Fuel Tubing

(mid-November 2006) There have been three spiral packs of Aluminium tubing sitting in my shelves for the past 12 months, and I have been wonder exactly how I would attack this unknown area. The time came when iI had finished the wing fairings - the next job was to "install the fuel vent and fuel delivery lines". The tubing is straight Aluminium tubing with a wall thickness of about 3/64". It is soft and reasonably pliable. It will form the plumbing for fuel delivery from the tanks to the fuel pump (3/8"). and it will also form the fule tank vent (1/4"). I have somespiral spring steel "benders" - devices that slide tightly over tubing and distribute the stresses and strains when it is being bent. I also have a Parker flareing tool. This device 37 deg flares the finished end of each piece of tubing so that it fits snugly withing the proper AN aircraft tubiong attachment fasteners.

Stop and Think

I had a long hard look at the plans and the plane at this stage. I had never done any fuel delivery tubing work before. The consequences of not doing it properly could be catrastrophic. Moreover, it was not immediately obvious to me exactly how to approach these tasks. There are many issues to consider:

  1. How to get perfect ends on each tube, and perfect flares
  2. How to get each piece of tube bent into shape and inserted in position
  3. Remembering to put all the loose bits onto the tube before flaring each end
  4. Measuring the various lengths between ends and bends
  5. Bending the tube without crimping

The following pieces of tube need to be cut to length, inserted, bent and attached to various unions and fittings:

  1. Fuel delivery from Left tank to fuel tank selector fluid switch (3/8")
  2. Fuel delivery from Right tank to fuel tank selector fluid switch (3/8")
  3. Fuel delivery from tank selector switch to fuel pump (3/8")
  4. Fuel tank vent to fuselage skin union (one on each side - 1/4")
  5. Fuel tank vent lin inside fuselage to vent fitting in forward floor skin (one each side- 1/4")

 

Fuel vent tubing 1

I decided to tackle the "fuel tank vent to fuselage skin union" tubes first - because they were short, and carry air, not fuel. Image 1 below shows the relevant drawing. This piece of tubing is about 8" (200mm) long and joins the tank vent 90 deg union to a union fitting which passes through the fuselage, enclosed by the wing fairing (see drawing, image 1). This tubing runs forward over the black tank attach bracket (image 2) and into a union in the fuse in front of the bracket. The first major considertaion is where to drill the hole to take this union. The bracket is supported inside the fuse by a piece of angle (image 3), and the wing fairing covers an area in front of the bracket in which this union must be inserted. Van's don't tell you this, and their drawing suggests a lower position. However if you put the union in place too low, it would lie across or even outside the position of the wing fairing, and this would not be good for obvious reasons (we do not want fuel vent tubing sticking out from a wing fairing)). In the end, I measured the limits of the wing fairing position (image 4) and decided to place the bulkead union and its support washers in the middle of the available space (outlined in red in image 4). This was probably a little high, because i did not then have enough space between the union position and the lomgeron above the union within which to fit a nice even bend around the longeron. I had to take the tubing forward and up at a 45 degree angle to allow sufficinet space for the bends clearing the longeron and rudder cables. Image 5 shows the support washer in position, and image 6 shows me using the right-angled drill to drill a pilot hole in position. This was widened to 7/16" with a Unidrill inside the fuselage (image 7), and image 8 shows the union in place, with its thick aluminium support washer (outside).

fuel vent drawing

fuel tank attach brkt..

and support angle inside

position of fairing in red

support washer posn

drilling the hole

widening from inside

union in position

Image 1 below shows the right-angled union seen from inside the fuse, pointing up and a little left. Image 2 shows what NOT to do when cutting tube to length. DO NOT USE a rotating tubing cutter. It work-hardens the tube, and leaves a large inward-pointing lip which does not flare properly. Image 3 compares a tubing end made with the tube cutter (right) with one made from tubing cut with a hacksaw, and finished on the a belt-sander and Scotchbrite wheel. The left tube end has a clean flat end with no distortion, unlike the right tube which was cut with a roller cutter. Image 4 shows my Parker flaring tool and it's instructions. Image 5 shows the business end of the tool. Two rotating elements have dies ground to various diameters (1/4" is in position). These clamp the tubing in place - shown in open position. There is a "cone" beyond these dies which, when the tubing is clamped in place, is used to mould the "flare" on the end of the tube. It rotates and moves gradually into the tube, forcing a "flare" shape with a shiny 37deg internal surface which perfectly matches the fittings' conical surface. Image 6 shows a first attempt to cut a length of tubing to fit between tank vent union and fuse wall union. It was too short!! So I cut another length a little longer and it fitted fine (images 7 & 8).

