Category Archives: Fuel tanks

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.

The fuel tanks, mains and outboard leading edge aux tanks – which I outsourced – have needed a fair bit of work. Both aux tanks had leaks, which was disappointing. To help fix the leaks, I bought a cheap hand cranked vacuum pump on E-Bay. Having marked the leak points with the tank under pressure (using soapy water), I cleaned and prepared each leak point for pro-seal. I then had an assistant crank on the vacuum pump, while I applied pro-seal over the leaks. The vacuum pulled the pro-seal into the voids really well. After curing, I once again pressure tested the tanks, using a water manometer to set up around 1 psi of pressure. The water level went up and down a few inches, with changes in temperature and barometric pressure, but across a number of days it was evident that there were no more leaks.

I made up fuel lines for the aux to main tank connections, and vent lines for the aux tanks. The main tanks are vented through the aux tanks. The “usual” vent fitting is present in the mains, in case I ever have a problem with an aux tank and want to revert the plumbing back to normal. This fitting will simply be plugged, and in normal operations the main tanks vent to the auxiliaries. I have provision for transfer pumps, which are only there as a backup in case an air leak (e.g. a leaking fuel cap) on the main tanks prevents the normal syphoning of fuel from the aux tanks into the mains. There is a float type fuel level sensor in each tank.

I tested the level senders in each tank, measuring the resistance of the sender with the tank normal (empty) and inverted (simulating full). All senders are working fine, which is just as well since they are pro-sealed in place.

Each outboard aux tank holds approximately 65 litres, so full fuel for the aircraft is around 357 litres providing a no-reserve endurance running LOP of around 9 hours. Most RV-10’s in Australia have extra fuel tanks, not in order to stay aloft for such long durations, but because fuel can be a problem across large distances in the outback. Most of the time I don’t expect to use the auxiliary tanks, but on occasion they’ll be invaluable.

I still have a fair bit of cleanup to do (clearing away pro-seal spills and touching up some riveting) and some priming, before mounting all of the tank assemblies onto the wing spars. That won’t happen until later on in the month because I have a few weeks of build hiatus coming up.