If you would please, take a look at the attached schematic and tell me why I'm venting fuel overboard during flight. For calibration purposes, here are the essentials:

Plane - 80" WM Extra
Engine - FPE 3.2 gas
Tank - 800cc (27 oz) rectangular, mounted on the CG
Vent exit - through the belly of the fuse, just aft of the main gear

The check valve, tee, and fuel dot in the vent line were not there originally. I noticed the venting problem and added those pieces, assuming it was a "siphoning" problem. This after trying several configurations of vent line, to include turning it 90 degrees, with the tube facing forward as shown in the schematic. The check valve has slowed the rate at which fuel is venting, but it has not stopped it.

This is my first gasser, and at first, I thought it might be "normal". But after talking to some very experienced modelers who have built literally dozens of gas powered planes, I don't think so... they're saying I shouldn't even need the check valve. I can live with this problem, but just don't understand why it happens. There are absolutely no tuning or "flyability" problems with the engine... it starts and runs flawlessly. I am considering the installation of a smaller tank, but I can't imagine that it would have any effect on this problem. (I used the stock WM tank that came with the kit, converted to gas with appropriate tubing and stopper. It was intended to feed a large glow engine and is really about twice as big as I need.)

Your comments and suggestions are very welcome!

Sounds like you are generating presure inside the tank. Have you tried this option.

Is the vent line routed higher than the tank before it goes down? If not, give that a try. Once the fuel starts leaking out when the fueld is sloshing around in flight, it will continue to siphon out.

I use a "T" connector to split the line with the Carb and Fuel Dot. I use a toothless forcep to clamp the line to the carb when fueling and defueling.

RC-Av8tor, I would use the line set-up that is posted by mecam. Do not use the check valve in the vent line on gas engines. The vent inside the tank should be as close to the top and as far forward as possible. It also helps if the front of the tank is just a little higher than the back in level flight. The Walbro carb on you FPE will not allow gas to pass when the engine is not running so there is no need to pinch the line during fueling. Gas has a much higher evaporative pressure than glow fuel, its the wooosh you here when you open the tank in your car, some liquid venting is normal but it shouldn't be more than about a teaspoon a flight.

The two configurations (posts #2 and #4) have already been tried. The original configuration was the two line setup shown in post #4. I went to the three line when I realized that I was opening the supply line to the engine every time I refueled... I didn't like that idea, because my filter was on the supply clunk. In any case, this decision had nothing to do with the fuel venting problem, but for the record, the two line setup lost fuel out of the vent just like the three line setup shown in post #2.

The check valve was added later, and has helped the overboard venting problem, so I would like to keep it in the system. I can't see that it could hurt, as it allows air into the tank to replace the fuel pumped out by the engine, but at least slows down the venting losses.

I am going to try Flyfalcon's idea (an overhead loop on the vent line), as mine leaves the tank and immediately dives straight down to the overboard vent tube. I am also going to move the vent line check valve closer to the tank, as it is currently just ahead of the overboard vent. This move would (in my mind, at least ) allow a smaller amount of fuel to be "trapped" in the vent line by the check valve. I suspect the valve lets at least a portion of the trapped fuel out, when it eventually opens to let air in the tank. With or without a check valve, I'm envisioning the vent line being filled with fuel for instance, on a long "braking" downline. Then, when the plane goes upright again, the fuel simply falls out of the line. I'm sure there are other "sloshing" manuever combinations that would allow the same thing to happen. BTW this plane is straight and level exactly twice on every flight; for about 5 seconds after takeoff and for about 15 seconds before landing only .

Now... having said all this, if normal loss is a teaspoon per flight, then I didn't really have a problem to start with. (Of course, that's not what "The Big Boys" told me. ) But if that is the case, I'll simply run a longer vent line, maybe along the curvature of the main gear, to simply keep the gasoline from accumulating on the bottom of the fuse.

Anyway, your time and thoughts are certainly appreciated.

Thanks!

Av8tor: Read what the gas Heli boys do. Use the check valve(could add a springloaded one) and wrap the vent line to make a complete circle around tank. Also one guy put a stopper in vent line with just a pinhole for vent. Worth a try. Thanks Captinjohn

av8tor, i have the same problem with my 40cc ZDZ in an ultimate bipe.

i'd like to know what you find out.

David Soniat - Tampa, FL

captinjohn - I found the heli thread you refer to. Hmmm... very interesting. As you say, might be worth a try, since the only cost would be that of several inches of gas line.

Use the concept in post 4 but make sure that the vent line is not in the air stream. If it is, you will suck fuel out of the tank.

how can it not be in the air-stream?


that seems to be the problem with mine. the airstream sucks the fuel out. i tried bending it so that it faced the airstream and the air "rammed" into it - but that didn't seem to help...........


D Soniat

I don't think a check valve is a gas system is a good idea. You would be amazed at the amount of pressure that can build on a 80 degree day when using gasoline, especially if the gas was relatively cool when it was pumped in. I think it not only may change fuel metering at the carb, but should it blow a line or split the tank it could be a disaster.

Outcast - The pressure build up is something I thought about when I installed the check valve, but so far no problems. Of course, I also haven't seen anything above about 60 degrees ambient, either.

On the other hand, based on a conversation in the thread alluded to by captinjohn, use of a check valve on the gas helicopters is apparently not uncommon.

Put it inside the cowl or at a point in the airframe where the air will dam up and not flow. If it is in the airstream, it will syphon it out.

outcast: I hear what you are saying. But......I think when you fill up the tank in your RC gas plane/Heli I would assume it is going to be flown right away. Should not be a heat problem. Also any tank should not be filled right to the top. Should have room for expansion. I do think every field should have a fire extinguisher just in case. Goes with any fuel bing used near electricity. I think the tiny pin air pressure hole is the best idea, combined with a loop around the fuel pump. Capt,n

If you can cap the vent line, you can fill it and keep it filled.
This is what I do and I don't have spills and the smell is minimal.

Geistware, are you saying you fill the tank and plug the vents when your done flying? If so why would you want to do that? I have always been taught to drain my tank and plug the vent.

Captianjohn, Put some gas in a small soda bottle, cap it then shake it like the plane will taxing out on grass, then remove the top {wear eye protection}, it may change your mind. I know that people do this and have no problems, just like some mount the throttle servo on the fire wall without an opt link and have no problems, and I'm not saying there wrong. I just like to avoid things that may be a problem.

Outcast,
Gasoline will not expand/contract as much as air under different temperature/pressure differences. Filling the tank with gas not only helps minimize the pressure/vacuum buildup while in storage, it also helps prevent moisture from entering the tank and fouling up whatever fuel is in there.

outcast: You have a very good point. I am going to do some re-thinking on this. This is good.... all these ideas coming together. That is the way we all can fine tune this interesting hobby. I think it can be said...we are all learning. Thanks Very much to all. CaptinJohn

I understand this and I agree with you, in fact I always parked my real aircraft with full tanks just for that reason. But we're talking about blocking vent ports with either plugs or check valves, and the more fuel in the tank the less room for expansion. We all know you can't compress a liquid so should it reach the failure point of the weakest link, whether that be a hose, the tank plug, or the tank itself, man is it gonna make a mess.
Please understand I'm not trying to start an argument, I just don't think it's a good idea.

Gas is not like glow fuel.
Glow fuel will absorb moisture if left in the tank and/or evaporate out. Gas will not absorb watter as easy. Also gas when sealed can last a long time unless it is real cold in which case, the oil may separate from the gas. I look at it like I do with any gas device. You keep the tank full to prevent moisture from condensing in the tank. Right or wrong, this is what I do!