Fuel draining at uneven rates

[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

[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?