fuel vent union

DON'T DO THIS!

left=good, right=bad

flare tool

1/4" setting

too short!

better now

vent fuse union

 

Fuel Tubing

The 3/8" fuel tubing is a lot tougher and harder to bend than the 1/4" vent tubing. Three lengths are required. Two pass from the fuel tanks, through the fuselage sides (Image 1- before making right angle bends), making two right-angled bends before running under the covers in front of the main spar to the fuel selector switch inputs. A third length runs from the fuel selector switch outlet, along the forward floor of the cabin, to the fuel pump which is mounted near the central firewall. This third length seems the simplest, so I started with it. Image 2 shows this length lying along the cabin floor, connecting to the fuel pump, with a spiral tuning bender in place. Image 3 shows this length of tubing on the bench, after it had been bent with these spiral tubing benders. Image 4 shows the final product, a complete tubing run from fuel selector switch to fuel pump. Amazing! Image 5 shows a piece of ordinary packing foam being attacked with a standard laboratory "Hole Cutter". The silver hole cutter is rotated and moved into the foam, removing a perfect round tunnel of foam from the block. This was cut lengthways and opened to allow the previously-formed forward fuel tubing to run through the foam. This will support the tubing and isolate it from floor and cover sheetings (image 6).

The next job is to make the two lengths which pass through the fusealge wall on each side. First I used a 1" hole saw to cut holes through fuselage sheeting and " gear attach web" sheeting on each side (see later). I then trial fitted the large 1.25" grommets (meant for these holes) which support the tubing as it passes through the fuselage sheet (image 7). In image 8 you can see two of these grommets in place - one for the fuse wall, and one for the cover plate located inside the fuselage. The tubing then has to receive TWO right-angled bends so that it runs neatly through the fuselage and down through the cover plates and along the fron surface of the main spars.

3/8" tubing thru fuse

3/8" tube to fuel pump

using the spiral bender

blue foam supports..

.. replaced (better foam)

in position on floor

flared with grommet

ready to bend

 

I did this (right-angled bends) by carefully measuring the distances involved (image 1), and just bending to suit (image 2) using the 3/8"spiral bender. You have to disassemble the cover plate support brackets from the front spars. They have support holes and plastic grommets for this tubing. I fed the tubing through these bracket holes before re-installing. A GENTLE curve (image 3) in the tubing helps to get it into position without crimping. Further coaxing renders the tubing into its final shape, and the ends going to the fuel selector switch have to be flared almost in position (image 4). Image 5 shows how tubing finally passes though the cover plate edge, via a grommet, hiding a multitude of sins!. Image 6 shows the tubing for the other side being fitted. It was bent into final position (image 7), fittings were added and the tubing finally flared. It needed a little finesse to bend the tubing into position so that there was no stress or strain required to mate the surfaces of flared tubing and blue joint to the fuel selector. No worries!

measure length to bend

bend on bench

position

final bend to fuel switch

cover in place - nice!

left hand side too

final bend

flared and connected

 

Fuel vent tubing 2

The fuel vents are connected to quite long runs of tubing which run up, along, and down the forward cabin walls, emerging as two modified 45-degree-angled union fittings which poke out into the slipstream and, I suppose, pressurise the fuel tanks. Image 1 shows these fittings, after modification. Image 2 shows how I had to tilt the right-angled union carrying venting though the fuse wall, in order allow enough room to bend the tubing around a longeron and the rudder cables. Image 3 shows an intermediate stage of making these runs - note that the 45 degree bend on the right is TOO CLOSE to the rivet hols on the adjacent bulkehead. I had to modify this run to be able to get to these holes. Next time I would leave more room around all rivet attachment holes. Image 4 shows two pieces of fly-wire gauze cut to shape ready to be epoxyed onto the 45-degree end fittings. Image 5 shows one whole vent run, removed from the cabin for final bending, and almost complete. Finally, image 6 shows the cabin side of one of these fittings. The 45-degree profile is underneath this side of teh fitting, and placed so that the slipstream will pressurinse the fuel tank. Finished fuel tubing - not as hard as I thought it would be.

vent intakes

oblique - too close to longeron

fuel vent tube initial posn

a little bit of gauze to filter

bent and on the bench

finally connected