Giant scale gasoline systems

Aug 05, 2014 3 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.

3 Responses to “Giant scale gasoline systems”

  1. Rick Alba says:

    Using Du-Bro fuel line barbs (two sizes available) soldered on the brass tubing ends provides extra insurance against tygon swelling and still using the wire ties.

  2. jeb says:

    You forgot to show the detail of the “T” fitting and the fueler.

  3. jeb says:

    Oops, I guess you did mention the fueler. My bad.

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