The most common solid fuel lines used in metal airplane kits today are the fairly soft 3003 aluminum tubing that comes in rolls that are usually a bit more than a foot in diameter. Usually, builders try to straighten that tube out (successfully, using a variety of tricks) to make nice straight runs. Tight corners are done with tubing benders. But how about nice medium-to-small-sized loops?
Our F1 Rocket project uses a fuel vent line appropriately referred to as the “rocket vent” because it has been used successfully in Harmon and F1 aircraft for decades. It consists of a couple of loops of quarter-inch 3003 tubing at the wing root, connected to the vent line that comes out of the inboard end of the tank (after running inside the tank from the outboard end). The loops provide a “trap” to allow air in and out, and (mostly) keep fuel in. To fit inside the wing root, the loops need to be about 5 or 6 inches in diameter … so how do you make nice, consistent circles?
Look around your shop … certainly, you have something the appropriate diameter! In my case, my eyes fell upon a lightweight tailwheel that was sitting on the shelf. The soft tire looked just like a bending mandrel to me, so I chucked it up in the bench vise. I then simply wrapped the tubing around the outside of the rubber and just like that, I had a couple of complete loops plus partials at each end to which I could add AN fittings to connect to the vent line on the tank and a vent fitting on the bottom of the wing. The rubber did a good job of keeping the tube from slipping as I bent it … almost like it was a ready-made form for the job!
FAA Part 23.951 and 23.955 concern design requirements for fuel venting systems in certified aircraft. The guidance is performance-based and does not specify a calculation for vent size related to fuel flow specifications. Nevertheless, airframe manufacturers often size the vent line using the same tube diameter as those used for the fuel tank supply line to the fuel valve.
This is not always the case with Experimental aircraft and your photos show a very common fuel tank vent line for the RV line of airplanes as well as other makes. What has been your findings with fuel tank venting particularly in light of the emergence of unleaded fuels and changes to vapor/suction characteristics?
Your’e correct that tank venting is basically performance based – and I have never heard of an E-AB that didn’t have adequate performance from its vents (unless they were plugged by debris or forgetting to remove a cap….), despite them traditionally using smaller tubing than the tank outlet. The truth is that air is considerably less dense than fuel, and the flow rate of either (unless you are feeding a rocket engine) is actually pretty small, so its not surprising that the typical 1/4” diameter lines have proven to work fine in thousands of airplanes. Just keep the bugs away and don’t forget to remove the cap before flight!