Fuel draining at uneven rates

[rosslr]

Hi all,

I know there has been a few posts on this over the years and I think I have read most of them. However, I have noticed that the right wing tank empties at a faster rate than the left tank. It shows sometimes over a 1/4 of a tank difference. I have had perfectly smooth conditions and kept everything level and balanced. So far I have done the following:

checked and straightened the vents on the filler caps
made sure the vent holes are clear
kept the ball centred and wings level
disconnected the hose from the left wing tank to the header tank to check for full and free flow


Is this normal? Any tips to check anything else would be appreciated.

cheers

ross

[jrevens]

Ross,
I'm sure that others with much more Kitfox experience than I have will chime in, but I believe that it is normal more often than not. All of the various forces involved when flying, including gravity, are factors. I think also, if you look at the fuel system, that the length of fuel line between the wing tanks and the header tank is longer from the left tank than from the right one. You may also have more turns or bends on the left one. This can result in a slightly greater pressure drop through that line. Anyway, that's one reason why I decided to add shut-off valves on those lines - to manage fuel flow, especially on long cross country trips. Additionally, if the airplane is parked on a surface that isn't level, fuel will flow from the higher tank to the lower one. In that case a valve can be shut off to prevent that. With those valves, I intend to make them an ALWAYS check before flight item.

[airlina]

Hi Ross, It is a bit disconcerting for new kitfox pilots to watch the fuel drain from one tank - you always wonder will that tank run dry or when does the other tank start feeding? I know that was my reaction when I started flying mine 12 years ago. Also walking around at OSH one year ,I saw fuel peeing out of one of the fuel tank vents on a freshly fueled kitfox. The plane was parked on a slope and the high side tank was pushing all his expensive fuel out of the low wing. So how to fix this-well John covered it pretty well, the fix for me was to install 2 inline fuel shutoff valves on both tank to header, fuel lines. This allows several things 1) inflight fuel balancing, especially helpful on cross country flying. Before the fuel valves were installed, I would fly uncoordinated to try to balance fuel loads, now I shut the appropriate valve and burn from the heavy tank. 2) Shut one of the valves after parking on uneven slopes so your kitfox doesn't pee while you are away. 3) maintenance- sometimes during maintenance work I need to isolate the wing tanks from the fuel system, and these valves allow this . Now for the cautions! Make sure the valves are large enough and do not restrict flow(good idea to redo fuel flow check at engine after installation). At my advanced age it is easy to forget I shut one or both valves, so whenever I touch one of them I have a big red flag (one of those remove before flight jobbies) that I clip on my throttle be it on the ground or inflight. Also to avoid inadvertent movement ,I made a simple locking device on each valve. Have fun Bruce N199CL

[Dorsal]

FWIW, I have also had this issue in my plane but believe it to be related to air bubbles. It only happened when I first started flying and/or after I clamped off the main line to drain the header tank (test low fuel warning). I would fly some sustained hard slips both sides and the system would stabilize.
Also do the quick check of parking the plane level, the fuel tanks should balance and the ball should be centered (you have probably already confirmed that the tanks and the ball agree in static conditions).
In my experience a half a ball of uncoordinated flight leads to visible differences in the tanks.
My planes requires slight pressure on the right peddle (like resting you foot on one peddle) to maintain balanced flight when fully loaded. If I don't do this or my tall sleeping son rests his foot on the left peddle I get uneven tanks.

[redbowen]

I have the same issue in my series 5. I found that the fuel line from the left tank, which is longer, had a sag in it right before going into the header tank. Removing this low point helped the issue. However, I usually fly solo and that puts the left wing low, so the right tank will always empty first. But check that fuel line. I attached a photo, hopefully you can see it.

[Dave S]

Bryan's comment about a sag in the fuel line is absolutely correct - the design of the fuel system requires that the fuel lines are running continuously downhill; and, the vent line is running continuously uphill to keep from capturing a bubble.

I found that the feed line from the left tank was prone to sagging, even if only a little. To help maintain a corrrect downslope in this area, I zip tied a couple 1/8" gas welding rods to the hose in this area between the cushion clamps.

Early on in testing, I found a little difference between the right and left side - my assumption is this was due to the fuel cap pitots pointing different directions other than 100% straight into the wind since the difference went away after bending the pitots. I believe this has multiple causes including different amounts of fluid friction between the left & right sides as has been mentioned.

It would be interesting to do some tuft testing near the fuel cap pitots to determine if "straight ahead" is actually straight with the air flow to know if there is wind flow off the windscreen and off the prop which might affect this - but I don't have the camera for that:mad:.

