Fuel tank vents. How big?

cardiffrob · Post by **cardiffrob** » Sun Aug 09, 2009 2:28 pm

My aircraft crashed just after take-off on the last flight (about 5 years ago and with a pevious owner) for an unstated reason. I've just noticed that there is no fuel tank vent and that the tank cap, as fitted, had no breather.

What size bore of vent tube is needed? I presume it doesn't need to be exactly the same as the fuel line due to the lower friction of air over the friction of fuel. I have space to drill the fuel filler neck and to run a rubber hose to below the wing to act as a fuel overflow drain and a breather that sits in an area of higher ambient pressure. Would a 2.5mm bore hose be enough?

ColinC · Post by **ColinC** » Sun Aug 09, 2009 8:55 pm

Rob,

I don't know how relevant it, is but I just checked the Taylorcraft wing tank cap which has a curved vent pipe in the centre. That is 1/4" od - approx 3/16" id.

tnowak · Post by **tnowak** » Mon Aug 10, 2009 8:17 am

Rob,

The Vagabond fuel vent (1/4" OD tube in tank cap) is curved 90 deg to face the airflow.
Tony

Dave Holl · Post by **Dave Holl** » Mon Aug 10, 2009 8:29 am

Gents
I thought that the curved pipe in the fuel cap pointing forward was for pressurising the fuel tank and a seperate pipe from the top area of the tank was the vent both the jodel and mk7 kitfox have this setup.
Dave

cardiffrob · Post by **cardiffrob** » Mon Aug 10, 2009 10:58 am

Doesn't a forwards-acing vent just ask to get bunged up by a bug or freezing rain being hit at high speed? I always though that vents went into an area of high pressure but off from the direction of flight. What is the general opinion? What has everyone else got?

cardiffrob · Post by **cardiffrob** » Tue Aug 11, 2009 9:33 am

I should have mentioned that the cap is also a fuel gauge sender unit so it is no longer possible to add a vent to the cap. It is a FRED and the cap and neck are from a Morris Minor but the cap is flat at the top and the sender has been bolted through this. There is enough neck to drill through for a small vent or to add a stub at the front.
There is a piece of windscreen washer sized plastic pipe already in place that might have once been part of a breather system but it was just hanging in space and there wasn't any connection to the tank.

Hmmmm.

Charles E Taylor · Post by **Charles E Taylor** » Thu Aug 20, 2009 7:18 pm

You should consider using 2 vents for some redundancy.

Too many failures have happened due to blocked vents, both Fuel Tanks and Static Ports have this problem.

Take Care.

Charlie

gasax · Post by **gasax** » Wed Aug 26, 2009 11:40 am

If you are going to get technical the density of air is about 770 times less than water so 500 odd times less than petrol. However viscostiy also has an effect so a simple area comparison whilst probably good enough is not particularly accurate.

A vent in the neck if lead to suitable area (where fuel can drain without gathering in anything or being blocked or sucked out) should be fine. Plastic washer piping is not the material to use!

The majority of vents are lead to either a wingtip or lower fuselage area.

At 100kts the dynamic pressure is less than 2% of the static pressure - so forward facing pipes are actually vents, the level of pressurisation can be matched by a person blowing....

cardiffrob · Post by **cardiffrob** » Thu Aug 27, 2009 8:07 am

It is a FRED with a wing arrangement like a Pietenpol. If it has only a stubby vent then it might overflow down the wing and drip into your face. The thin plastic pipe is there to direct any overflow to the side of the wing just above your head and to one side. It faces forwards and is in a high pressure area of the wing shape.
I guess I need a second stub ahead of the tank filler and pointing forwards. Not going to be eassy to get anything in.