January 27, 2020

Running Fuel Lines through Fuselage

I mentioned in an earlier post that I decided to go with hard lines for fuel and brake lines. There are definitely a few challenges to accomplish this, since the kit was designed to use rubber tubing , so routing is a little tricky. I’m using 3003 Aluminum Versa-tube in 3/8″ OD size for both supply and return fuel lines. While you can bend this tubing by hand, I’m using a proper tube bending tool especially for some of the 90 degree bends that are needed. I pulled some close-out panels off the shelf and temporarily put them in place to make sure the tubing will fit in the space available.

Close-out panel that goes below the front seats
Holes through which the fuel lines are to be routed

Per the build manual, the supply line should get routed through the bottom hole in the photo above, and the return should be routed through the top hole. The close-out panel just behind the holes doesn’t leave much room, especially for the return line. To make things fit, I added some bends to the tubing – about 25 degree bends on the supply side, and 60 degree bends on the return as shown in the next photo.

This photo shows how I shaped the fuel lines to fit behind the closeout panel

After figuring out the bending, I next had to figure out how to secure the lines where they pass out to the wings. Originally, I planned to put bulkhead connectors here, but there’s not enough room (again, due to that pesky closeout skin). So, instead I came up with a little bracket to which I could attach cushion clamps, which keep the tubes in place. I will flare these ends and install a transition to a rubber hose for the connection to the fuel tank. I’m also going to put the fuel filters in this rubber hose segment between the fuselage and wing, rather then inside the cabin per the kit manual. To me, the cabin seems like a bizarre place to put them – it’s hard to access, and has the potential for leaking fuel inside the cockpit.

Custom bracket accommodates a pair of Adel clamps for securing the fuel lines where they exit the fuselage
Success! The fuel lines fit comfortably behind the closeout skin

I was forced to add one break in the lines behind these closeout skins, due to a sharp corner the lines have to go around to enter the center tunnel through a small opening (circled in the next photo) below the gussets. I didn’t think this through, and now I realize I should have just drilled out a few rivets holding the gussets so that I could route the fuel lines continuously up to the fuel selector valve. It’ll be a pain, but I’d rather redo this now instead of having a leaky joint in a hard-to-reach location. I don’t have the fuel selector valve yet , so I’ll have to wait until it arrives before I can fix this, since I need to install this valve and measure the exact length of tubing needed to get from the valve all the way out to the wings. The fuel selector should be shipping from Andair in the next week, so I won’t have to wait long.

The corner that the fuel lines have to bend around. You can see the blue union just behind the circled area that I want to eliminate.
Looking at one of the unions just in front of the autopilot servo location. I’m going to redo these lines so that I don’t need this union joint

Another challenge I encountered during this exercise was flaring the tube ends for the union joints. Although I bought a highly-rated Ridgid 377 flaring tool several months ago, the flares were coming out like crap. The flaring tool (in the next picture) has a clutch that releases once you torque down the screw far enough. However, I discovered through a little reading online (and a little deduction) that the torque value this tool is calibrated to is for much harder materials like stainless steel; by comparison, this 3003 aluminum is very soft, so I was cranking down way too hard, resulting in flares where the horn was paper thin and not well formed. I was ready to toss this tool and buy a Rolo-Flair instead, but I figured out I need to stop torquing down the screw well before the clutch releases. Most other tools don’t have a clutch, and it’s up to you to figure out when to stop screwing down based on resistance. So, I’ll give this tool another try, and will make a bunch more practice flares before I redo the tubing.

The Ridgid 377 flaring tool I’ve been using
A sample badly-formed flare.. the aluminum has been squished down way too much, resulting in the horn being paper thin, too big, and unevenly shaped

Brake lines are next. I think I need to temporarily put in the dash, so that I can temporarily put in the throttle plate, so that I know where to run the brakes lines up to the brake handle and parking valve.

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