Fuel draining at uneven rates

[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.

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

[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".