Tip: Put more vent holes in the high part of the tank ribs!!!

Tip for all those building their own fuel tanks.

Put a good sized vent hole at the top of each internal rib of the tank at the high point. (Think of how your wing will be sitting when your on the ground getting fuel, this point will be different on the TD vs the Tri-Gear)

Otherwise you will spend many minutes/hours over the life of your plane waiting on the air to work its way out of the tanks so you can fill them all the way up...

I second that. Especially on taildraggers. Having just converted the 9A to a 9, it clearly takes longer to get the air out.

greg

Yes indeed.....wish I'd have thought of it when I had the tanks open.

I hadn't really thought about a fix to the "slow fueling" issue (probably because I didn't build my tanks), but it sure would be nice to come up to a solution to this little "nuisance". I almost always pump my own fuel, even when the fuel station is full serve - just so I know I am getting the tanks full.

Paul

FWIW,
Having had the slow filling tanks on various RV's, I put in larger holes at the lower and upper sections of the rib for fuel and air flow in my last set.
You cant find a fuel nozzle that flows faster than my tanks can fill now.
This is of particular concern cause I often have folks filling my plane when Im not present and I more often than not used to not get filled up. Could leave as much as 5gallons a side of air when I checked a top off. Not good.
That problem solved with holes at the bottom and top for flow.

I agree. Those with stock tanks need to always verify the "topoff" provided by the line guy. They almost always never get em full unless I warn them about it. Either that or they dump a ton of fuel out on the wing and ground by pumping too fast!

Sure wish I had known about this issue when I was elbow deep in pro-seal!

Maybe Van's fixed on recent kits?

So I saw this thread and of course had to go back and check my tank construction from about a year ago.. I was really kicking myself for not thinking about this during my build.. But it looks like there is a hole already in the stock ribs for this purpose? In the pics from the pointer below you can see that besides the vent line hole there is another 1/4 inch or so hole at what would be the top of the tank rib.

At least there is something there, but I suppose another hole, or larger hole would help speed things up..

http://www.704ch.com/2009/12/left-tank-proseal-it-is-as-bad-as-they.html

Fuel moves to the outside quick!

I did the same (larger holes at top and bottom of the ribs). No problem fueling, but there is a reason for those baffles (with smallish holes)! I do see the fuel level in one tank reduce a lot, due to it moving outward quickly, when not flying coordinated. That made me wonder what happens on a long slip. Will all the fuel move to the outside bays, leaving nothing for the engine to sip from?

So now, when I need to slip to loose altitude, I do it in the direction so that the fuel in the active tank does not move away to the outside bays.

Regards, Tonny.

Mine had those holes as well. They are not big enough to do the job. They eventually vent the air....if you wait. Something most line guys won't do.

I suppose the fix to the fuel sloshing at low levels in uncoordinated flight would be to put a little flapper door made from a couple pieces of scrap Al hinge material on the inside of the hole - basically a one-way valve that would let fuel in (toward the pickup) when fueling but keep it from going the other way too fast if you are slipping or otherwise uncoordinated.

And I see no reason why one couldn't enlarge the hole in the top (or add another), unless there is a structural issue.

cheers,
greg

It is possible to unport the fuel pickup when doing a slip with low fuel level even with the stock holes. Don't ask me how I know....

I now slip with the tank that I am using on the high side.

We are talking about two different issues here. Fuel can flow pretty freely from side to side in the stock configuration. Like Greg said above, some put in a flapper valve to stop the fuel from doing this. I did not think about that either until my tanks were built.

When filling, air gets trapped in the top area between the ribs and displaces fuel until it works its way out. This can be a slow process with the stock holes.

Pilottonny said:

I did the same (larger holes at top and bottom of the ribs). No problem fueling, but there is a reason for those baffles (with smallish holes)! I do see the fuel level in one tank reduce a lot, due to it moving outward quickly, when not flying coordinated. That made me wonder what happens on a long slip. Will all the fuel move to the outside bays, leaving nothing for the engine to sip from?

So now, when I need to slip to loose altitude, I do it in the direction so that the fuel in the active tank does not move away to the outside bays.

Regards, Tonny.

Click to expand...

For those who enlarged their holes: What size do you recommend?

As I recall, I used my 1" stepper bit and punched it through.
SO for me it was 1" diam hole.

Rats, this thread comes about 2 weeks too late for me. My left tank is sealed up and leak-free. Haven't started the right tank yet though - would you recommend doing this mod on the right tank only? Could be frustrating having one tank fill fast and the other slow though. Not sure what to do.

Advice??

Frustrating or not, I would do it to the second tank. This gets to be a pain after a few dozen fillups!

do it to your second tank:

then fill your slow one first, go fill the fast one, and if you really need the extra fuel go back to the the slow one

i did both the flapper and the bigger hole in all the ribs in my tank. I have no issues whatsover in flight or filling. Something well worth the effort to do.

oh and I drilled to the same size as the conduit for the wiring in the wing instead of 1/4 on the holes.

Thanks for the advice, I'll take a look at the ribs this evening. Can anyone think of a way to mod a completed tank by removing the access panel? I've already taken that off once to fix a leak, it's not so bad.

