Giant scale gasoline systems

Jun 05, 2012 9 Comments by

On a recent rebuild project of a Ziroli 77 inch Stearman PT-17 biplane, I replaced the the fuel tank and plumbing. Here’s how to install a new gasoline fuel system in your plane.

(Above.) For the Zenoah G-38 engine powering the Stearman, I chose to install a 20 oz. fuel tank. I also used Sullivan’s heavy Duty gasoline tank hardware kit.

(Above.) I used a 2-line tank installation and used a DuBro fueler fitting shown above.

(Above.) Notice I am showing Tygon fuel line as well as clear plastic tubing from the hardware store. Both are acceptable. I use at least 1/8 inch ID (inside diameter) tubing.

    

(Above.) Here’s the Sullivan fuel tank hardware and the tools I use to cut and bend the brass tubes. K&S and Du-Bro make these tools and they greatly simplify the job.

    

(Above.) To determine the length of the upward bend of the vent tube, just estimate it by placing the tube as shown, fro the lower edge of the stopper opening and the top of the tank. Some tanks have a raised dimple on the top of the tank for the vent tube to fit up into. Just make sure you do not install the vent tube so it contacts the inside surface of the tank, Vibration will cause the tube to damage the tank wall. (Above right.) Here the tube has been bent to 75 degrees using the bending tool.

    

(Above.) Assemble the stopper and tube assembly and estimate the length of the outlet and vent tube lengths. They should be just shy of the end of the tank’s extended lower section, so the tubes do not push against the firewall when installed in the fuselage.

    

(Above.) Cut the tube to length with the K&S tubing cutter then use a sharp X-Acto knife to clean out the end of the tubing cut. The rotary cutter compresses the tube and this mustt be opened out so fuel floe is not restricted.

    

(Above.) Cut the output tube to length and use fine sandpaper to smooth the ends of the tubes so they do not chafe the flexible fuel lines. (Above right.) Here’s the complete stopper and clunk assembly. Cut the length of the pick up tubing so the clunk just clears the back of the tank and is free to flop around as the model moves during flight.

(Above.) Here’s the completed tank with fuel lines attached and secured with cable ties. Make sure to leave the fuel lines extra long so you can feed them into the model and out of the firewall and vent holes.

    

(Above.) Feed a flexible pushrod into the firewall hole and snake it into the fuselage. Place the tank in the wing saddle and attach the fuel out tube to the end of the pushrod and pull the tubing back out of the hole as shown (Above right.)

(Above.) Here you can see the fuel tank installed on a support plate (right side of the wing saddle.) I use stick on double sided foam tape and heavy duty Velcro to hold the tank in place while making it easily removable if needed. Also remember, when looking into the bottom of the wing saddle, your tank should be upside down.. When the fuselage is upright on its gear, the tank will be rightside up.

 

When you install the fuel fitting to full and empty the fuel tank, make sure to install the fuel lines so the fuel flows into the tank when the fueling probe is attached to the fitting. A fuel filter should be installed between the fuel  fitting and the carburetor.

OIL-TO-GAS RATIOS

Often you hear about the fuel’s oil-to-gasoline ratio. But what is it, exactly? Simply put, it is the amount of 2-stroke engine lubrication oil (measured in ounces) added to a gallon of gasoline. Here are some common ratios used with RC gasoline engines.

Here’s a quick reference chart for mixing up your fuel’s gas and oil ratios.

RATIO                   OZ./GALLON OF GAS

100:1                     1.28

90:1                        1.42

75:1                        1.7

64:1                        2

50:1                        2.5

40:1                        3.2

32:1                        4

24:1                        5.3

16:1                        8

 

With a couple of flips of the prop to prime the engine, It took only a few flips of the prop to start the engine. Check the fuel flow and watch for bubbles in the fuel lines. Any bubbles indicate you may have a pinhole somewhere or a loose fitting bleeding air.

Let the engine warm up for a few minutes and then adjust the throttle trim for a solid reliable idle. Adjust the low end needle-valve so you have a smooth transition to full power. That’s it! The first step to having a successful flight is installing your fuel system properly! Hope you found this helpful! What are you waiting for? Go fly something!

From the Magazine, Gerry Yarrish, How-tos

About the author

Senior Technical Editor About Me: I have a lifelong passion for all things scale, and I love to design, build and fly scale RC airplanes. With 20 plus years as part of the Air Age family of magazines, I love producing Model Airplane News and Electric Flight.

9 Responses to “Giant scale gasoline systems”

  1. Ken Park says:

    Hi Gerry

    I just installed a new gas tank in my 26cc gaser model – The one extra step I was shown was to add barbs to the brass tubes. This easily made by wrapping very fine telephone type wire around tightly 4-5 times and soldering them into place. The other thing is the vent line should be looped over the top of the tank before it exits the plane. This prevents gasoline spilling out while inverted.

  2. Gerry Yarrish says:

    all good points Ken, thanks for your comments!
    GY

  3. Michael Boland says:

    Nice job, but just one little point.
    The fuel filter should be on the other side of the filter, otherwise anything that gets filtered during the fill of the tank will get flushed straight into the carby when the motor is run.

  4. Michael Boland says:

    Sorry, I meant
    The fuel filter should be on the other side of the filler valve

  5. Gerry Yarrish says:

    Thanks Michael. Actually I also have a filter in the fuel supply line that plugs into the fueler fitting, so there’s little chance of debris getting into the carb. Plus you have the internal filter screen in the Walbro carb too… Clean gas is good!
    GY

  6. Aaron Greer says:

    I noticed that the photos do not show a zip tie on the internal fuel tube for the clunk. Why are the zip ties used on the outside tank connections, but not inside on the clunk tube?
    Thanks.
    Aaron

  7. Larry Hebert says:

    I fell the need to Michael stated. Your fuel filter between the filler value and tank does no good at all. Everything it might catch will be passed to the carb. If as you said you are supplying perfectly clean fuel by filtering it before it gets to the filler value then why put a filter there at all. No a filter is needed before the gas enters the carb. And yes the carb does have a internal filter but when it plugs with debrie it is a unbelievable hassle to clean. Not to mention it might decide to plug and kill the engine during a flight. The external filter can be cleaned or replaced with little trouble. So what Michael stated is the only correct way.

  8. Gerry Yarrish says:

    You guys are of course correct! I installed the fuel filter in the wrong location. It would block sediment from going into the tank, but then allow it to enter the engine. I will reinstall and take a new photo for the post! Thanks for keeping me honest guys! Have a great flying season!

  9. Bryan says:

    Hey Gerry,

    What servo are you using to activate the fuel throttle?

    Thanks,
    Bryan

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