Fuel draining at uneven rates

[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.