Sincerely,

Dave S

[jiott]

I have also had the same issue more or less since day 1, however it has never caused the engine to hiccup or stumble. When the tanks are low, sometimes one tank will show nothing in the sight tube and the other will show 1/4 tank or so. I quit worrying about it; just because one tank drains faster than the other does not mean that the slow tank is not draining at all. It still drains nearly as fast as the fast tank, which is plenty to keep the engine running at WOT.

I agree with Dorsal that 1/2 ball uncoordinated for a period of time will cause visible differences in draining rates. After I finally tweaked my rudder trim tab to fly ball centered hands and feet off, it noticeably helped balance the tank draining rates, although still nowhere near perfect.

I also think that the corkscrewing prop slipstream has a lot to do with the actual ram pressure applied to the fuel cap vent tubes. I bet if someone wanted to experiment with tweaking the angles of those vent tubes they could very nearly balance the draining rates. Bend the one on the slow tank more into the corkscrew slipstream, and bend the other away from the slipstream. Don't bother with trying various restrictor sizes in the vent tubes, because its ram pressure you are dealing with, not air flow rate.

[redbowen]

Originally I thought it had something to do with the header tank venting to the right tank only. Any thoughts on this causing the right tank to drain slower?

[rosslr]

Thanks for the replies guys. I think all the reflections/ideas you have made probably contribute to the situation. I did make sure that all the fuel lines are continuously down hill with no sag. And it appears to happen even in really smooth conditions and I concentrate on ball exactly in the middle and wings level. Jim, I tend to think that your thoughts about the slightly different airflow over wing near the fuel caps could be a contributor to this. That just might account for the reports I have read about some guys finding the left tank drains faster and some the right tank.(?) for me, it is the right tank that drains faster. I guess my main concern was whether or not the remaining fuel will feed the engine if one drains first ... you re-assure me Jim ...but I need to find out I guess with height above an airfield!

Thanks again for your thoughts - I am going to experiment with a little vent bending and will let you know the results!

cheers

ross

[Esser]

Are your hoses the exact same length? Maybe make the one supply line longer then the other to increase the friction losses...

[gregsgt]

When I flew a 172 the tanks would also drain at uneven rates when the fuel was selected at both.

When I flew a 210 you always would land on the tank that had more fuel.

I guess I don't understand what the concern is. Seems normal to me.

I know my kitfox has the header tank vented to the right side tank and that tank seems to empty quicker as well.

[jmodguy]

I flew RC for many years and some of the ducted fans had saddle tanks that fed a header. The setup of the tanks was critical so you didn't run one of the saddle tanks dry and cut your flight short. The rule of thumb was to make sure the lines that fed the tee to the header were as close as possible to the same length so the saddle tanks would drain evenly. Worked pretty well.

[n85ae]

It's the single vent line, I ignore the difference. It will run both tanks empty
before you fall out of the sky ... Takes a bit of getting used to, but unless
you bore a hole in the other tank and put dual vents in, good luck fixing it.

Jeff

N85AE, Series 5, IO-240B

[jrevens]

It's the single vent line, I ignore the difference. It will run both tanks empty
before you fall out of the sky ... Takes a bit of getting used to, but unless
you bore a hole in the other tank and put dual vents in, good luck fixing it.

Jeff

N85AE, Series 5, IO-240B

If you say so, Jeff, but I'm not sure that line can have much to do with it. I guess that when the fuel level in the wing tank is above the level of the vent line fitting, the effective line size between the wing tank & the header tank is increased by the addition of the 1/4" vent line. However, when the fuel is below that point in the wing tank, it should make no difference. Both wing tanks already have more than adequate, individual pressurized vents. Am I missing something? I could be wrong of course - it's been known to happen on a regular basis.

[SkyPirate]

Mine seems to drain evenly, both tanks usually read the same, unless I let my co pilot take the controls and not tell them to mind the horizon, keep the wings level, its pretty easy to fly with 1 wing low and not notice it, and I always check the static tubes on the gas caps, if any blockage occurs creating a vacuum in the tank could be disastrerous , seeing fuel in the tank but starving the engine..

[n85ae]

It's all about hydraulics. What you say makes sense, but it's not correct.
There are several issues with the design which cause the problem. One,
the header tank being offset to the CP side of the aircraft - This creates
different length feed tubing. (Fluid flow is determined by viscosity of fluid,
diameter, and length of tubing). Next the header vent, is the second
contributor.

Anyway, that might not be a good explanation, but if a person were
really determined to understand what is happening, the key is in the word
"hydraulics". :)

Regards,
Jeff



If you say so, Jeff, but I'm not sure that line can have much to do with it. I guess that when the fuel level in the wing tank is above the level of the vent line fitting, the effective line size between the wing tank & the header tank is increased by the addition of the 1/4" vent line. However, when the fuel is below that point in the wing tank, it should make no difference. Both wing tanks already have more than adequate, individual pressurized vents. Am I missing something? I could be wrong of course - it's been known to happen on a regular basis.

