venting petrol tank

[colin bassham]

I am in the middle of putting a 26cc petrol engine in my T240. I have vented the tank to a brass tube in front of the windscreen. My worry is losing fuel when flying upside down during aerobatics.

[John Greenfield]

Colin

If you want to avoid losing fuel when flying inverted take the vent pipe down to the bottom of the tank by extending it inside the tank with a piece of brass tube or similar. It will still happily work as a vent and when you are upside down the pipe end will be above the fuel so it will not run out.

Happy Christmas

John

[colin bassham]

Thanks John , That sounded good till I realized as I fill the tank through the dubro valve air wont come out the vent , It would be petrol. Thought about a one way valve, which allows air in ,stops fuel spilling out, but this also wont allow air out when filling. I blanked off one side of the dubro so I could get direct run from carb to tank. Colin

[paul hughes]

If you take the vent pipe from the top of the tank and run it to the underside if the fusalage you should be ok.

[Chris Lane]

I have used the "stunt tank" layout we used to use on control-line models for my Super Greenley. This uses two vents. There are pipe fittings on both the top and bottom of the tank. Two more pipe fittings are provided in the top and bottom fuselage skin. Tygon tubing connects the top tank to the bottom skin vent and vice versa. Fuelling is done through the top skin vent and de-fuelling through the bottom vent.

[Glenn Masters]

Colin - just a note of caution re. the dubro fill-valve. Some work well, but others - and i've seen a few , will have you scatching your head over poor running / lean and overheated engines. I recommend a simple two vent set up, one to fill and the other to show full and then vent. Cheers Glenn

[colin bassham]

Chris I like your Idea, but I guess that involves making your own tank from scratch. Glen I know what your saying, so thats why I am only using it as a filler, the carb has direct connection to the tank. I have now added a one way valve which can be disconected whilst filling the tank to allow air out. Then it is reconnected to fly, the tank can breeth through the one way valve and no fuel can escape. Even a small amount dripping out an open vent has got to be bad for the enviroment in these days of eco awareness. Colin

[Chris Lane]

Colin; no need to fabricate your own tank. I just buy the plastic tank and some extra fittings from Glen although I did turn up the model outer skin fittings on my lathe as I wanted specials.

Two pictures attached; the first is the tank for my Hostetler Cessna with the two vent fittings installed. There is nothing inside the tank - they are just hollow screws with big flanges. The second is the complete installation in my Super Greenley. You can see the brass top skin fitting I made right at the top of the second picture.

Happy Christmas guys - Chris

[colin bassham]

Hi Chris, interesting, but every time you go inverted the pipe will fill ,then as you come upright half whats in the pipe goes back in the tank and half goes out the vent in theory. Can you say for sure no fuel will escape during rolls, loops , inverted ect, also wouldnt you have to defuel from the top skin vent which is connected to bottom of the tank? ( opposite of what you put above) Colin

[paul hughes]

If you wish to collect every little bit of fuel which might escape down the vent pipe, why not fit a small can (tank)inline on the vent pipe. any fuel running out of the main tank will be trapped in the second empty tank. if this tank is plumbed correctly the fuel collected while inverted will run back into the main tank when flying straight and level. just a thought!

[Chris Lane]

Hi again Colin,
No, I can't guarantee zero venting of some fuel, but the tank is being sucked by the engine and the fuel in the tube which, at a given moment, is going to the "top" of the tank will suck back into the tank. As Paul says you could put some extra volume in the tubes but I doubt if it is necessary.

Your absolutely right; fuelling and defuelling are BOTH done using the top skin (=bottom tank) vent connection - sorreehh

Chris

ps. unless you fuel your model with it inverted!!

[colin bassham]

Thanks Paul , not a bad Idea, If it was made so it didnt drain back completly it would be a good indication that fuel was escaping. You would of course need two one on each vent. You can tell I'm a Yorkshire man ,we dont like to see our money wasted on fuel going down the vent ! The way Ive done it with a one way valve nothing can escape, the only downside would be on a scale model , because you need it to be accesible to disconect whilst filling the tank.
Thanks Chris for your help the photos show it well. (how do you put photos on here?) MERRY CHRISTMAS Colin

[Steve Mitchell]

Hi Chris, interesting, but every time you go inverted the pipe will fill ,then as you come upright half whats in the pipe goes back in the tank and half goes out the vent in theory. Can you say for sure no fuel will escape during rolls, loops , inverted ect, also wouldnt you have to defuel from the top skin vent which is connected to bottom of the tank? ( opposite of what you put above) Colin

I assume your aware that you should never de-fuel a petrol tank.

If you do it will cause the clunk and the diaphragm in the carb to dry out and make the motor impossible to start. Bung up the overflows and you'll be fine.

Steve

[Bob Thompson1894]

dead right Steve- not bad for a sailor! One of the reasons I went to petrol was the simplicity of bunging up (or coonecting together) the inlet pipes so the model could be taken indoors without dripping fuel everywhere. Try that with glow!

[colin bassham]

Thanks Bob and Steve, will take note of that. But Can you tell me of a venting system that spills absolutly nothing duing aerobatics. My one way valve will work ,but it needs disconecting to fill the tank.

[Bob Thompson1894]

never really thought about that one! You would need a (maybe) fuelling valve that lets air in but wont let fuel out. The Dubro ones do that when you dont want them to! I would have thought that a SMALL opening is all that is needed just to stop the tank collapsing?