opjose

Check valve = too much pressure ?

I've have two separate planes with what appears to be the same minor problem due to the presence of a check valve.

One plane is a Big Stik .60 with a SuperTigre G75 engine. This plane has a typical two line setup. One line is the fuel feed line terminating at the carb on one side and the clunk on the other. The other line is the pressure/vent line, one side is attached to the muffler fitting the other goes to the vent tube in the fuel tank.

The other plane is a Super Skybolt ARF with an O.S. .91 Surpass III non-pumped engine. It uses a three line UniFlow system. One line connects at one end to the clunk, and the other to the fuel inlet/needle. The second line connects on one side to the muffler ( for pressure ), and on the other to a second clunk. The third is a filler line that goes to a vent tube on the fuel tank, and hangs down from the plane while being capped off in flight.

Both of these planes have small check valves installed between the muffler and the fuel tank. In the Stik the valve sits in the line between the muffler and the pressure/vent tube. In the Skybolt the valve sits between the muffler and the second ( UniFlow ) clunk. The valves are set to permit pressure into the fuel tank.

The problem is that I seem to get a very high pressure build-up.

If I accidentally disconnect one of the lines w/o first unpressurizing the system, fuel sprays with good force all over the place... even if the plane has been sitting for a while.

Fuel also tends to seep from the carb on my side mounted Skybolt because of the relatively high pressure.

I've also noted that in tuning the engine with the check valve in place I have to practically close down the needles...

So much so, that I am completely UNABLE to blow air into the carb line ( Yes I use a clean tube for this... ) even with the throttle COMPLETELY open on both planes!

In other words the pressurization is feeding the fuel in completely at high enough force to still make this all work.

So :

- Is this normal?

- Should I eliminate the check valve altogether?

- If so how else can I prevent fuel from leaking out in the UniFlow setup while I fill the tank, other than by clamping off the muffler line?

- Finally I quite suprised that there is this much pressure buildup. In other discussion people are frequently citing that muffler pressure tends to be VERY low, under 1-2 PSI, this contradicts this... comments?

RCKen

My Feedback: (9)

Opjose,
I would say to eliminate the check valve. Unless you have a YS engine that needs a check valve to even work you shouldn't need one in your setup. The having that valve there is causing pressure to force the fuel into the carb of the engine, and they weren't designed to run that way. Muffler pressure alone should be ok. At least give it a try without the valve and see how it works, but I'm going to venture a guess in that you will probably have better luck without it.

Hope this helps

opjose

Thanks for the reply.

Will do.


- Any idea how to solve the fueling problem with the Uniflow setup short of clamping off the muffler line each time I fuel the plane?


I also see the check valve recommended for Uniflow set ups a lot.

Thanks again.

AIRDEL

I use a closed loop back to the fill jug for filling then when it overfills it goes to the jug. Then you can back it off a bit to pull it out of the exhaust tube.

Hope this helps,

Del

Wild Foamy

ive never had need for a check/non-return valve, return the engine to base and remove the valve then try it

opjose

Yeah, with the Uniflow setup without the check valve, as soon as you start putting fuel into the tank it then equally begins to spew out of the muffler line.... which is why I guess everyone recommends the valve.

AIRDEL

Just a question, Why the second clunk? Why doesn't it just go to the top of the tank like a typical system?

Please Excuse my ignorance.

Del

RCKen

My Feedback: (9)

Opjose,
Ok, let me make sure I understand your setup. You have the engine on it's side (engine head to the side and the muffler below the engine). When you try to fuel it the fuel spits out of the muffler vent line?? That's not right. Is the vent in the fuel tank at the top of the tank? No matter how you have the engine mounted fuel should not come out of the vent line until the fuel tank is full.

Ken

opjose

Thanks yet again.

Not quite...

The set up is configured in a Uniflow setup.

Both the pressure line and of course the fuel feed line are submerged in fuel.

A third vent line is uncapped when I am filling the tank and it does run to the top of the tank, and I've verified that it is unobstruted and in the correct position.

I have a World Models fueler between the tank and the carb, so I am able to fill the tank from the carb feed line effectively intercepting the connection from the tank to the carb, though from outside the cowl...

One of these...


( BTW: I like these a lot since they permit me to perform pinch tests on cowled planes w/o problems )

So basically fuel goes IN through the carb feed line ( yes I have a filter there, but it does not come into this whole thing at all, as it is between the WM filler and the carb itself... ).

The open vent line should permit pressure relief in the tank and indicate a full tank condition when it spits out fuel.

What inevitably happens instead is that fuel sprays out of the muffler even though the tank is NOT full. This starts to happen once the muffler/pressure clunk is submerged in fuel.

You would think that this should not happen, but I think that the tubing ( I'm using standard medium silicon tubing on all of the lines ) does not permit fast enough pressure exit when I'm using an electric fueler, so the UniFlow setup causes fuel to start gushing out of the muffler instead.

Yeah I can clamp the muffler line during fueling, but I wondered what everyone else was doing with their UniFlow setups or am I just have a weird case.