[happiestflying]

So glad I found this thread. Same issue on my IV Speedster; am reassured by the "you won't fall out of the sky until both are empty" comments, but will probably look into equalizing feed line lengths and shutoff valves too.

I wear a belt AND suspenders.

[jiott]

I agree that "hydraulics" is the reason for this, however if you run a few numbers you will find that at cruise conditions of say 4 gal/hr the fuel flow rate thru each hose feeding the header tank is only 0.033 gal/min. With a 5/16" ID hose this flow rate is far too small to have any measurable frictional difference due to a length difference of about a foot. The difference in head of an inch or two will have more effect and will tend to cancel out the flow rate difference. Greater effect is caused by a difference in ram pressure on the cap vent tubes.

The key here to eliminate worries is that the slowest draining tank is still capable of draining at a rate well above what is need to keep the engine running at WOT. Also you don't need to worry about the engine sucking air even if one tank goes dry, because you have a 1 gallon header tank at the tee which always stays full (fuel to the engine is pulled off the bottom); this is unlike the RC plane mentioned with dual tanks and the lines teed together with no header tank-in this case the engine could possibly suck air bubbles if one tank went dry.

Sorry for the long dissertation, but I like this stuff since my pre-retirement background is "hydraulics".

[jrevens]

I like a good technical discussion... and I really appreciate your experience and knowledge of this subject, Jim. Would you agree with my assertion that the header tank vent line has absolutely nothing to do with this issue for all practical purposes?

[HighWing]

The key here to eliminate worries is that the slowest draining tank is still capable of draining at a rate well above what is need to keep the engine running at WOT.

A story. On one of our annual Idaho Back Country flights, a couple of guys from the SF Bay Area joined us mid week in Idaho. One of the guys flew a Model IV - 582 powered and the other guy flew a Series V powered by a Franklin - I suspect his first real cross country. When we met up, I think in Elk City, the Franklin guy was scared (You know what) flying all over these mountains - worried by the topic of this thread. With eight airplanes we had enough tools to trouble shoot his airplane, to totally disassembling his fuel system, checking each component and doing independent fuel flow tests on the wing tanks - Nothing amiss, Nada!

The worries were eliminated but at the cost of a half day flying. This phenomenon has been going on forever. Some guys put in valves to see things even. I like simplicity and just go with the flow. With that in mind, coming home from Idaho once, I decided to eliminate the fuel stop at Winnemucca, NV due to pretty good tail winds. Descending from the Sierra crossing, I had a chance to check my low fuel indicator system as I had the original aft ported fuel tanks and a long descent would unport the tanks interrupting fuel flow to the header tank. Yes, the low fuel system worked fine and at 4 gallons per tank all was even on both sides.

[rosslr]

Like John, I appreciate this discussion so thanks to you all for your contributions. If I understand it correctly, there would appear to be no downside to making the feed hoses from each tank the same - even if the difference in lengths is a minimal contributor to uneven flow. The other challenge appears to be getting the cap vent angles correct on both sides so they are receiving even pressure from their angle of meeting the airflow. I imagine that the offsets would be minimal so will experiment a little with that.

Again thanks guys.

ross

[Floog]

All things being equal, atmospheric pressure and gravity work on this simple fuel system. Just fly straight and level. Fuel seeking it's own level is going to have more effect than ram air in those small vent tubes in my humble and non scientific opinion. The sky is falling all over Washington state today so I'm doing this instead of flying.

[jiott]

It will be interesting if Ross experiments with vent tube angles. I have never heard of anyone doing this before. One idea I had was to put a short length of some soft bendable tubing like copper over the end of the cap vent steel tubes (tight fit with good seal) to be able to try many different angles without work hardening the steel tubes. I might try this myself one of these days, just to try to verify the cause, not because of any worries.

[jiott]

Yes John, I tend to agree that the header tank vent tube has nothing to do with this issue. It is simply a path for any air bubbles trapped in the header tank to work their way up out of the system without having to try to go upwards against the flow of fuel in the main hoses. This vent tube is nearly always unported unless you have the tanks topped off full. The tube remains full to the level of fuel in the right tank, but there is zero flow rate in it when it is unported, thus allowing air bubbles to rise.

This fuel system is actually quite sophisticated IMHO, it has been developed and tweaked over the years to what it is today as I understand it. At times I wish for dual shutoff valves for maintenance purposes only; like recently when doing my annual I decided to replace a piece of vent tubing near the header tank. I had to go thru the process of completely draining both wing tanks before I could open up a fitting at the header tank-how nice it would be to only have to shut of a couple of valves. I know I could have clamped off the two hoses, but I have a personal aversion to tightly clamping these critical hoses-just me. But this kind of maintenance rarely occurs and I rather keep the system simple-just me.