Oh, and would a flapper be all that necessary in any ribs besides the one that would seal off the inner-most bay? I mean, that's where the pick-up is and there should be plenty of fuel there to last through whatever sideslip you want to do, right? BTW, I'm building a 9A, not an aerobatic model, so maybe that makes a difference...

I guess I'm just too laid back.

I built my tanks per the plans over 18 years ago and have never had a problem filling them. Yeah, they might be slightly slower than my car tank, but I typically want to fill a little slower anyway to prevent overflowing.

How slow is slow?

I am ready to close my first tank. How long does it take to top off a tank with the factory holes? Wondering if I should be making this mod.

Mel said:

I built my tanks per the plans over 18 years ago and have never had a problem filling them.....

Click to expand...

My experience mirrors Mel's. I usually use self serve pumps to top off and when doing so, insert the fuel pump nozzle, pull the trigger, fill the tanks to near overflow, see a few burbs of air released, tease the trigger a bit and I'm done. No time is wasted and the tanks STAY completely filled. If the total time it takes me to fill the tanks (which seems perfectly reasonable to me) seems too long for others, perhaps my perception of time is different.

Bottom line is I do not perceive any problem filling the tanks the way Van's designed the fuel tanks and until reading this thread, never even knew I was supposed to have a slow filling problem with the plan's built fuel tanks fitted to the -6A or -8.

It's a matter of seconds.

DaAV8R said:

I am ready to close my first tank. How long does it take to top off a tank with the factory holes? Wondering if I should be making this mod.

Click to expand...

Don't be concerned about slow tanks because the reality of the situation is as Mel describes it. On my -10 and my previous -6A, as soon as the gas reaches the ring, stop. Within seconds, not minutes, it goes down a little and top it off.

Best,

Rick6a said:

My experience mirrors Mel's. I usually use self serve pumps to top off and when doing so, insert the fuel pump nozzle, pull the trigger, fill the tanks to near overflow, see a few burbs of air released, tease the trigger a bit and I'm done. No time is wasted and the tanks STAY completely filled. If the total time it takes me to fill the tanks (which seems perfectly reasonable to me) seems too long for others, perhaps my perception of time is different.

Bottom line is I do not perceive any problem filling the tanks the way Van's designed the fuel tanks and until reading this thread, never even knew I was supposed to have a slow filling problem with the plan's built fuel tanks fitted to the -6A or -8.

Click to expand...

And with my 96' kit, 6A...........I share the same view as Rick, Pierre, and Mel. It's never been a real problem. It took longer to fill my Honda motorcycle due to air.

L.Adamson --- RV6A

Building on

I think based upon the comments, I'll skip the larger holes. If it fills too slow, I'll just drill some holes in the top of the tank

DaAV8R said:

If it fills too slow, I'll just drill some holes in the top of the tank

Click to expand...

Just placard for "Positive G Aerobatics ONLY!"

Seriously, after owning/fueling a Grumman Yankee (dang tricky, those little tanks!) for more than 20 years, the RV is not a real issue for fueling. My standard fuel stop is 9 minutes, from engine stop to engine start at a self-serve pump. That often includes a waste-water-dump break....

Paul

I remember those vent holes as being very small (1/4 inch?), I wonder if some of the slow-fueling issues are because some of the holes might get partially blocked by proseal during construction. The holes are very near the rib flanges which would expose them to some accidental blockage.

John

The speed does not bother me much when I fill the plane. It is when others do that is most painful since I usually end up with about a gallon less per side than what they hold.

RVNineA said:

......I wonder if some of the slow-fueling issues are because some of the holes might get partially blocked by proseal during construction.......

Click to expand...

Until I reviewed this thread, I was not aware slow filling was an issue with some RV's. Some builders are of the school of thought that you can not apply too much proseal. In light of that misguided notion, I too wonder if the misapplication of proseal *may* be a cause of slow refueling some builders report. For those still building the fuel tanks, making certain those holes located just below the top flange of the internal fuel tank ribs remain open and unobstructed seems a good idea.

hmmm

Well I'm building my 8 tanks as I found this thread. I have looked very closely at the said problem, and I'm not sure holes need to be enlarged or added. Seems like a proseal issue could be a cause as stated before. Being from the old school (new term for me, age done snuck up on me) I tend to "not" remove metal from airframes unless there is a very very good reason, and I dont add unneeded weight to an airframe if at all possible. Of course needed weight is debatabe. Needs, to me are mission oriented. Well thats my 2cents on the matter.

bird

The problem is a very simple one.
Once the tank get past half full, air can not exit the bays at the rate that fuel can enter. In fact its not even close.

If you want to think of it in the reverse. Imagine trying to fill your tanks through that tiny vent hole. Thats what your trying to do.
You can only put in what the bay can vent. And dont forget its only that one vent hole. The other slightly larger hole is for the tank vent line. And some may be getting more venting at the nose ribs. Some may have prosealed em.

So look at that big hole in your fuel hose and under pressure, look at that large flow hole in the aft bottom tank, and look at the air vent hole. You should be scratching your head.

