. Im confident I could recognize the onset of this failure now much much sooner and increase my chances even more of a successful outcome.

It took me a few thousand feet of glide in the goo to declare emergency, point my nose at the closest airport which I was probably not going to make 90 degree turn away from heading, get the equipment configured for a new approach (there are a lot of buttons to push), fly the plane, hold best glide, and get my systems all working towards the new destination before I could then work the problem of engine stoppage. First step was switch tanks. She fired back up. Engine ran long enough to land in minumums.
 
Wow, Kahuna - that is scary

It would be nice to know were the low (water) was frozen in the system, but we could only speculate.

From this account I concluded the following:

If the blockage was before any cross over between the two sides, then you would not have had the other tank to restart your engine with. I would not add a cross over!

If the blockage was after any extra vent (valve, etc) then the results would have been the same. Not worth adding a extra vent unless you know for sure that blockage will not form between it and the tank.

Adding a hole behind the current vent, is not likely to keep ice from sealing off that location for long, if at all.

A couple of thing that might be worth considering:

Before flight that might go anywhere near freezing temps, blow out the vent system (IE pre-flight). But using you breath, although clearing some moisture from the system, might also add moisture to the system. This should be done with some source of dry air. (What to use? Canned air for cleaning keyboards?)

If I were still building I might think of adding a small loop in the wing root and placing a check valve at the highest point.

Any comments? Pro or Con.

Kent
 
kentb said:
If I were still building I might think of adding a small loop in the wing root and placing a check valve at the highest point.

Any comments? Pro or Con.

Kent
Click to expand...

Kent this is an interesting idea. Lets talk this through. Can you elaborate on the issue your trying to solve with this? Im not following.

One senerio. Im picturing this little ball of water striking my above freezing outside vent tube (warmed by the cowl air mostly) traveling along the vent system with outside pressure pushing and my fuel tank pulling, through my warm toasty cabin, into the wingroot where is cools some, and along the inside of the cold tank where it freezes before exiting into the fuel area. This is my best guess of what happened to me. Whats the check valve going to do? Or are you after a different set of crcumstances?
 
Kahuna said:
One senerio. Im picturing this little ball of water striking my above freezing outside vent tube (warmed by the cowl air mostly) traveling along the vent system with outside pressure pushing and my fuel tank pulling, through my warm toasty cabin, into the wingroot where is cools some, and along the inside of the cold tank where it freezes before exiting into the fuel area. This is my best guess of what happened to me. Whats the check valve going to do? Or are you after a different set of crcumstances?
Click to expand...

Why not get a piece of clear tubing, and see what it takes to get a drop of water to climb straight up that 24" rise or whatever it is. I'm not even convinced it can do that. At least not yet. :)

L.Adamson --- RV6A
 
L.Adamson said:
Why not get a piece of clear tubing, and see what it takes to get a drop of water to climb straight up that 24" rise or whatever it is. I'm not even convinced it can do that. At least not yet. :)

L.Adamson --- RV6A
Click to expand...

The calculation of a vert column is a known ~.85psi on the vertical 2', much much less on a drop, but I dont think that helps any cause in the absense of vertical, which we have here, fluids act funny particularly in smaller cross sections. Where are our fluid dynamicists?
 
Kahuna said:
The calculation of a vert column is a known ~.85psi on the vertical 2', much much less on a drop, but I dont think that helps any cause in the absense of vertical, which we have here, fluids act funny particularly in smaller cross sections. Where are our fluid dynamicists?
Click to expand...

One thing for sure, is that the end of the "tube" will not be in the fluid to maintain a constant flow. It will be like sucking a drop through a 2' vertical straw, pulling it out of the water & then seeing what it takes to pull that drop up & over the hump.