[PapuaPilot]

Uneven fuel burn is a very common problem on planes that can burn off 2 tanks. Once you figure out which tank is going down quicker you can fly with the ball slightly out to that side. Often 1/4 ball is enough to even the burn or even transfer fuel to the other side. It is much easier to do on planes that have rudder trim so you don't have to hold the rudder.

This worked with the Caravans and Kodiaks that I flew.

[n85ae]

Argh, it's not complicated.

It's hydraulics, and the vent has much to do with it, and so does the
different length tubing to the CP side offset header.

The biggest mistake most people are making, is thinking of this system as
a liquid level system which it is not (in flight). It's an alive hydraulic system
with fluid flow.

The only way to balance it is - One install vent to both tanks, AND/OR
increase the size of the fuel feed tubing on the longer length feed line
This typically the pilot, side from tank to header. OR lengthen the CP
side feed to match the flow rate of the pilot side.

Super simple explanation - Fuel will take the path of least resistance -
The shorter feed from the CP side, with the double lines (feed and vent).
this will cause the CP side tank to drain faster. At some point the head
pressure (the weight of the fuel in the fuller tank) will balance against
this. Which is why when you approach zero, both tanks empty at the same time.

However when you sit on the ground, there is no fuel flow, so what
happens? The fuel cross feeds, and levels itself.

Jeff

[rosslr]

Thanks for all the posts guys - I have learned a lot - yes, really Jeff!

cheers

r

[jrevens]

Argh, it's not complicated.

It's hydraulics, and the vent has much to do with it, and so does the
different length tubing to the CP side offset header...

...Super simple explanation - Fuel will take the path of least resistance -
The shorter feed from the CP side, with the double lines (feed and vent).
this will cause the CP side tank to drain faster...
.
Jeff

Jeff,

Not to beat a dead horse, but with all due respect you are not correct. I'm know that Jim (jiott) can defend himself, but I would just like to repeat a couple of points...

First of all, Jim (jiott) explained that the different lengths of the feed lines on the left & right tanks and the ID size and flow rates that we are talking about, are a non-issue for all practical purposes - no measurable frictional difference. I believe that Jim is a retired engineer who dealt with hydraulic design for a living.

Secondly, Jim & I both explained that the vent line does not and can not feed any fuel to the header tank when the level in the wing tank is below the level where the vent line connects to the wing tank (which is up at the very top of the tank of course). The feed and vent lines are not a set of "double lines" that are both feeding the header tank.

[kitfox2009]

Wow! This has turned into quite an discussion.
Put me down as being in the "fly straight and level" camp. This seems to pretty much solve the problem (if there ever was one) in my Vixen.
Cheers Don

[PapuaPilot]

It really is that simple. Fly straight and level, if that doesn't give an even burn then kick the ball out just a little to one side to compensate.

Also, make sure your ball is perfectly centered when the plane is on a level surface.

[n85ae]

Haha, I'm not wrong. But I have better things to do than argue about this.

Regards,
Jeff



Jeff,

Not to beat a dead horse, but with all due respect you are not correct. I'm know that Jim (jiott) can defend himself, but I would just like to repeat a couple of points...

First of all, Jim (jiott) explained that the different lengths of the feed lines on the left & right tanks and the ID size and flow rates that we are talking about, are a non-issue for all practical purposes - no measurable frictional difference. I believe that Jim is a retired engineer who dealt with hydraulic design for a living.

Secondly, Jim & I both explained that the vent line does not and can not feed any fuel to the header tank when the level in the wing tank is below the level where the vent line connects to the wing tank (which is up at the very top of the tank of course). The feed and vent lines are not a set of "double lines" that are both feeding the header tank.

[Danzer1]

The only way the header vent could have an impact - is when it is releasing air from the header to the tank - and even then it is only going to be the same volume as the air released to the tank - negligible!

The amount of friction loss difference between such a minor amount of tubing length is also negligible. Measure and drain both tanks through those lines and I'd bet you the difference might not even be measurable! 3/8" pipe friction loss for 1 foot of pipe at 5gph would be less than .002 feet of head.

The big difference would be the relative angle of the airflow into the tank vent inlets. My suspicion is; in flight with prop wash - the air angle on each tube is not dead ahead. Only way to check is to tuft both vent areas and observe/video from above in flight - static would only give an approximation.

Greg

[HighWing]

At some point the head
pressure (the weight of the fuel in the fuller tank) will balance against
this. Which is why when you approach zero, both tanks empty at the same time.
Jeff

Who knows why, but Jeff has it spot on in the above quote. Thinking, I could go down the list of my flying buddies over the years who have in some cases thousands of hours in their various Kitfoxes - None on this forum to my knowledge. In my mind's eye, I can see the look on their faces if they were to read this discussion. This has all been argued before - many times - and I guess it will be argued again, because there is no answer that will appeal to everyone. And perchance there was an answer, I seriously doubt it could ensure even fuel flow in every Kitfox built thereafter.