Some folks dont care.
Some folks do.
The fix for the ones that do is let more air out. Bigger air vent hole.

Kahuna said:

If you want to think of it in the reverse. Imagine trying to fill your tanks through that tiny vent hole. Thats what your trying to do.
You can only put in what the bay can vent. And dont forget its only that one vent hole. .

Click to expand...

Not entirely true. I would think that air will flow through a given hole much quicker than liquid fuel. Also, initially the air doesn't have to "find the hole" as the fuel must. Having said that, I would think that opening up the top air venting holes 50% or so and or adding one more could only help and not alter strength significantly.

my .02 worth.

Bevan
not flying yet, YMMV

You guys are making a mountain out of a mole hill.

My tanks are built per plans.
My holes are not pro-seal plugged and I have the pics to prove it.

Bottom line is unless you want to wait on the air to escape from the small vent holes on an RV7 TD when filling, enlarge the holes.

My experience is that 8 out of 10 line guys will not completely fill the tanks on a RV7 TD with tanks built per plans. Your results might vary.

If you don't care about either of the above then leave them alone and build on.....if your tanks are already built, live with it just like I am. There is no reason to tear your tank apart to make this mod.

Enlarging these holes will have zippo effect on the life of the tank (within reason of course). These tanks when put together with sealant are built like battle ships. If you have concerns, just ask Van's they will most likely tell you it is OK. After all, there are already much larger holes in these ribs by design, the aft bottom for example.

This is just my real world, recent experience building and flying and putting a ton of gas in a RV7 TD that is built 98% per the plans. I also watch another RV7 TD and a RV6A with quick build tanks deal with the same issues on a weekly basis.

Is this a deal breaking, heart stopping, what in the world are we going to do issue? Nope, just a nuisance that can be solved easily in the build phase thats all.

The REAL venting problem..

It is NOT the problem that air can not flow out through a 1/4" hole as fast as fuel can flow in through a 1" hole. It surely can -- IF there is air on both sides of the hole so the air can flow easily, and lower air pressure on the outboard side of each rib.

The actual problem with the venting is that fuel is trying to flow in through the same 1/4" hole! Now, the escaping air has to bubble its way through the fuel "blocking" the hole. Draw a sketch of the tank and ribs, at a dihedral angle, and you will see this. Each bay has an air bubble in its outboard corner, against the rib outboard of it, but the same rib is flooded in the next bay out. Same situation for each bay. The flooded bay is essentially raising the pressure at the hole (because of the 'head' of fuel above the hole), so it takes some slightly higher pressure in the air bubble to force its way through to escape.

Ever try to fill a small-neck bottle quickly under a faucet? At a certain flow rate, the flow will spray all over the place instead of going into the bottle, because of the airflow trying to get out of the bottle. The internal air builds up pressure until it burps out through the hole. Ever wonder how high-speed bottling machines deal with this? They put a straw in the hole, giving the air a path to escape while fluid goes in the rest of the hole. Its a really small straw, because without the back pressure, the air can flow really fast through a small hole.

A gallon of air can flow through a 1/4" hole WAY FASTER than a gallon of fuel can flow in through a a couple of 1" holes, as long as there is not a higher pressure on the other side of the hole.

So, larger vent holes help because they allow more space for fuel to flow in and still have some room for air to flow out, through the same hole, without building up any pressure "head". Another way to do it would be to put a tube through each vent hole, ending right close to the rib on the inboard side, and extending across to the outboard side of the next bay. This would surely be overkill, fixing a problem with a more complicated solution than is needed. But it would work really well.

My solution? I do my waste-water dump first, so I don't get impatient with the sound of running fuel into the tanks and start knocking my knees together. Without that added stress, I am happy to wait the extra 30 seconds it takes for the air to gurgle out.

You have it exactly right Steve in regards to why....

Just watch out when you are not the one refueling your plane.

Kahuna said:

....And dont forget its only that one vent hole....

Click to expand...

Kahuna,

I'm confused about your assertion that we are dealing with only ONE vent hole. I count two. In addition, as you can
see in the photos, even with a snap bushing in place, the generous size of the larger vent tube hole does provide
significant venting. Couple that hole with the TWO factory vent holes provided in each rib, I would think that properly
assembled and obstruction free, in the aggregrate those THREE areas are designed to provide sufficient venting.

Note in the first photo how the filet of proseal I layed down can (potentially) obstruct those vent holes. In any case,
I do not find slow filling to be a characteristic I am familiar with. Based upon my routine experience at the fuel pumps,
naturally, I have to suspect some builders who report slow filling fuel tanks may be inadvertently obstructing one or
both factory provided vent holes in one way or another.

Rick,

The one in the back is under several inches of fuel (see Steve's post above) in the condition where the venting starts to get slow. The one for the main tank vent is 90% blocked by the line and snap bushing and with surface tension and the like once it is under the fuel level will do little to help. The one in the middle is the one that I would enlarge if I was to do it again.

You guys can argue all you want. It is a proven fact that these tanks vent air slow during the last 1/3 of the fill if you build them per the plans.

This will be my last post on this subject. To each their own.