L.Adamson
 
Thank you for your enthusiasm with the subject

I was the guy who originally posed the question. Again, my fear being that the small area screen inlets would ice up quite readily. One see's the logic as to why a certified airplane vents one cap, runs a crossover to the other vented tank which breathes thru a standpipe.
After all your discussion; I will not place a rubber grommet between the vent line and the fuselage, instead, will leave this open and allow cabin air to flow. Between the heat of the fuel and the heat of the cabin it is reasonable to assume that this area of the vent line will remain warm. I will place an Andair check valve, CK250 (Aircraft Spruce catalog number) with AN-4 fittings on the very top of the vent line run as it loops thru the fuselage. I will use an AN824 tee to splice into the vent line along this run and place the check valve at the 90 degree point of the AN tee, with the arrow pointing on the check valve "in" toward the line. I will do this only on one side (RH), just so I have the peace of mind that there is a back up automatic method of venting a tank other then the venerable externally exposed scupper.
I am no engineer, just another builder (91219). I think this is conservative and am using known aircraft quality components (Andair and AN fittings).
Once again, my sincerest appreciation for allowing me to discuss this with you all.
 
skyyking said:
I was the guy who originally posed the question. Again, my fear being that the small area screen inlets would ice up quite readily. One see's the logic as to why a certified airplane vents one cap, runs a crossover to the other vented tank which breathes thru a standpipe.
After all your discussion; I will not place a rubber grommet between the vent line and the fuselage, instead, will leave this open and allow cabin air to flow. Between the heat of the fuel and the heat of the cabin it is reasonable to assume that this area of the vent line will remain warm. I will place an Andair check valve, CK250 (Aircraft Spruce catalog number) with AN-4 fittings on the very top of the vent line run as it loops thru the fuselage. I will use an AN824 tee to splice into the vent line along this run and place the check valve at the 90 degree point of the AN tee, with the arrow pointing on the check valve "in" toward the line. I will do this only on one side (RH), just so I have the peace of mind that there is a back up automatic method of venting a tank other then the venerable externally exposed scupper.
I am no engineer, just another builder (91219). I think this is conservative and am using known aircraft quality components (Andair and AN fittings).
Once again, my sincerest appreciation for allowing me to discuss this with you all.
Click to expand...

Look at the schematic for the Piper Arrow. Seperate vents. Scroll to at least page 3

http://www.aerotech.net/stuffforCFI/8162S_Fuel_System.pdf

L.Adamson --- RV6A
 
skyyking said:
I was the guy who originally posed the question. Again, my fear being that the small area screen inlets would ice up quite readily. One see's the logic as to why a certified airplane vents one cap, runs a crossover to the other vented tank which breathes thru a standpipe.
After all your discussion; I will not place a rubber grommet between the vent line and the fuselage, instead, will leave this open and allow cabin air to flow. Between the heat of the fuel and the heat of the cabin it is reasonable to assume that this area of the vent line will remain warm. I will place an Andair check valve, CK250 (Aircraft Spruce catalog number) with AN-4 fittings on the very top of the vent line run as it loops thru the fuselage. I will use an AN824 tee to splice into the vent line along this run and place the check valve at the 90 degree point of the AN tee, with the arrow pointing on the check valve "in" toward the line. I will do this only on one side (RH), just so I have the peace of mind that there is a back up automatic method of venting a tank other then the venerable externally exposed scupper.
I am no engineer, just another builder (91219). I think this is conservative and am using known aircraft quality components (Andair and AN fittings).
Once again, my sincerest appreciation for allowing me to discuss this with you all.
Click to expand...

If you cant warm the vent all the way out the tank, what is the purpose of trying to warm the wing root? w/i an inch or 2 of entering the tank, its quickly going to become the tank temperature. Guess if you never fly below freezing its no big deal. Im routinly in the teens, and have been as low as -25. Its going to take a lot of BTU's to overcome the the moving air in the fwd wing root. You would be surprised how much air movement is going on in there.

All the tees and check valves add complexity to a design that already has sufficient redundency. Probably not going to hurt. But a failed checkvalve could quickly be dumping fuel in your cockpit.

mud dobbers cloging your vent at SnF will heat that fuel. A failing checkvalve that you dont have a good way of dealing with at annual, will dump fuel into your cockpit.

Just a few thoughts for you.
 
You need to install the grommet. Leaving that out does nothing. I also think the check valve is not necessary. You are better off leaving the system as designed. Be sensitive to the symptoms of a plugged vent. As always any loss of power should always be followed by switching fuel tanks. Know the conditions that are conducive to icing. (temps <freezing and visible moisture). Routinely clear your vent lines by blowing through them (with the fuel caps off).

Here's a quote from the Arrow fuel system description posted above "The vents should be checked periodically to ensure they are not obstructed and will allow free flow of air". That should be done on our aircraft as well. Traditionally we just check for blockage at the inlet, however there could be water trapped upstream that may be the source of problems.
 
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