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Old 04-06-2011, 09:20 PM   #1 (permalink)
carey shurley
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Join Date: Apr 2004
Default Gas Helicopter Fuel Tank Plumbing - My Way

I致e read quite a few posts about whats the proper way to plumb the fuel system for a gasoline helicopter. Like most model helicopter things, there痴 no one way to do anything There are always many and often all will work.

So I知 going to offer my technique as an example, its not the only way but I virtually guarantee that it will work correctly every time.

Generally the fuel tank is provided with the model so I知 not going to spend much focus on them. If for some reason you have to provide a tank, use a Dubro or Sullivan HDPE tank that is thick and hard. I値l explain why later.

Unless they are already molded into a custom designed tank, the first thing you値l have to decide is where to put the fittings. Depending on the exact setup you値l need three or four:
  • The fuel pickup feeds the carburetor
  • The fuel return from the primer circuit
  • A tank vent the carburetor has a negative pressure fuel pump, it will pull fuel from the tank so it must be vented or it will collapse
  • A fuel fill this is optional depending on how the tank is vented more when I discuss venting
The exact position for these will depend on how the tank mounts, frame layout, tank location etc. You値l want the fuel pickup fitting to be as near the center/centerline of the tank so that the fuel clunk can move to the most extremes of the tank. Unlike glow models its position won稚 really affect the fuel flow because of the fuel pump involved.
The other fittings should all be on the top of the tank since the model is usually sitting on its skids when you put fuel in it.

Parts Used List
  • Fuel Tank - Any Dubro or Sullivan RST style
  • Tygon fuel tubing
  • Viton fuel tubing - Zenoah Part # 1799-85400
  • 90 degree inlet - Miniature Aircraft part #MA0410-90
  • Fill/Vent/Return - Miniature Aircraft part #MA0405
  • Fuel clunk - Walbro/Zenoah #125-527-1
  • One way valve - Stens/Echo #610-079
  • 3/4" hole plug - Jasco - Ace Hardware
  • Aluminum tank stopper - Sullivan #478
  • T fittings - Sullivan #486
  • Fuel plugs - Thunder Tiger #PV0251
Here痴 an couple of examples of my tanks. You can see the pickup fitting on the side and the other 3 on the top.

The fittings I use are from Miniature Aircraft and the part numbers are:
Inlet - 90 degree #0410-90
Fill/Vent/Return #0405

FYI with two piece fittings like the ones shown, I use neoprene washers (available at Ace hardware) to help seal them to prevent any leaks. These aren't required but as shown they don't leak.

Typical fuel tank with fittings

Another view of tank fitting positions with one way vent installed

Some older designs use tanks with removeable screw top caps. I致e seen many people fuel their model by turning it on its side, removing the cap and just pouring in fuel. If your model uses one of these tanks and its accessible, that痴 another option for filling the tank (and you wouldn稚 need a fill fitting)

Whats inside?

Clunks - Typical (Left) - Stens (Center) - Walbro -preferred (Right)

So typically glow models use a rather simple clunk like the one on the left. This will actually sort of work in a gas helicopter but I don稚 recommend it. Because there is always more vibration, this type of clunk tends to dance around on the bottom of the tank. As you get lower on fuel, its much easier for it to pick up air and cause the motor to get leaner. I prefer to use a filtered clunk which is very common with most small gasoline motors. I致e shown two types, the one in the center is a Stens/Poulan #610-381 which uses a plastic sleeve and comes stock with the Spectra-G helicopter. This will work fine for a while but over time the porous plastic gets dirty and will slow the flow of fuel. Its not really cleanable. I don稚 recommend it. The one on the right is a Walbro/Zenoah #125-527-1 which uses a felt filter. Its cleanable and is slightly heavier than the stens. I recommend that you only use this filter.

You値l need to plumb the inside with a gasoline tolerant fuel tubing, I recommend the tubing that comes with Zenoah motors, its Zenoah Part # 1799-85400. Due to the size of the clunk fitting, you値l need to use 1/8 ID tubing. Tygon in this size is fairly stiff and doesn稚 let the clunk move around well enough and it can wind up out of the fuel supply in certain model positons. The Zenoah tubing is very flexible and comes like this: (the stopper and clip are easily removeable)

Zenoah Part #1799-85400

Usually in a glow setup you run a very short line such that the tip of the clunk can just touch the tank in any position. Because these filtered clunks are much heavier use enough fuel line on the feed that the clunk can literally move anywhere in the tank. A typical length would be 6-8. Heres an example of how that should look.

Plumbed clunk with loop

What if the clunk won稚 fit through any hole on the tank?

