alasdair

I have a question.


My current O/D sport jet (called JayLow, as it's a low wing version of the JayTee in my avatar) is powered by a Jets-Munt VT80 and is fitted with a DuBro fuel tank (80 oz or 1.8 litre I think).
PROBLEM I seem to have fuel seepage from the vent, judging by the evidence on the bottom on that side (port). Also, I have noticed that when I do a slow roll, or an inverted segment, there seems to be a whiff of fuel, a white streak maybe six to ten feet long, coming from the model. I don't always notice it, but maybe it is always there. If it is travelling 190 to 200 ft/sec we are talking about a fraction of a second, 1/20th or so.
I always imagined that once the engine was running, especially at high power setting when I'm doing rolls, it must be sucking air all the time and so no fuel could be ejected.
QUESTION What could cause a brief ejection of fuel when inverted? And what is the cure?
The Tygon vent line is unusually short. Would making it much longer, with a loop up to the tank top, be a solution?
Any ideas from all you clever experienced chaps out there?

sewbusy

I was told in another thread that my method will put a strain on the pump. However, my KT235 never had any problems while competing in JET AEROBATICS and I certainly don’t have any fuel leakage such as you mentioned above. This was learned flying in Unlimited in IMAC for 28 years….so I brought over this tried methods into jets when I started competing in F3S over 4 years ago.
Use it if it interests you, or not, if it doesn’t.
Wayne

FenderBean

Need to make some loops in the vent line like above, the white smoke during a roll is not fuel coming from the vent. You will see a lot of motors do this

luge_racer

Not sure why this would put strain on the pump since it's the vent line and typically only flowing air.

I always put a loop or two on my vent line.

joeflyer

My Feedback: (48)

In addition to having a loop I also exit the fuse with a brass tube cut with a bevel facing the air flow.
That puts a slight pressure in the vent line preventing fuel from migrating out.

alasdair

Many thanks for the tips guys.
I'll try an extended vent line, maybe not as long as Wayne's, to begin with, just a big loop, up to the top of the tank and down again.
On my other jets the vent pipe comes from the top of the tank, up a little and down. No obvious venting. But on this DuBro tank it comes straight from the bung to the fuselage bottom.
I also thought of a small tank, a fuel trap, like a bubble trap in reverse. But I'd need to make sure it didn't fill when refueling.

alasdair

Update on my fuel venting problem.
originally my vent pipe ran from the bung in the DuBro tank to a simple nipple in the fuselage bottom. I used the small Tygon tube. In the photo below the vent tube is behind those wires.


Following your suggestions, I added about 15" of the larger Tygon and looped it up and around, and down to a forward-facing vent.



That seemed to cure the venting. However I cut the extension much shorter, only 5" or 6" extra Tygon, but still looped it up high, and that worked fine too, still no sign of venting.


Then I tried looping the excess down and around, and still it wasn't visibly venting, but looked neater.

So my next step is to replace the tape covering the U/C wire which has been discoloured by the venting fuel and oil and see if I have cured the problem long-term. Thanks for the advice/suggestions.

RCFlyerDan

My Feedback: (54)

Having the vent exiting to the rear creates a Venturi affect. This simply isn’t done, even on full scale, due to your results and issues.

Tom Cook

Check out jetmodelproducts website. Near the top of the home page are the parts to deal with fuel venting problems.

alasdair

Yes, I agree. But this vent of mine is forward facing. See the other photo, of the underside, with the wing leading edge showing

alasdair

Yes, thank you. I assume you mean the FV-6 @ $28.80

The fitting I used was a cheap thing from Hobby King, and more importantly, already in my stock. I mounted it forward facing in place of the simple up/down double nipple.
I like to use a nipple as, to avoid spillage in the car, after flying I fit a vent blocker (with a 'remove before flight' flag).

mongo

My Feedback: (15)

the little plastic line fuel plug thingies that the mfgrs ship on new fuel pumps, will work as a "plug" to put in the vents JMP sell for transport/storage. just the right OD for sealing the vent.

Turbotronic

I have done this for many years, it has the advantage of being able to see the tank filling without spilling fuel all over the place. In some cases it also eliminates the stopper.
The fuel left in this small tank will get used up first, almost like a taxi tank
Andre

Tom Cook

Hello Andre,
I have made this kind of "vent line tank" for quite a few years. They use staggered tubes and mount vertically. Built with the same parts used in our Air Traps. It does not really need to be any larger than our small size Air Trap. The guys that fly the most aggressive style totally test them.

alasdair

Hi TOM and ANDRE,
Have you a photo and/or plumbing diagram to show how you connect it? Sounds interesting and might save me some experimentation.

AEROSHELDON

My Feedback: (99)

I would also like a photo of this. I put this together and I am going to try it out but wonder what others are doing and if there is a better solution.

Turbotronic

Sorry no pic but its really simple, imagine small fuel tank with fuel pickup at low point and air input at high point. I also added an anti siphon valve to the vent,
but you would need to bypass it when refueling so as to let air out.
A good anti siphon valve could work on its own but sometimes the leak a bit. I mainly used it so I don't spill fuel when refueling but also to not get fuel all over my car should I forget to plug the vent.