What does it accomplish

T

TimDavie

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Jun 18, 2020
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So I haven't thought a lot about it but I really don't know what this does.
I'm not ever sure why the ram air vent is there since the tank has check-valves.
Cessna has the vent in addition to vented caps. Tanks Always draws unevenly seems like because of it.

I have theories but I know a lot of you will know for sure.
 

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Ha. Think wasps in the hangar would be a greater threat for crawling up there. Guess they knew what they were doing....
 
TimDavie said:
Ha. Think wasps in the hangar would be a greater threat for crawling up there. Guess they knew what they were doing....
Click to expand...

Maybe...

One of the things I dislike the most about a Citabria is the lack of a fuel tank selection valve. Any time I take a cross country in my 7KCAB the right tank gets a lot lower than the left tank. For example, the other day I started with full tanks and after 3.8 hours it took 21.4 gallons to refuel - 15.3 in the right tank and 6.1 in the left tank.

That's an artifact of the tank vent and associated plumbing.

As I understand the Citabria fuel system each fuel tank has two outlets, one near the front of the tank, and one near the rear of the tank. The two front lines run down the structural tubing below the front spar and meet at a tee above the shutoff valve. The two rear lines run back and down behind the cabin and tee together at the bottom of the airplane (which is why there needs to be a drain there as that;s where water from the rear of the tanks will eventually end up) and a single line runs from there forward to join the teed-together front lines. The fuel proceeds through that point through the shutoff valve and then to the gascolator.

The vent is located under the left wing, and is plumbed into the outboard end of the left tank. The inboard end of the left tank has a vent fitting that is connected across the top of the cockpit to the right tank with no check valve between. Consequently the right tank is vented to the left tank, and the left tank is vented outside. It all works more or less ok - as long as the fuel caps and their gaskets aren't leaking and/or have not been mistakenly replaced with vented caps. Both of those will screw up the tank pressures and cause uneven flow to even greater degrees.

You can check the vent lines by blowing through the vent line while having a helper listen at the tank filler holes with the caps on and with the caps off. You should hear air flowing when the caps are off. However, if you hear air flowing anywhere when blowing into the fuel vent with the caps on, you have a leak.

You also need to ensure that the uneven fuel in the tanks will cross flow and even out over night. That's because the rubber sections of the fuel lines that connect the aluminum fuel lines can get get soft and swell internally, restricting the hose enough that they'll expand and appear to function when you blow in the lines and pressurize them, but collapse when that pressure is removed and prevent the tanks from equalizing.

None of that is necessarily unique to the Citabria, the Cessna 140 parked next door to my Citabria has a single vent located behind the wing strut to keep bugs and water out and they can also have issues with unequal fuel flow.

Lots of aircraft have fuel systems that don't feed evenly.

Which is why I've always preferred aircraft with a fuel selector valve so I can choose where the fuel is feeding from.

In my case the system is functioning properly (or at least with no detectable flaws) and I'm still getting greater fuel flow from the right tank on the order of about 2.4 gallons per hour.
 
I don't understand the reasoning of having two outlets per tank. As you mention....front ones that T just before the shutoff valve, and the other rear ones that make the big rear-ward loop and join together.
 
the way I understand it, we all have the plumbing for inverted fuel. whether or not we have the header tank to complete the system depends on the individual plane but the plumbing is there from the tanks to make it possible.
 
Normally with two wing tanks you need two outlets per tank and a "both" position, tying all four to the engine. With a header tank you do not need a "both" -c.f. Super Cub.
Reason - unporting at unusual pitch or slip attitudes during minimum fuel operation.
 
A few thoughts:

1) As Bob indicates, you need fore and aft outlets in the tanks to ensure fuel flow in prolonged flight at high nose up or nose down attitudes at low fuel levels, and with left and right tanks, at least one of the inboard mounted ports should have fuel regardless of right or left bank (more on this later).

2) If you have separate fuel tank selection, like on a Supercub where the options are "right", "left", and "off", you need a header tank (or two) to ensure fuel will feed from either tank, while the aircraft is banked. The Supercub fuel tanks have a single outlet, however, the Supercub also has two small header tanks (about the size of a small propane torch bottle) for each wing tank, which ensures that is the "low" wing tank is selected it'll still feed fuel via the header tank for a few minutes. In addition, one header tank is located forward in the fuselage (fed from the left tank) and the other is located aft in the fuselage (fed from the right tank), which ensures that fuel flow continues in a climb or descent for at least a few minutes, even at low fuel tank levels that might unport the outlet for the tank.

It's not totally idiot proof however. I do recall making a sustained 45 degree banked left wing low turn with the left tank selected for 4 or 5 minutes until the header tank went dry and the engine quit. At 200 ft it was an attention getter. Rolling level got fuel back to the wing root in the left tank where it flowed into the header tank, fuel lines and engine in not much more time than it takes to describe it, and the engine started right back up.

3) A single header tanks also works. For example, in my 7KCAB there is a 1 1/2 gallon header tank under the instrument panel (only 3/4 gallon of that is available for inverted flight). Since it feeds from both tanks, if my right tank goes dry, the 1 1/2 gallons available in the header tank will provide enough fuel for about 7 minutes of operation in a left wing low turn even at full throttle, sea level, full rich (12.5 gph) operation.

It'll also work with a fuel selector valve, much like a Supercub. There's only one header tank but it's fed by both wings and by for and aft fuel pickups in each tank, If field approvals were not so hard to get in this area, I'd seriously consider installing a "left", "right", "both" and "off" fuel selector in my 7KCAB to allow me to run both tanks, or run the left or right tank to balance the fuel during a long cross country.
 
A coordinated turn should not unport either tank (pitch not considered). It is a slip that can get you in trouble.
 
I'm just thinking about it after reading your replies, the inverted fuel is strictly from the header tank, no? the plumbing to the wing tanks is what it is but there's no inverted capability at the tanks with or without the header since the wing tanks are down below the header when inverted.

i was under the impression the dual pick-ups at the tanks were related to inverted fuel, my bad.
 
Bartman said:
I'm just thinking about it after reading your replies, the inverted fuel is strictly from the header tank, no? the plumbing to the wing tanks is what it is but there's no inverted capability at the tanks with or without the header since the wing tanks are down below the header when inverted.

i was under the impression the dual pick-ups at the tanks were related to inverted fuel, my bad.
Click to expand...
Yes, the inverted fuel capability comes from the Bendix fuel injection and the header tank.
 
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