Now suppose the tank that came with the kit doesn稚 have a hole large enough to use one of these felt clunks? Don稚 let that stop you, its very important for consistent operation to use one of these clunks. The felt soaks up the fuel even when there is very little left. You値l have fewer unexpected lean runs. So it won稚 fit what do you?

Clunk won't fit? Cut a hole and plug it

On the top of the tank, in a flat area, cut a セ hole. Then run the fuel line through the normal tank hole and then through this large hole. Connect the clunk, pull it into the tank and then insert the normal fuel pickup plug in the hole where it goes. Go to the Ace Hardware store and get a セ hole plug. You値l find it in the yellow Jasco hardware bins that most Ace stores have. Get some silicon RTV, apply it around the edge of the plug and seal it into the hole. That痴 it, done.

Plumbing the tank/carburetor

So how to plumb the tank to the motor. Here痴 an example that I put together. The lengths of the tubes are only for example, any part can be pretty much any length but I壇 keep the fuel pickup to less than 12

For the fuel inlet, you want to run a straight line, no filters or T痴 from the fuel pickup on the tank to the fuel inlet on the carburetor. If you use this as a fuel line especially with a T fitting, you値l get air bubbles in the lines that even priming the fuel system won稚 remove. However when you get it in the air the vibration will pull it into the carb. I致e seen that cause a hiccup. With the straight line this can稚 happen

When you pump the fuel primer, it pulls fuel through the carburetor and then out through the primer outlet. I have seen these simply left bare or have a tube that simply lets the excess dump on the ground. I prefer to let it return to the tank and it also makes a good line to put fuel in or to use a filling vent. (explain more in a minute). So plumb the fuel return on the carb to a fuel return fitting on the tank. Put a T fitting in this tube, run the T line somewhere on the model and insert a fuel plug of some sort in the end of it.

Look a little further down and I have some obvious examples. These are the only connections you need to the carburetor

Vent/Fill plumbing

Now onto the vent. No matter what you have to vent the tank because all of these carburetors use a negative pressure fuel pump, they are pulling fuel from the tank. The fuel input and primer circuits are sealed so if you don稚 provide an air vent for the tank when you get the motor running hard it will suck the tank into itself, go very lean and then quit running probably when you least expect it.

So you need to use a vent. There are two popular ways
  1. Coil some fuel tube as shown and connect to a vent fitting
  2. Use a one way valve Stens/Echo #610-079 the XCell Spectra includes one on the kit

Vents - Loop (Left) - Stens - preferred (Right)

There are some dependencies here based on what you do so be careful with this part.

To summarize the choice here:
  1. If the tank is hard plastic I recommend the one way valve and an additional fill fitting on the tank
  2. If the tank is soft (flexible), use the looped tube and you won稚 need the additional tank fitting.

Plumbing with loop vent - no fill needed

The simplest way to vent the tank is with the coiled tube. Its simple and easy. In my opinion its not very neat but it is simple. It will also work perfectly regardless of whether the tank is flexible or hard plastic.

In case you池e wondering why its coiled, if you just run a straight line gas will simply run out when you invert the model not only making a mess but it could easily ignite on the hot parts. So using 2-3 coil loops pretty much keep any from ever making it all the way out. The coil can be any size just make sure you loop it at least twice

Another advantage of this vent is that since its always venting, you can use the tube on the T fitting from the fuel return line to fuel the tank instead of making an additional fuel fitting on the tank.

Plumbing with one way vent - fill needed

The neatest way (in my opinion) is to use the one way valve. Virtually EVERY lawn implement you have will have one of these little vents on them. They are cheap and easy to get.

It does have two prerequisites:
  • It won稚 work with a soft plastic/flexible tank. It takes a certain amount of vacuum pressure to open the valve. Over time the valve gets sticky and it takes more pressure to open. With a flexible tank, the tank will literally distort instead of venting. This will cause a slight lean condition that痴 very hard to diagnose. The Dubro and Sullivan RST tanks are hard enough. The Nalgene bottles that some mfgrs use normally are not.
  • The other is that it痴 a one way valve, air can稚 get out. So when you fill the tank you have to have another open line. The T fitting on the return line provides one of the two you need. But you値l either have to remove one of the other lines to fill into or my suggestion is to add a fueling fitting on the tank connect a tube and then plug the end. Now you致e got a vent and a fill both with removeable plugs. Locate them somewhere accessible on the model and fueling becomes simple.
The bottom line:
  • There are many ways to plumb the fuel tank
  • This way is not the only one but it is virtually guaranteed to be easy to use and problem free
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Carey Shurley
Proprietor - Gas Powered Helicopters

Last edited by carey shurley; 03-19-2012 at 05:55 PM..
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