jeff4912

Sig 4 Star 60 gas tank fits so tight in airplane that I cannot loop the vent line on top of the gas tank.

If I bring the vent line out of firewall and then down and out the bottom, will I have problems?

Would running the vent line out the firewall and the up and over the spark plug wire and then down help?

av8tor1977

My Feedback: (6)

I've never had a problem with just routing it straight out and down. It wouldn't hurt for it to exit as high as possible, and then head down and out.

AV8TOR

w8ye

My Feedback: (16)

As has already been said, I never had one minutes problem just running the vent line out of the fuselage and down.

Mpizpilot

My Feedback: (45)

Looping just helps to keep it from draining out while sitting

av8tor1977

My Feedback: (6)

If the tank vent inside the tank is bent upwards to touch, or nearly touch the top of the tank as it should be, then fuel cannot drain out. I've never had fuel drain out of my planes, not even when I hold them nose down. Since the carb line is closed off by the carb when the engine is not running, there is no venting action to allow fuel to come out of the tank vent when held upside or nose down. Maybe a few drops, but that's it.

AV8TOR

jeff4912

Sounds good you guys, as always, thanks for the quick replies!

Zeeb

My Feedback: (41)

I beg to disagree; I tried putting the vent line on my 2.6 Comp-Arf 260 straight out the bottom and when I landed there was fuel all over the bottom of the fuselage from the vent line back. Looping the line on top of the tank fixed the issue.

So if you decide to just run the line straight out, be sure and check it when you land and see if it's been siphoning. Now if you're not going to be doing any aerobatics, especially negative G stuff, it'll probably work.

w8ye

My Feedback: (16)

I think some of you guys are missing a strong point here.

I've been around model airplanes for more years than most of you have been on earth.

The vent line opens inside the tank at the very top. The fuel suction line goes (now days) to a flexible clunk line that moves up and down, according to gravity, towards the rear of the tank.

From my experience with these planes, the only failure of the vent was when you stood the plane on it's nose. Fuel would spill out the vent. I always cut the end of the vent outlet at an angle facing into the slip stream.

The loop people are talking about is just a length of flexible fuel line that connects to the vent outlet of the tank and goes along the highest point in the fuel compartment all the way to the back of the tank and then back to the front and then down and out in the engine compartment. With the vent loop, fuel doesn't spill with the plane resting on it's nose.

Zeeb

My Feedback: (41)

We're not missing the point here let alone a "strong" one and it would seem that you've become, shall we say "touchy" since becoming a moderator and slamming my threads which deal with something you left from other posters. Just because you're older than most of us does not mean you're any smarter.

That said; among other things I have A&P licenses and know about aircraft and what works, having a Commercial Pilot's license brings with it some knowledge of aerodynamics and I DO know something about working with model airplanes including turbine powered models. I will tell you that when doing aerobatics the nose of the model DOES point straight down at times and no matter what you've done with bending the vent line toward the top of the tank, it will siphon out of a straight out line like nobody's business. Even with the loop on top of the tank, cutting the line and an angle and all the rest of the stuff you're harping on, it will still siphon a bit out onto the fuselage when flying aerobatics.

w8ye

My Feedback: (16)

I don't have any trouble with the vents even without the loop.

So you must not follow the usual procedure?

Bob Pastorello

My Feedback: (198)

This is EXACTLY correct, and to avoid fuel siphoning during aerobatics (i.e. - ANYTIME the airplane is moving with the nose LOWER than the rear of the tank, downlines, half-Cubans, Immelmans, ANY of those and multitudes of others like ANY rolling maneuver) the ONLY fix is to loop the vent line such that with the nose pointing DOWN (or any angle approaching down) the vent line "high point" actually is then ABOVE the rear of the tank. It will NOT siphon fuel out of the vent when configured this way.

It really is true.

w8ye

My Feedback: (16)

To get back to the original post and the gentleman's nice Four Star 60, he is not going to have any trouble by not running vent the loop to the top rear of the tank compartment and back to the front and out the bottom.

If he was wanting to fly nice 3D with a need for the vent loop that 3D people use - he needs a different - more capable - 3D type model airplane.

The Four Star 60 is a nice casual plane to own. I've had a couple.

Truckracer

My Feedback: (19)

On every glo powered engine I ever owned ..... and many of these go back to the days before we used mufflers, I just ran the vent out the bottom of the airplane and had zero problems. Of course an exception would be conventional muffler equipped engines where the vent just connects to the muffler. I would assume the later is the case with the OP's 4 Star.

On gassers where there is more room in the fuselage, I use the loop back and over the tank then out the bottom of the fuselage in the now conventional gasser style.

Both systems seem to work well for their intended applications.

pilotpete2

OK, the OP seems to be running a small gas engine in his 4 Star 60 (OS GT15 maybe?). Having flown a 4 Star 60 for many years I know how hard it would be to get the vent loop setup in that plane!
Also, using the smallest ID fuel line will help slow down the draining, since, when fuel drains out the line, air must flow in through the vent line in equal volume, since there is no other opening in the tank to vent to atmosphere. Since there is air always in the vent line, siphoning is not possible, it's just a simple gravity drain. At least that's what I remember from Hydraulics class with this old Greek Professor...Archimedes I think his name was
Pete

jeff4912

I am running an Evolution 15cc gas engine and have thought of everything I can to fit a loop around the tank, but I don't think it will fit.

