Fuel tank vents

[Old Koreelah]

I recently learned a useful lesson about venting fuel tanks; perhaps other can benefit.

 

A persistent fuel flow issue nearly prevented me getting to Ausfly.

 

During takeoff and climb to 5000' the engine would not miss a beat. After about ten minutes of level cruise flight it would run short of fuel and go into morse code. Bubbles were visible passing along the fuel line towards the carby. Fuel pressure would fluctuate wildly then drop to zero. Switching on the boost pump and swapping tanks did nothing to improve the intermittent engine running.

 

After a complete fuel system check, including pressure testing all lines and vents and pumping fuel from each tank for several minutes, I thought it must be OK. I hadn't found a definitive cause of the problem, but packed up and left for Ausfly.

 

After 20 minutes the bubbles appeared in the fuel lines and morse code started again. Sadly returned to home base and started packing aircraft on carrier. Local Ag pilot wanders over and hears the story.

 

His first suggestion was to check the location of the tank vents. (During a recent wing refurbishment, I had moved the vents from just on the top of the wingtips to just under- a distance of 40mm.) I rushed home and came back with tools and material, and added a small aluminium "hood" over each vent to deflect the tip vortice. Problem solved. Engine didn't miss a beat on the trip.

 

 

[Downunder]

So the vent was in a vaccum or just "disturbed"air?

 

 

[Old Koreelah]

So the vent was in a vaccum or just "disturbed"air?

Can't really say for sure. The vents are half way back along the straight wing tips. When they previously exited on the top there was no problem. When I exited the new vent tubes just around the edge, slightly under the wingtip, the problems began, but only after about twenty minutes of level flight. Presumably the vortex created a slightly lower pressure there than where they originally were on top- the opposite of what you would expect. Most tank vents face forward to add a little pressure to the tank and prevent fuel syphoning out. The matchbox-sized hoods I added face forward and seem to have the same effect.

 

 

[Marty_d]

Most tank vents face forward to add a little pressure to the tank and prevent fuel syphoning out.

That's interesting - the chapter on Savannah tank install shows a diagram of where the Rilsan tube from the cap vent goes through the wing, out the lower skin and the end appears cut on an angle so it faces aft.

 

 

[Old Koreelah]

If it works, don't change it. The problem is you can't see air and it sure moves in funny ways.

 

 

[Mick]

The problem is you can't see air and it sure moves in funny ways.

Do you mind if I use that next time my landings are less than perfect?

 

 

[Old Koreelah]

Do you mind if I use that next time my landings are less than perfect?

Copyright applies, fees will be paid. I'll make a motza!

 

 

[kgwilson]

I have been advised to vent my wing tanks under the wing with the vent tube cut at 45 deg facing into the airflow to provide positive pressure to the tank. I have seen a number done this way. It sounds logical to me but I have not researched the subject at all.

 

 

[Old Koreelah]

I have been advised to vent my wing tanks under the wing with the vent tube cut at 45 deg facing into the airflow to provide positive pressure to the tank. I have seen a number done this way. It sounds logical to me but I have not researched the subject at all.

I presume that vent tube comes out of the top of the tank before curving down under the wing.

I put a lot of thought into the vents for my wing tanks. If I crash and flip over they will trickle fuel onto the ground as far from me as possible. I used the largest diameter tube I could fit. During a crash impact my fibreglass tanks will cop a huge G-force and the large vent tubes should help dissipate the shock wave. They exit on the inner side of the tank so that fuel doesn't pour out during side-slips or if I park on a slope.

 

 

[kgwilson]

I presume that vent tube comes out of the top of the tank before curving down under the wing.

Yep. I wouldn't get too far if they came out the bottom.

 

 

[Kyle Communications]

That's interesting - the chapter on Savannah tank install shows a diagram of where the Rilsan tube from the cap vent goes through the wing, out the lower skin and the end appears cut on an angle so it faces aft.

No Marty the tubes are cut so they face forward

 

 

[Marty_d]

Fair enough. The pic in chapter 25 is a bit misleading then.

 

 

 

[Kyle Communications]

The line at the back is the main spar so the tube cut is actually facing forward....otherwise you get a suction which can pull the fuel out it needs to be forward into the air pressure

 

 

[Marty_d]

I agree with you... but the picture is wrong then. The main spar is shown on the left of the picture, and the hole the tube goes through is, as you say, aft of the main spar. But in the picture the tube is cut with the cut edge facing aft. They should have drawn the cut side on the forward side if that's the way the cut should be facing.

 

I think I'll use a bit of aluminium tube thru the top and bottom skins with a bend facing forwards at the bottom, and the flexible tube from the cap clamped on at the top.

 

 

[Kyle Communications]

about half of the Sav builders and owners fit alu tube off the top of the filler caps facing into the airstream the others followed the manual and put them through the wing to underneath but you still need to cut the tubes so they face the slipstream

 

 

[Marty_d]

I like the idea of putting it through the wing so any overflows don't go all over the paintwork. Kind of like this....

 

 

 

[Downunder]

I like the idea of putting it through the wing so any overflows don't go all over the paintwork. Kind of like this....[ATTACH=full]27835[/ATTACH]

I like that idea. If I fully fill my tanks and the weather warms up they do spill out the breathers staining the paint.

I have seen some aircraft with bad staining when not washed too often.

 

 

[Marty_d]

Guess another advantage (on a high wing anyway) - any untoward venting would be more visible if it came out underneath.

 

 

[Kyle Communications]

Marty what was the chapter from you showed the pic....it must be a 701 manual not a Sav manual...the English is too good :)

 

 

[Bruce Tuncks]

The fuel tank on my kit Jabiru came packed in polystyrene pillows, with one ot these ( I later found) stuck in the vent tube.

 

Well it passed all the flow tests but after a few weeks, the avgas fumes turned the polystyrene into a gummy mess which actually blocked the vent completely. I reckon this was a one in a thousand thing to happen.

 

I was flying when there was a loud bonk as the top of the fuel tank oilcanned in. The fuel pump sure was sucking good.

 

So I turned on the electric pump and came down as soon as I could.

 

Back on the ground, it was hard to get the filler cap off but when I did there was another bonk as the tank came good.

 

I've been careful of that vent ever since.

 

 

[Marty_d]

Marty what was the chapter from you showed the pic....it must be a 701 manual not a Sav manual...the English is too good :)

Nah, definitely the Sav - someone sent me chapter 25 in PDF format when I was looking for info to shoehorn the Sav tanks into the 701 wing.

 

 

[Dafydd Llewellyn]

I'm not familiar with the Savannah setup; but fuel tank venting is a more complex problem than most people realise. If you face the vent forward, to get some ram pressure on it (the amount you get at 100 knots is actually around 1/10 of a psi, which is pretty piffling given the minimum fuel pressure required by a Bing carbie), it's liable to get plugged by an insect - (or ice, if you were fool enough to get into icing conditions) unless you locate it behind something; take a look at the tank vents on a typical high-wing Cessna; they face forward, but they're located in the lee of the lift strut. Also, it's prudent to have more than a single vent; the Cessna setup has a vent on each wing, connected to the corresponding tank; but the tank airspaces are interconnected, to prevent siphoning if one vent has less pressure than the other - so one vent blocking will not spoil your day - that's cheap insurance, in my book. The vent system gets very complex - if it is to meet all the relevant requirements - if the engine can draw fuel from more than one tank at a time. The vent system plumbing needs to be arranged so it has no low spots in which water could freeze. It's worth while looking up FAR 23 on the subject.