[n85ae]

5 feet of 3/8" tubing with a 3 ft. drop flows about 2.9 gpm, whereas a 2 ft.
shorter section flows 2.2 gpm. Take into consideration the pressure
differential between the tanks, and the impact the header vent line has
on this.

Yes there are VERY real reasons there's an imbalance. It's because it's a
dynamic system. It's not so simple as the plumbers among us would
believe.

The Earth is flat also, I have flown over to the edge and looked. I guess
not all of us can be engineers and do math. In any case, it's better to use
heresay, stake burning, and crucifixions. :)

Jeff

[Danzer1]

At some point the head
pressure (the weight of the fuel in the fuller tank) will balance against
this. Which is why when you approach zero, both tanks empty at the same time.

Yes it would, but the question is, why is one fuller in the 1st place and how to equalize it.


5 feet of 3/8" tubing with a 3 ft. drop flows about 2.9 gpm, whereas a 2 ft.
shorter section flows 2.2 gpm. Take into consideration the pressure
differential between the tanks, and the impact the header vent line has
on this.

I don't know where you get your numbers from, but they are way off. Could be you are reading Pd/100 feet of pipe and not correcting for that.

Also, these systems do not flow anywhere near 2 to 3 gpm - 2 to 3 gpH would be much closer. The Pd at 3 gph would be negligible. Pd is exponential to flow rate not linear = higher Pd with higher flow.

There should be no pressure difference in the tanks if they start out at an equal level.

Maybe the answer is - people making more left hand patterns instead of right hand? :)


It's because it's a
dynamic system. It's not so simple as the plumbers among us would
believe.

Hydraulics is hydraulics - doesn't matter the application, just the configuration of the system type. 20+ years owner of mechanical engineering firm specializing in hydraulic design. Thanks Jeff! BTW, plumbers work on toilets.:D

Greg

[n85ae]

Yes I calculated the rates incorrectly, but the principal is the same.

Actually, the REAL problem with this discussion, like many. Is it centers
around shooting down the other guys idea. ALL of the planes have the same
problem, and while I'm not an expert, I think my reasoning is sound as to the
reason.

I would like to see somebody propose an idea that isn't just shooting down
my ideas, or isn't that everybody has mounted there turn coordinator out of
level?

Clearly if somebody did 20 years worth of mechanical/hydraulic engineering,
they ought to be able to do the calculation that explains it :) Or maybe
it's more in the area of fluid dynamics (i.e. too hard)?

[Danzer1]

Yes I calculated the rates incorrectly, but the principal is the same.

So off by a factor of 60 (gpm - gph)? or factor of 50 5'-3'=2'/100? or both? plus the exponential difference of Pd between flow rates and length = negligible!

Greg

[n85ae]

Should we start exchanging flaming messages, until the list admin locks the
thread or bans us? That's the usual resolution to thread argument isn't it? :)


So off by a factor of 60 (gpm - gph)? or factor of 50 5'-3'=2'/100? or both? plus the exponential difference of Pd between flow rates and length = negligible!

Greg

[jiott]

I am really enjoying all this. We are all just putting forth our differing ideas on a very interesting subject, and no one here except n85ae seems to be taking any disagreement personally, certainly not me.

I still stand behind everything I said, and I did propose an idea to do some testing with the cap vent tubes at various angles into the wind.

N85ae's theory is correct (except for the "double feed line") in my view, but the problem is in the ASSUMPTION that frictional pressure drop effects are much larger than reality. I believe that ram pressure in the tanks and side forces due to uncoordinated flight or one wing being low have a much greater effect on this issue. Let's hear from some who have actually tried equalizing feed hose lengths-did it make any difference? I also believe there have been a few who have added a vent line to the left tank-did it make any difference? I propose another test idea: simply clamp off the header tank vent tube and fly for an hour or so to see if that vent tube has any effect on this issue (I don't believe there would be any problem in doing this other that maybe trapping a few air bubbles in the top of the header tank).

Great fun! Let's just try to be engineers and toss around technical ideas. There are no winners or losers, just possibly some solutions to a non-problem. At least it makes us all think and learn more about our airplanes.

[mr bill]

Amen, and to all--a good night

[rosslr]

Well, just woke up over here in OZ and see you guys have been busy over night( your day!). As the person who re-ignited this thread, I am appreciative of all your inputs as usual - from the specialist engineers, the experienced kitfox flyers and the the novices (myself). I am not an engineer. But I work with engineers in getting to build collaborative ecosystems with other disciplines. Mixing it up with diverse viewpoints - not only fun but it usually yields outcomes nobody had in mind when we started - as long as people listen, clarify and challenge ... respectfully.