So assuming that, I am planning to run the vent line out the firewall and down to the drain hole on the bottom of the 4 Star.

Assuming I do rolls,Cuban 8's etc... but no 3D(since I am not good enough to do that anyway) will I only see some dribbles out the bottom or will it stream out .

I am planning to use a flush mount connector to the bottom of the plane that the vent line plugs into.

I do not usually pull negative g's or fly for extended periods inverted.

Threw in some pics...for fun.

av8tor1977

My Feedback: (6)

My gasoline powered airplanes are rarely flown straight and level for more than a few seconds. I love doing aerobatics. I have not had any significant fuel seepage/drainage out of fuel vent lines exiting directly from the tank to the bottom of the fuselage. I too have more experience in full size and model airplanes than I can count as well, and don't care to argue. It just has never been a factor in my very long modeling experience. I have more issue with unburned smoke oil sometimes making a small accumulation on the bottom of the airplane and tail.

AV8TOR

w8ye

My Feedback: (16)

Jeff,

Looks good

F.O.R.D.-Doctor

I've had it several ways with gas setups. I loop over the tank when room allows or out and down when room is at a premium. For those times when my out and down did drip some fuel.
Which was obviously seen when transporting and or tipping the plane nose down with a half tank or more.
I put a circular loop in the drain line as it exits the firewall. The size of the loop is usually dictated by the fuel line size. But typically 1 to 2 inches or larger if possible.
I believe that the longer you can make the drain line and possibly incorporate a loop the better. I also think that while flying at any angle the airflow across the vent tube
presents the possibility of fuel siphoning. For this reason I have sometimes filed an angle in a piece of brass tubing and face it forward into air flow. This serves
as a minor way of pressurizing the tank. Anyway that's my 2 penny's.
Regards

LarsL

I was getting a little confused myself as I was reading through all of the posts, that is until I saw here towards the end that the OP is using an Evolution gasoline engine with a pumped carb. On all of my glow planes that use muffler pressure to the fuel tank, there is no vent tube per se. and I of course don't loop that muffler pressure tube. If I run a third line for fueling, that also doesn't get looped as it gets plugged after fill. As far as the gas engine planes go, looping of the dedicated vent tube is the way I go but I haven't had any space constraints that haven't allowed me to do so.

Lars

raptureboy

This is a very interesting thread to run into, as last night I was talking to a club member who is very experienced in 2 stroke racing engines, and carburator design about this very thing. I just had an engine out shortly after pulling an upline and roll. I managed a dead stick back to the field. He asked me about my vent line on my plane, if it had a loop over the tank and I said no but did have a loop over the firewall then down out the bottom. He explained to me that in an inverted position the fuel would cover over the vent tube and that there would be no venting at that moment, and the carb would pull a vacume on the tank and not be able to draw fuel causing an engine out. He explained that most carbs on the smaller engines, in my case DLE 20, only pull about 1lb or less of vacume and can not overcome the restriction and will starve for fuel. By having a bigger loop there would be more air in the line overcoming the loss of venting. Made total sense to me so I am going to try it and see what happens as this has been a common problem for me with my other gassers which are mostly 60 size warbirds running DLE 20's. I wil lget back here and report on my results after doing some tank work.

av8tor1977

My Feedback: (6)

Hmmm..... That's strange. I have several 25 to 30cc aerobatic planes, and they do unlimited vertical without quitting. My vent lines have no loops.

AV8TOR

pilotpete2

Hey Jeff,
Nice job on the 4 Star 60!
Good luck with the new pumped version of the EVO 15. I have the original non pumped version, first on my old 4 Star 60 that flew great until its expiration date hit;-(.
I now have it on a 4Star 64 ARF, great combo.
I'm with av8tor1977, you won't have any problems with fuel draining out in flight. Just don't leave a significant amount of fuel in the tank, and then store the fuselage nose down. The tendency for fuel to drain out the vent line varies with the size of the tank. A larger tank creates a higher head pressure due to the higher level of fluid above the vent. The smaller (12oz?) in the 4 Star will result in slower leakage than you'd have in a giant scale model.
Best of luck with your maiden, I hope the new design muffler is better than the original. My Evo 15cc has the OS muffler for the GT15, as the original was a poor design.
Pete

pilotpete2

Hi raptureboy,
Your friend is way off the mark on his idea that the fuel covering the vent somehow prevents air from entering the tank through the vent line. The air still comes in through the line and enters the tank as bubbles.
There are many misconceptions regarding how fluids behave in a fuel system. More than once, I've heard folks say that you need the loop over the fuel tank to prevent fuel from running out while flying inverted. Just think that through for a moment. When inverted the vent line, where it exits the bottom of the fuselage, is now UP and higher than any other point in the fuel system. No way fuel is going to flow out the vent while inverted, especially when air is being drawn in as the engine draws fuel;-)
Pete

jeff4912

Thanks for the compliments about the 4 Star and the info about the vent line issue.

Should be maidening in the next month. Don't have a lot of free time right now and still have a few things to do to finish the 4 Star.