Fuel tanks part 7 and trailing edges

A few weeks between blog entries but lots of progress on the fuel tanks.

I've been pondering the cut outs in the tank for the fuel senders and the fuel filler necks.  Unfortunately neither of them can be cut in the upper tank skins using a standard hole saw (or at least one we have in the shop) so I needed to use the fly-cutter.

I adjusted the centre of the fuel sensor hole a little more forward on both tanks so the fuel sensor gets as close as possible to the front wall of the tank which is also the deepest part of the tank.  This also gets the mounting plate to a location on the upper skin that is as flat as possible, important for sealing the mount to the tank. 

I wanted to come up with a way of sealing the sensor plate to the tank that will not only be fuel tight but allow me to open the tank for inspection if needed in the future or if I ever need to service/replace the fuel sensor itself - not likely, but nice all the same having access to get a hand or tool inside.

I'll be securing the fuel sensor assembly to the tank using "well-nuts".  These neoprene fasteners are used in the automotive industry to join parts together and seal holes.  They are a neoprene tube with a shoulder washer and captive nut inside:

To test the plan I grabbed a couple of scraps of aluminum to represent the tank skin and the fuel sensor plate.  The well-nut gets inserted in the hole up to the washer head (also helpful as it can't fall inside the tank).  A 8/32 machine screw is threaded into the well nut and as it tightens down to compresses the washer head to seal the top of the tube.

The magic on the backside is that as you tighten the machine screw, the captive nut inside tightens up against the underside of the hole, compressing the neoprene tube and forming a seal on the underside of the hole.  Good and fuel tight, but I'll add some thread sealer on the machine screw to be sure and my machine screws will have wider heads and be shorter in length.

Laid out the fastener lines on the underside of the fuel sensor plate.  The well-nuts require a larger hole for insertion so I needed to ensure edge clearance would work.  The fastener line will approach but not clear the sensor mount tube, but I can get a well-nuts close enough to each other to ensure a tight seal around the perimeter.

Usually the fly-cutter is used on a drill press.  The tank is too bulky to mount safely on the drill press bed.  I checked online and see several people cutting the sensor holes in the sides of their tanks using a hand drill - the key is to go real slow and keep the cutting head well lubricated with cutting oil.  Mine worked out fabulously with the hand drill.  Speed control is important here as a tear in the skin would be disastrous. 

Back drilled pilot holes in the mounting plate at the corners first then placed it in the newly created hole.  Once squared in position, used the pilot holes as a guide to match drill the tank:

Cleaned away most of the Sharpie lines and retraced the outer edge of the mounting plate.  Then extended landmark lines from the corner pilot holes.

Mounted the sensor plate onto the tank, transferred the landmark lines onto the front of the plate and marked out the fastener locations:

Drill the balance of the fastener holes up to A3 using clecos to secure it as I went.

Very happy how this turned out.  Waiting until I get an order of well-nuts in stock before I drill these out to the larger size.  Once those are done, I'll cut out the access hole in the skin that the sensor plate will cover.

Now that the final size and shape of the fuel tank is established, it's time to make the fuel bay in the wing ready to accept and secure the tank from moving around.  First up, two U shapped channels are added to the spar as front tank supports.

Thin cork strips secured to the support channels with contact cement to prevent vibration damage on the front wall of the fuel tank:

Rear support angle gets the same.  These front and back supports have some spring to them holding the tank securely from moving back to front in the wing bay.

With the tank now in final position in the wing bay, I was able to peel back the top skin, reach inside and mark the location of the tank drain hole in the lower skin of the fuel bay (tank pulled up and forward to take this picture)

Here is the initial hole.  Once I secure the tank completely, I'll flip the wing over and final size the hole to match the drain plug.  Also needed to open the passthrough hole a bit to give more clearance for the fuel port fittings.

Flipped the wing over on it's back again so I could add the trailing edge to the wing and the last of the flapperon support arms.

​The reason to wait on the last flap support arms is the complex joint where the lower wing skin, trailing edges and rear strut support meet.  The inboard trailing edge needs to fit over the rear strut attachment first, then the outboard trailing edge, then the lower wing skins on top.  Order of operations stuff keeps me up at night sometimes!  With this done I can final rivet the lower wing skins and flap support areas that are already in place.  The trailing edges are left clecoed together for now until final adjustments of the trailing edge position.

Wing back upright again with top wing skin lifted up to add the final flap arm in place and adjust the trailing edge.

Last flap arm placed in position for riveting

Trailing edge adjusted for straight and correct up/down relative to the flap arms, then clecoed to the rear channel.  Fuel bay skin also put in place to match up with the trailing edge.

Cork strips added on the floor of the wing tank bay.  The fuel tank will rest on these after the contact cement that holds them in place dres.

​Some light weight to prevent the cork strips from curling up before they set.

