Over the past week or so I’ve done a lot of work on the left wing leading edge assemblies – the main fuel tank, and the outboard leading edge assembly which incorporates an auxiliary fuel tank. Although I outsourced the construction of these, there were a lot of problems which I’ve had to address – leaks, mis-alignments, riveting etc. I finally got to the point where I was happy with each assembly and it was time to combine them with the left wing box section.
I did a trial fit of the two assemblies, to make some adjustments and ensure they fitted with each other OK (I had already done a trial fit of each assembly to the main spar). I then primed all of the overlapping surfaces, and the open bays. They were supposed to be primed during (outsourced) construction, but that didn’t happen either. I was a bit late shooting the primer on during a winter’s afternoon here, and the temperature dropped rapidly, so I set the tanks up with a heater to keep them warm. The primer cured OK. The biggest problem with priming occurred the day before, when I drained the rusty water out of the compressor. There was quite a bit in there, so I got impatient and unscrewed the drain valve a bit too far – it flew off and with pressure in the compressor, water went everywhere and cost me an hour of cleanup time. I won’t be repeating this mistake anytime soon.
After priming, I installed the stall warning vane, microswitch and wiring. I also installed the auxiliary transfer pump. During normal operation, the main tanks are vented through the auxiliary tanks, so fuel in the auxiliary tank will syphon through into the main tank. The transfer pump is only there as a backup in the event that an air leak into the main tank (leaky fuel cap for example) prevents the auxiliary tank fuel from being drawn into the main tank. Rather than land with fuel in the auxiliary tanks, the transfer pumps can be used as a backup to empty the auxiliary tanks into the mains. The fuel lines are accessible through the stall warning inspection cover.
With the two leading edge assemblies bolted together, and the fuel lines in place and properly torqued, I pressure tested the entire assembly – both tanks and fuel lines. After fixing a minor leak due to a bit of proseal interfering with a fuel drain O-ring, the entire assembly held pressure with no leaks!
Then came the acid test. Will the two leading edge assemblies and the wing box section fit together properly? I set them all up on the work benches, they came together with surprising ease, and fitted to perfection. I modified the wing stand, fitting it with a section of carpet now there is a leading edge on the left wing, hefted the wing into the wing stand, fitted all the tank bolts and screws, and cleco’d the outboard skin in place. Now it looks more like a wing!
Next step is to rivet the outboard assembly in place.
A few other notes on the auxiliary tanks:
- I fitted a few #8 platenuts to the flange of each rib adjacent to the tank bays in the auxiliary tanks. NAS1801 screws secure these through the main spar.
- The auxiliary tank is not easily removed, since it is riveted in place. This is obviously a compromise, and is one of the reasons I did a lot of pressure testing of the system. If a fuel leak occurs in the aux tank rear baffles, the options for repair are limited – drill everything out or cut an access panel in the bottom. The fall-back is to remove the fuel lines and convert the wing back to a standard arrangement with an empty, unused auxiliary tank. To this end, the vent fitting has been included in the main tanks even though it is not used (blocked off) at present.
- Each auxiliary tank holds approx. 65 litres. A float type sender is included, there is a drain valve and of course a separate fuel cap.