I have already taken a punt on the vent angles but waiting for a nice calm spot of weather to keep the other variables equal. As for the hose lengths - I am thinking of evening them but will have to work out a continual downhill run for the right tank. I get the unbalanced flying contribution but I am still interested in achieving an even draw down, other things being equal. I hear what some of you say about it not being an issue and that the left tank will still fed the engine if the right is dry but try telling that to my hawk-eye flying partner! I will be landing to refuel or flying with a increasingly nervous punter in the right seat!

I was actually wondering if it would be possible to design a vent as a weather cock so that it automatically points the vent into the prevailing airflow? Silly idea?

cheers

ross

[Av8r3400]

Solution:

I put a valve on each side. If one flows more than the other I close the valve. Easy peasy.


Seriously, I don't know a Kitfox or Avid that doesn't flow one over the other. Some are "left tanked" some ore "right tanked". They all do it.

Like was stated earlier, to fix the asymmetrical flow is as easy as stepping on a rudder peddle.

[redbowen]

So I investigated the fuel vent angle and was surprised to see the left hand tank pointing noticeably towards the wing tip. This is the tank that always drains last.
First picture
So I corrected the angle and will give an update on the flight test when it stops raining. Second picture.

[Dick B in KY]

I have flown wing high, wing low, ball out of center both ways and have yet to run out of fuel due to one tank being lower than the other. If it's that much of a concern, stop and refuel.

Dick B

[Danzer1]

Should we start exchanging flaming messages, until the list admin locks the
thread or bans us? That's the usual resolution to thread argument isn't it? :)I see no need nor would it be appropriate, it is a healthy topic of discussion - that's all.

There is a pretty easy test to determine the friction drop difference between a 3' and 5' length of 3/8" hose in an open system. If anyone has the spare time to bother - here goes:

Scenario:

Tanks have equal head pressure at start of "flight" and are filled equally.

We are talking about an "open" (vented) system without a return or pump.

Assume engine burn at 6 gph or 3 gph per tank and hose. (You can't flow more than the engine burns) and 6 gph is an easy number to use for this purpose.

Test:

Take an open reservoir larger than 1 gallon and hang it up high with a 3' length of 3/8" hose on the outlet.

Put 2 valves on the hose.

1 to start - stop flow for timing purposes.

1 to meter the flow to achieve filling a measured container with 1 gallon of fluid (water is close enough) in 20 minutes (3 gph flow rate). Might take a few times to get it right at 20 minutes.

Now take the valves and put them on a 5' length and repeat the test - NOT moving the position of the metering valve from the 1st test (3gph per 3 feet).

Now time how long it takes to fill that same 1 gallon container with the 5' hose.

I'd venture to guess, it will take about 10 seconds longer. Or .025 of a gallon per hour = 6.4 tablespoons per hour.

Have fun and thanks for flying Danzer Air!:)

[jrevens]

I think this discussion from my perspective has gotten a little confused. Engine stoppage because of one tank running out before the other isn't, and never was my concern. I also understand clearly that there are enough variables involved that it is almost a sure thing that most will have uneven flow rates comparing one tank to the other - I've always known that. In my mind we were discussing what does and what does not affect uneven draining of fuel from the wing tanks.

I think I just like to be able to manage the fuel system if I want to, whether to service something downstream of the tanks, or to avoid having to fly in an uncoordinated condition for long periods of time to even things out if I want to. I like to have a nicely balanced airplane - might not be any big deal in a Kitfox, but still desirable to me. If you have roll trim, or a wing leveler, again - no big deal on a long cross-country. My "other" airplane is beautifully balanced, and I can actually make it roll or pitch by just moving my head when trimmed-out in cruise. I like to keep the weight distributed as evenly as possible, so when I'm by myself I guess it might actually be good to have more weight/fuel in the right tank than the left one of a Kitfox. Again, I just like to be able to manage it that way (with valves)... to each his own. I really appreciate all the comments - it's good to learn about these airplanes vicariously through the experiences of others more experienced with them.

One other thing - I wouldn't have probably even thought of valves initially if it weren't for my good friend Stan Specht, who recommended them highly to me. Stan has flown his 80 hp Mod. IV Speedster well over 2000 hours to every State in the union but Hawaii. He's also flown to almost every Province in Canada, and we'll most likely get to those couple he missed when we make our Alaska flight in a year or so. He's flown into every public & many private airports in Colorado. He knows all about flying a Kitfox cross country. I really respect that guy, and he's the reason I decided to build a Kitfox (well, that and the fact that my dad had an attraction to them before he passed away in 2000). I'm gonna' love this little airplane so much!

