tank cap

[efil 01 74]

 

i've got an old style tank cap . there is a valve in center cap: it can let go air inside but no outside. do i need to get a vent tube on top cap or let it as it is? that is the question...

 

 

[1avidflyer 1983]

The ram air tube when it is faced forward, supplies pressure to the fuel in the tank, which may keep your engine running if the fuel pump fails while in flight.  I would put the tubes in.  JImChuk

[nlappos 413]

The tube is punched right thru the cap, and it points into the wind so it pressurizes the tank at cruise speed a tiny bit, enough to keep the fuel pushing down the fuel feed lines. People who lost a cap in flight have reported a stoppage of fuel flow, so that slight pressure is a real help.

[efil 01 74]

how do you manage to cross valve?

[efil 01 74]

i'm thinking the fuel evaporation add some pressure inside if the no return valve does the work

[Dusty 126]

You need to let pressure out as well as air in,If it isn't vented as the fuel/vapour expands with heat you may get a pressurised tank,which may cause problems.

[efil 01 74]

ok thanks

i will use the oil cap i 'm not using to do the tube welding on it cause it has not valve and so, i keep the fuel cap as it is in case of..

[1avidflyer 1983]

The tube is punched right thru the cap, and it points into the wind so it pressurizes the tank at cruise speed a tiny bit, enough to keep the fuel pushing down the fuel feed lines. People who lost a cap in flight have reported a stoppage of fuel 

That picture looks vaguely familiar.  Way more junk on that wall now though.  JImChuk

[NorthIdahoAvidflyer 374]

You don't have to weld the cap. I went to my local auto parts store and bought two short aluminum brake lines. I bent 90 degree bends in them.  I drilled a hole through each cap and used JB weld to attached them inside. They work like a charm.  

[nlappos 413]

Proof #2 that no welding is needed, this is mine, with Permatex Fuel Tank repair epoxy putty.

[Turbo 162]

The wing's boundary layer is surprisingly thin, order of 1 mm, so the vertical part of the vent does not need to be very tall.  I'd give it at least a couple of cm, though.   Having a forward-facing vent means if you happen to fly into a rain storm you will collect a little water.  To counter this I would overshoot on height a little, then come back down to put the forward-facing part parallel to the local wing surface, not the freestream direction. The amount of pressure you get from a forward-facing vent is around 1 kPa, not much, however with two wing tanks with similar vents you can be sure they will be very close to the same pressure.  The total pressure is the easiest to enforce.  By comparison, freestream static pressure is much more difficult to get right.

[Matthewtanner 44]

Maybe it’s because I have a learning disability...but pictures?

[Matthewtanner 44]

Maybe it’s because I have a learning disability...but pictures?