Fuel draining at uneven rates

[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.