[HighWing]

I have already taken a punt on the vent angles but waiting for a nice calm spot of weather to keep the other variables equal. As for the hose lengths - I am thinking of evening them but will have to work out a continual downhill run for the right tank. I get the unbalanced flying contribution but I am still interested in achieving an even draw down, other things being equal. I hear what some of you say about it not being an issue and that the left tank will still fed the engine if the right is dry but try telling that to my hawk-eye flying partner! I will be landing to refuel or flying with a increasingly nervous punter in the right seat!
cheers

ross

A couple of things - maybe three on the quote above. I do understand about Hawk-Eye. I have one of those too, but now she just watches from the ground. But, the low tank will never fully empty before the fuller tank begins to flow. It is a matter of head pressure as Jeff mentioned. Regarding the fuel routing. Frankly, I never liked the down the cross tube behind pilot and passenger heads routing. I run mine immediately aft above the quarter window, then a gentle bend brings it forward again to the header tank. I like that routing for two reasons. 1. the fuel line is now where it is close to impossible to be stressed or damaged from loading the baggage sack and 2. I place my glass fuel filters above the quarter windows so I can monitor actual fuel flow from time to time. On my inflight low fuel indicator test, I saw on both sides the big bubble form in the filters. I am wondering if that routing would create opportunities for lengthening the right feed line while maintaining the desired continuous downward flow.

One more thought not related to the above quote. Valves in the lines would most certainly cure the flow imbalance if monitored regularly. I chose to go with what nature provides primarily, because, I don't want a sputtering engine to remind me I closed the low tank valve and forgot to open the valve to what was previously the low tank. Most accidents are caused by pilot error, but we all know that.

[redbowen]

For those with Fuel valves on the feed lines to the header tank, there is a service bulletin #29 from Skystar that suggests not having those valves. I'm sure you all have found them safe, but here is the bulletin.




December 15, 1992

SERVICE BULLETIN #29

SUBJECT: HEADER TANK AND FUEL LINE ROUTING FOLLOW-UP

(Follow-up to Service letter #22) MANDATORY

TO: ALL Kitfox™ BUILDERS WITH HEADER TANKS

FROM: SKYSTAR AIRCRAFT

Service Letter #22 was an advisory to all Kitfox™ owners who utilize a header tank in conjunction with one or more wing tanks. It dealt with the possibility of fuel not flowing to the header tank from the wing tank if the wing tank fuel pick up is un-ported, causing air to enter the line. Then fuel flow stoppage to the header tank, can cause the header tank to drain; resulting in fuel starvation to the engine.

This Service Bulletin will deal with specific recommendations and instructions on how to deal with this potential problem. The overall object, is to have the fuel flow from the wing tank to a rear mount header tank and then forward, to the firewall with the greatest amount of droppage as possible (gravity feed with no high point) . It has also been determined that deleting the individual fuel valves from each wing tank, helps prevent, air entrapment in the fuel lines.

RECOMMENDATIONS

(1) : All front mount header tank systems must be removed and a rear mount header tank must be installed. (SkyStar Aircraft is offering a new, rotationally molded header tank for easier installation of a retrofit rear mounting position, at a special reduced price for this Bulletin.)

(2) : Each wing tank fuel valve must be removed from the headrack.

(3) : The fuel line routing, should flow directly from the wing tank to a rear mounted header tank. From the header tank, fuel line routing should travel down to the bottom of the door frame, forward (under the door frame) to the firewall, across the firewall to the shut-off valve.

NOTE: For those operating wing tanks in conjunction with the large, front main tank, the wing tank fuel lines should route:

forward from the wing tank, above the door frame work, down the diagonal bracing in the windshield area, and plumbed directly into the filler neck of the main tank. The wing tank fuel valve can be installed in the headrack above the door. Because of the capacity and fuel visibility in the main tank, the question of fuel flow stoppage from the wing tanks is not as critical. With the fuel routed into the filler neck, you will have a constant visual on all fuel.

Enclosed, you will find in-depth instructions for the mounting and retro-fit mounting of the rear mounted header tank. You will note that some of these instructions are in the form of revised manual pages. Those who have the new format construction manual should exchange these pages for those in their manual. The new pages are marked “Revision” 1.

If you have questions regarding this Bulletin or procedures, please feel free to call our Technical Support Department. The Header Tank Replacement Kit, front mount to rear mounted header tank, is part number #69141 and will normally retail for $119.99. Those ordering in response to this Bulletin will be able to purchase this kit (#69141) for the price of $76.00. Those aircraft which presently utilize a rear mounted header tank, may use there existing header tank but need to modify the routing, extra #47000 Fuel Line for this modification and the Replacement Kit can be ordered from the Customer Service Department. Phone: (208) 466-1711 Fax: (208) 466-7194

Rear Mount Header Tank (#44071) Installation

Preface

Read each step carefully to determine the specific application to your installation. Some steps may not, in there entirety, deal with your application; but, may contain points that you should be aware of and act upon.