AIRDEL

It sounds to me like the fuel fill line should have the clunk and the exhaust should be the vent.

Just my 2 cents.

Del

AA5BY

As others have said, having the muffler pressure line to the 2nd clunk or to a line at the bottom of the tank is a bit unusual and is why when filling the system needed the check valve. Better I think is to run the muffler pressure line as the top vent line and the bottom line or 2nd clunk go to a fuel dot or simply a line out the bottom with a screw in it for fueling/defueling.

Even better yet in that you are using the fueler is to use a two line tank with the pressure going to the vent port on the tank and the fueler to the clunk.

opjose

Eh the fuel fill line is the tank to carb line which does have the clunk.

The exhaust ( or rather the muffler pressure line ) goes into yet another clunk, as per standard uniflow setups.

I'd like to maintain the uniflow setup on this plane if possible but I ran into this snag.

I had it originally set up with a standard two line configuration which worked OK, but I want the consistency of Uniflow.

In testing the configuration with everything apart, Uniflow provided the best fuel transfer to the carb in a nose up attitude...


With the check valve it is MUCH too good!

I'm still rather amazed at how much pressure actually builds up with the valve in place.

I though this a fluke until my Stik did the same thing with a valve in place... ( to be removed ASAP! ).

AIRDEL

Again ignorance is taking over. I don't understand why the fuel fill line would go to where the vent would normally be. Why would the pressure(muffler line) be fed into the fuel? Do you have a link to a schematic for this system? Sounds wierd to me.

Maybe my thick skull just isn't seeing this correctly?

Del

AIRDEL

If this is the correct setup procedure I don't see how you can keep the fuel from coming out the exhaust (Clunk?).

opjose

Thanks but ...

Uniflow is far superior to maintain consistent fuel flow to the carb as it removes the effects of the fuel "mass" on the entire system.

It's actually requires the multi-clunk setup with the pressure clunk submerged in fuel... otherwise there is no benefit to this system.



I'm trying to use it for the obvious advantages it has.

The problem arose because of the effects of the check valve causing such high pressure build up.

With the check valve in place I can fuel the plane w/o problems, but then I have the pressure problem.

W/O the check valve fueling causes fuel to run out of the pressure line.

AIRDEL

What is the benefit or use for the vent if it is plugged. Can a pressure relief be put on this vent?

Thanks for the picture. I don't see the extreme benefit to this setup. But what does a new guy know.

See ya,
Del

opjose

You sound unfamiliar with Uniflow

Please see this thread: http://www.rcuniverse.com/forum/m_31...tm.htm#3131525

I was not a believer until one day I set up a tank with a Uniflow setup.

I put the tank on the flow about 1/4 full of fuel then blew into what would be the pressure line.

W/o much effort I could make the fuel come up almost 3 feet through the tubing thanks to Uniflow.

When I tried the same test with a stock setup I could NOT make the fuel fuel come up at all. I simply could not make enough pressure by blowing, this way... especially since the length of the tubing causes further restriction.

Uniflow is one of those things that you don't quite see how it should work, yet when you try it, it is rather amazing.

opjose

The vent is only uncapped when you fuel the plane.

That is what is supposed to solve my problem, but with the removal of the check valve things are bad.

What I need is a partial pressure valve... something that lets pressure build up, but not so much.

Anyone know of such an animal?

JohnBuckner

My Feedback: (1)

http://www.rcuniverse.com/forum/post...469&toStyle=tm

AA5BY

The answer then is simple...if you believe in the uniflow system... stick with it and unplug the vent line when shutting the flight battery off and replug it before firing up the motor.

JohnBuckner

My Feedback: (1)

My apologies but its about impossible to post here anymore involving anything over a few sentances without timing out.

opjose

Heh... you would think... but that's the problem and the very reason the check valve is recommended for Uniflow setups... ( see the other thread for someone with a similiar question as mine... ).

I always unplug the vent cap before fueling... and I still get the fuel gushing out if the check valve is not in place.

Has anyone ever tried poking a hole in the check valve membrane to make it more akin to a "restrictor" valve instead?

If the pressure is relieved over time then I could continue to use the check valve and there would be no problem.

opjose

I compose in my e-mail program, spell check ( If I remember! ) then paste into the reply box...

It helps when I am long winded.... which I often can be.

AA5BY

My simple question would be...what is the difference of blowing x amount of pressure into the vent line or the 2nd clunk line... x is x and will cause an equally corresponding response to the output line unless some or all of the response is at the 2nd clunk line when blowing into the vent line.

My point is... when running the blow in the line test... was the 2nd clunk line plugged just as the vent line was plugged for the test? I'm doubting it was.

And yes, there is a simple way to do partial pressure...use a tee and partially plug the vent side of the tee.

AA5BY

When I suggested sticking with the system and simply removing the vent plug when shutting off the receiver battery, I assumed the check valve would remain in the system.

If pulling the vent plug after flying is not suitable... then a bleed off as you say will work but keep in mind that some amount of fuel is likely to flood the carb before the bleed off neutralizes the tank. That is not something I would want.