Fuel tank venting ... does it need to be pressurised too?

[Guest rocketdriver]

Hi everyone ... and thanks for the help re parrallel or series piping for fuel pumps. Now the next silly question.

 

Is it necessary to pressurise rather than just vent a fuel tank?

 

My fuel tank has a float and stick level indicator, the stick poking out of the top of the tank and cowling .... necessarily there is an air passage via this opening into the tank. There is also a fitting with a bent pipe that rises out of the front top of the tank and is directed into wind rather in the manner of a pitot tube. Do I need this too?

 

This is the only orifice I have available for a fuel return line that I very much want to incorporate into the fuel curcuit especially with hot summer days and mogas fuel being a likely combination ....

 

Would appreciate your thoughts please ....

 

 

[Guest davidh10]

Doesn't having a fuel return line just add another point of potential failure that can result in fuel starvation if the pressure relief valve fails open?

 

P.S. My fuel tank is not pressurised. Just open to the atmosphere via two vent tubes (one from each high point in the tank) that join and the resulting tube points straight down where any excess fuel would be ejected into the air stream under the aircraft and below the engine.

 

 

[Guest rocketdriver]

Hi David

 

There is no pressure relief valve. Just a hose to hose connector with a restrictive opening that appears to be designed to allow the pump to deliver 1.5 lpm plus the returned fuel ...... I hope!

 

Regards

 

RD

 

 

[Guest ozzie]

The fuel vent is just that, a vent. If you were running a pressurized fuel tank there would be no vent. If that vent becamed blocked you would eventually starve the engine of fuel. This vent must remain open to the ambiant pressure out side the tank. If you are looking to add a fuel return line it should be a seperate fitting to the top of the tank. trying to utilise the existing air vent may cause you problems.

 

 

[facthunter]

The tank HAS to be vented of course, but regarding pressurising or otherwise.... Don't have a negative pressure especially with bladder tanks. A bit of positive won't hurt. Just curve the pipe from the cap forward a bit. Venting can be done with a bit of thought at the design stage. IF the vent pipes of each tank, are from the upper, outter part and track in towards the centre of the plane, they won't spill fuel if you sideslip in the air, or turn sharply when taxying. Nev

 

 

[Guest Maj Millard]

All good info above. Turning the fuel tank vent tubes foward into the slipstream works well to put some positive pressure on top of the fuel. Very popular on early aircraft.

 

It would provide a head of pressure which would assist the fuel to leave the tank, but it's prbabily not going to be much more than about 5 psi at the most. Might also assist the mechanical fuel pump a bit by providing a nice constant pressure.

 

Could be usefull with a gravity feed type system, and might assit in keeping the fuel from leaving the vent tubes during those loopy loops that pilots in the 20s and 30s were apt to do on occasions.

 

Can't come up with too many negatives, other than getting a bit of moisture force fed into your tank if you fly through rain................................................................................................................Maj...

 

 

[Guest rocketdriver]

The fuel vent is just that, a vent. If you were running a pressurized fuel tank there would be no vent. If that vent becamed blocked you would eventually starve the engine of fuel. This vent must remain open to the ambiant pressure out side the tank. If you are looking to add a fuel return line it should be a seperate fitting to the top of the tank. trying to utilise the existing air vent may cause you problems.

Hi Ozzie and everyone else .... Just to recap and rephrase my query .... I realise I must have a vent for the tank. Q. will the clearance around the fuel level indicator "stick" do the job? ... If so, I can reuse the current vent opening and fitting for the fuel return line. (If not, I have a problem with the fuel return as I do not have another available entry into the existing fuel tank ....) .... Perhaps the fuel tank cap can be modified to include a vent ...hmmm (it is a "standard" aviation type cap with a small "butterfly" lock that you lift and turn to open the tank)

 

cheers!

 

RD

 

 

[facthunter]

I have one of those cork float things with a rod sticking through the cap, on the Fisher. It is comonon on those 'traditional" EAA planes. It would be enough. Don't think there is anything else on mine. Check your fuel flow rate with tank full and also with it near empty, to be sure. Go for about 1.6 times the Full throttle sea level flow to the carb connection. Nev