1. Drain all fuel from the wing tanks and header tank. Try to eliminate fumes and ““ground”” the aircraft.

2. Remove baggage sack and seat back cover.

3. Remove the existing fuel line and #29001 Wing Tank Fuel Shut Of f Valve from each wing tank to the front rear mounted header tank. (For existing Rear Mount Modification, skip to Step 9.)

4. Remove the existing aluminum #65032 Front Header Tank from the firewall bulkhead.

5. Attach the #44071 Header Tank (new rotationally molded) to fuselage diagonal brace tubing, above and behind the control system. as per Fig CFO-4b.

6. Install #94002 Elbow Fittings in the #44071 Header Tank inlets and outlet. For single wing tank aircraft us a #94005 Plastic Plug in place of the #94002 Elbow. Use a good grade thread sealant on all threads. Be sure to use a wrench on the hex portion of the brass adapter bushing, to prevent its turning when tightening the Elbow. Orient the barbed, inlet Elbows outboard and outlet Elbow to the left. Also install a #94000 Vent Elbow using above procedure and orient the barbed end to the right. See Fig. CFO-4b.

7. Install the #44000 Rubber Fuel Line Hose between the Wing Tank Outlet and the Header tank. Route the fuel lines to insure a continuous drop, from the Wing Tank to the Header Tank. Secure these lines to the fuselage structure with tie wraps. Make sure the Hose is long enough to allow folding of the wings. Route the Hose under the headrack carry through brace as shown in Fig WO-3-l. (For those with an existing, Aluminum Rear Mount Header Tank, slip the hose over the inlet nipple and secure with a hose clamp.)

8. If not previously accomplished, install a return vent

[kmach]

Allot has changed since 1992 !

I for one , will not be removing my valves between my wingtanks and header. They are too useful , for inflight fuel management , uneven ground parked situations to prevent fuel transferring and dumping out of the low tank and for maintenance.

[happiestflying]

I too am having this problem.

My longest cross-country was bringing the plane home to Washington from Colorado, during which flight I had uneven flow but lots of fuel left in reserve, even after the longest leg which was 5 hours and 10 minutes. That was two years ago.

It seems that the problem is getting worse, so even though I muck around with full tanks, when I get home the left wing carries substantially more fuel than the right, and I don't have the courage to run them down to the point where the right wing shows nothing, just in case the opinions here are in error. Would rather just find and fix the issue, so will try the suggestions here and report back. Have tried the ball-off-center suggestions, but haven't yet re-aligned the gas cap pitot.

[Dusty]

This may have been mentioned before but check the fuel cap seals,
Yes I found out the hard way.:eek:

[rosslr]

Jeremy, I was posting the same Q's as you about 12mths ago. While I certainly think it is prudent to eliminate all possible causes such as the suggestions here, I found that one tank will go down to just under the 1/4 full mark and then they start evening out - just as one of the wise old soles on this forum suggested. They way I found out was not on a long cross country but close to home - very close to home and just went up high and flew around for about 3/4 hrs - all within gliding distance. I am now confident of how it will even out , although I have to admit, it does keep you vigilant watching the gauges!
And at the recent first annual, I removed the finger strainers in the tanks just to satisfy my curiosity about how clean or blocked they were ( they were clean, but now I don't fly along wondering!!)

Best of luck

ross

[happiestflying]

Ross, and Paul, thanks for the suggestions. I'll give those a try on the next flight(s), but perhaps I'll do some engine-out practise first, which is of course something I should be doing on a much more regular basis. Another great excuse to fly, not that I need one. And yes, you guys are more than half the world away, and I have no idea why I'm replying in the middle of the night.

[jiott]

I agree with Ross, they tend to even out at 1/4 or less, although I have run several times to less than 1/4 where the other tank shows zero and have never had an engine hiccup; I totally quit worrying about it.

I still believe the cause is flying with the ball somewhat off center, as well as slight differences in ram pressure in the cap vent tubes. I am still waiting for someone (maybe I will do it myself) to test various vent tube angles off of straight ahead to see what effect is has on fuel tank drain rates. I believe the testing could be easily done by slipping on a tight-fitting copper tube extension on the cap vent tube. Then bend the soft copper to various angles to test.

[n85ae]

Actually it's caused by Coriolis, if you fly in Australia the other tank drains faster ...

I never worry about it any more, mine never runs out of fuel because I
drink too much coffee.

Jeff

[airlina]

Actually it's caused by Coriolis, if you fly in Australia the other tank drains faster ...

I never worry about it any more, mine never runs out of fuel because I
drink too much coffee.

Jeff

The perfect solution Jeff!