The inboard strip will be fastened later once the final drain hole size is drilled out.  I'll add a collar of cork around the final hole to help support the lower skin here.

One of the challenges of an un-welded tank is fitting it in the wing bay without clecos.  In order to fit the upper wing bay skin over the tank, I can't have clecos sticking up, so I used a large amount of tape to secure the tank edges and fuel sensor assembly together temporarily so I can finalize tank position before sealing the tank up.

Like the bottom of the wing bay the space between the tank and upper skin will be filled and supported by cork strips, these ones will be cemented to the top of the tank once it is sealed.

Next was layout of the flush mount fuel filler neck and threaded mount flange - an order of operations that has killed off a lot of brain clock cycles in the past month.  The key here is to align them with the tank in position in the fuel bay.  The finished edge of the fuel neck needs to be far enough away from the edges of the wing bay skin, so I start the layout position there, keeping consideration of where the internal edge of the fuel tank are and how the threaded inner flange will mount in the tank.

My original layout called for a centering hole 60mm aft of the spar and 80mm inboard from the adjacent wing rib..

Once I had the upper wing skin in place, I realized that doing that would put the threaded tank flange too close to the side wall of the fuel tank to fasten and seal it correctly, so I moved the whole assembly inboard by another 10mm and drew another centering mark.

With the confirmed centre of the fuel filler assembly now confirmed, I drilled a A4 pilot hole in the upper wing skin through the gap between and on through the fuel tank itself.  This centering hole now aligns both the outer fuel neck and the inner threaded flange. 

The tank was removed from the wing again.  Using the fly-cutter and hand drill, a 60mm hole was created in the tank based on the pilot hole drilled in from above the wing skin in the previous step.

A quick check confirms the threaded male side of the filler neck nicely fits the tank

The upper wing skin is clamped onto the bench and the fly-cuttter is used to cut the upper flange hole.  This is 70mm in diameter, larger to fit the finished rim of the fuel filler neck and allow it to sit flat against the wing skin.

With the inner threaded flange taped in place, I placed the wing tank back in the wing and added the wing skin back over the tank.  This confirms the hole in the tank and the hole in the skin line up as expected.

Just for satisfaction, I threaded in the filler neck - this looks absolutely amazing!

Ideally, I want the tab that opens the flush fuel cap to face towards the rear like in the picture.  This has 2 important reasons and 1 cosmetic.

When in the stowed position, the tab is less likely to get caught in the slipstream over the top of th wing and inadvertently pop up - not likely, but why risk it.

Secondly, I want the vent tube I plan to add to the cap to be at the front of the cap facing into the slip stream.

Cosmetically, I want both caps facing the same way - it's just more pleasing to the eyes :)

The fuel neck threads into the tank fitting and will tighten down as it goes, so to have the tight position line up with the above three criteria for the cap, I needed to figure out how far the neck threads into the tank flange exactly, then fasten the tank flange to the tank at that rotation.

I know the gap between the tank and the wing skin will be filled by cork gaskets.  So I created  three gaskets that will eventually be the ones installed.

Glued together with contact cement and laid in place where they will sit on the top of the tank.

Another layer of cork on top of the three.  This 4th layer represents the wing skin aluminum (0.020) and a 1/16inch rubber gasket that will sit between the wing skin and the threaded fuel neck when it is in final position.

Once in tested position, I back drilled two of the tank flange holes - this is where the tank flange will be on final install.

Back drilled the rest of the tank flange holes.  To hold the flange in place for final fit testing with the fuel filler neck on the wing, I popped a couple of A3 rivets just to hold it in place.  These can be drilled out easily once I'm ready to add sealant on tank flange where it mounts inside the tank.

My mock-up worked perfectly.  When I get the 1/16 thick rubber gasket material the gap between the threaeded tank neck and skin will disappear and the cap tab sits exactly where I want it to - excellent!

The wing is almost ready for final fit of of the wingtip, trimming the inboard end of the trailing edge and other minor clean up items.

 
I've been feeling a bit of what it called imposter syndrome.

YouTube videos of other builders out there show amazing work and the speed at which they are accomplishing their builds is something else to behold.  It makes me feel a bit like my project is taking way too long some times and that I'm not actually doing anything or accomplishing anything.  I'm not actually building something amazing, I don't actually have the skills to do this, someone else is the builder - I'm an imposter.

But as some people close to me keep reminding me - I'm not an imposter at all.  I'm doing this and I'm creating most everything from scratch.  Those YouTube builders for the most part are buying a kit that is pre-formed, pre-drilled and just needs to be put together.  I'm creating a plane from pure raw materials.  Looking back, i've done tons of work so far.

I'm AM doing this - no one else.  I AM learning the skills and applying them.  I'm not an imposter - I AM THE BUILDER!

More to come, thanks for listening!