RV-8 Project N804PT

Auxilliary Fuel Tank Construction Suplement

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The following describes how I constructed the auxilliary fuel tanks in my airplane. It is written as assembly instructions, mostly because that is the easiest way for me to compose this essay.

The first wing is currently under construction, so expect some of these details to change as I revise my original plan.


The construction of the wing proceeds identically to the stock wing up to the point of drilling the leading edge ribs. At this point, only the inner-most rib (and joint plate) and the outer-most rib are drilled. The remaining ribs will be removed later in the construction. The leading edge is left in position until the main fuel tanks are constructed and fitted. This allows the main tank to be constructed in the normal manner (except for the venting system).

Now it is time to layout the initial holes for the aux tank ribs. There are three dimensions which are critical here: the aux tank baffle must be the same distance from the spar as is the main tank baffle, the spacing between the ribs must be correct so that the aft rib flanges will match the holes in the aux tank baffle, and we must not introduce any twist into the aux tank structure. The spanwise location is not nearly so critical; the dimensions I used were selected to avoid as many problems from the locations of pre-punched holes in the leading edge skin as possible.

The holes for the aft-most hole in each tank rib flange were drilled in the skin first. Using the line of holes for the innermost LE rib (the one with the joiner plate) as a reference, and moving out towards the tip, the rib-hole centerline spacings are 6", 8 5/8", 9", 9", 9", and 5 1/16". This leaves 7 7/8" to the outer-most LE rib holes. The end tank rib holes are located 2 5/8" from the aft edge of the skin (both top and bottom), while the intermediate tank rib holes are located 2 27/32" from the aft edge of the skin (both top and bottom). All holes were drilled slightly undersized with a 3/32 drill.

Next, the leading edge was removed from the wing and the tank ribs fitted. The original ribs are left in place as the tank ribs are fitted; I started from inboard and fitted the inboard tank rib first, then the first intermediate tank rib. At this point the LE rib between the two tank ribs is no longer needed and can be removed. The next intermediate tank rib is fitted next, then the next LE rib is removed, and so on. Here are some pictures of this work in progress:

Most of the rib holes were reletively easy to back drill using a right angle drill attachment, but the only way I could get to the two forwardmost holes was with a dremel tool flex attachment. The hole between the first and third rivets top and bottom will be drilled later, from the outside (measure the spacing on the main tank to get the location for this hole).

Once all of the ribs have been fitted, the aux tank baffle must be fitted. This is a critical step, because the exact hole locations between the skin and baffle will determine how straight the LE section will be. First drill only the center 1/8 inch hole in each of the aux tank attach brackets, and cleco the brackets and the baffle to the tank ribs (must be clecoed from the inside of the tank). The orientation of the brackets is not critical yet, but they will ensure the proper spacing between the baffle and the spar.

Now, the entire LE assembly is clecoed to the spar. MAKE SURE that the wing is straight (no twist) before proceeding to the next step. The end hole in each corner of the baffle may be reached from the outside of the wing, using a right angle drill attachement. Drilling and clecoing one hole in each corner will provide enough rigidity to remove the leading edge assembly and drill and cleco the remaining holes in the baffles. At this point the shape of the leading edge and tank is fairly well set and rigid, so the componets may be dissassembled and the remaining holes from the tank attach brackets through the ribs and baffle may be drilled. The best way I have found to do this is to drill and cleco the holes through the ribs and baffle which do not involve the attach brackets before the tank is disassembled, then remove the baffle and ribs as a unit, then drill the remaining holes.

Next, make sure that the brackets and baffle are well clecoed to the ribs with the clecos inside the tank, and put everything back together again. The brackets must be aligned in the proper directions so that the distances to the flanges can be carefully measured and marked on the leading edge of the spar. Once all of the locations for attachements between the spar and the attach angles have been marked, drill a small pilot hole in one location for each attach angle. Cleco the leading edge back on the spar, and mark each angle through the spar (from the aft side of the spar). Remove the leading edge and check that each marked hole is in the proper location for each attach angle. When you are satisfied with the location marks, drill all remaining attach angle holes in the spar. Once more the leading edge is clecoed to the spar, and this time I put everything possible back in position (end ribs, clecos to hold the end ribs in position on the spar, and the main tank) just to make sure that the leading edge was in the proper position. At this point the holes between the spar and the attach angles were all match drilled.

The tank can now be taken apart for fitting of internal components. I was a little worried about machine countersinking the thinner material of the leading edge, so I dimpled the leading edge and aux tank baffle instead. This makes final assembly more difficult, but with the thinner material it is not as bad as the main tank final assembly would be with dimpled skins.

A hole for the fuel filler cap must be made in the leading edge skin. I did this by first making a template from thin aluminum of the cap hole in a regular tank skin (including the rivet holes for the adjacent ribs and the cap flange):

This template is simply clecoed to the aux tank at the appropriate location:

Now the holes for the cap flange are match drilled and the hole cut-out can be easily marked (I did the initial marking and rough cutting with the plastic coating in place, then removed the coating and re-marked and finished the cut-out).

I partially opened the structure of the tank to make measuring and drilling the floor stiffeners easier (note the partially completed hole for the fuel filler cap):

The holes for the forward line of stiffeners are 12 3/16" forward of the trailing edge of the skin, and the holes for the aft line of stiffeners are 7 3/16" forward of the trailing edge of the skin. Once again, this should not cause a problem with the pre-punched line of rivet holes for the standard leading edge assembly. Here is what the aux tank looks like with the stiffeners clecoed in:

Access to the volume between the main tank and the auxilliary tank will be required for the transfer pump, tank vent, and fuel lines. A small hatch is constructed in this area:

This cover is constructed very simply with a backing plate with a cut-out in the center (outlines of the backing plate can be seen as the lines drawn on the skin in the picture above), with the actual cover held on by the same screws and platenuts which hold many of the other covers.

The NACA scoop for the fuel vent is constructed next. Since it is desired that no fuel or fuel fumes enter the actual wing structure, an airtight and fueltight box was contructed to sit inside the NACA scoop inlet and provide the ramp for the scoop. Here are the parts for the airbox (note that the doubler plate of .025 sheet does not follow the contour of the inlet at the trailing edge):

And the completed airbox:

The vent with airbox clecoed in place:

And an overview giving an idea of the location of the vent on the wing:

The section of the auxilliary tank vent inside the tank is effectively identical to the vent of a standard RV-8 tank. However, the vent for the main tank and the fuel pick-up for the auxilliary transfer pump are plumbed to the sump of the auxilliary tank. Here you can see the main tank vent (more or less vertical) and the aux transfer pick-up (with a finger screen similar to a standard RV-8 tank):

And here is the view of the same rib from the outside of the tank. A short section of tube will be plumbed from the tank vent outlet to the wing vent airbox after all riveting and sealing is completed. The main tank vent will be plumbed to the main tank, and the auxilliary transfer pump pick-up will be plumbed to the pump. Note the location of the removable panel allowing access to the plumbing fittings and tank capacitance sender BNC connector:

And a view clearly showing the geometry of the main vent and auxilliary transfer pump pick-up:

My landing lights are the Duckworks lights available through Van's. They are installed in the outboard bay between the end tank rib and the end rib of the leading edge, in this bay:

First, one of the discarded leading edge ribs was fitted alongside the end of the tank:

The line of rivets for this rib is 1" outside the line of rivets for the tank end rib. Only the first four rivet holes for this 'short rib' are drilled, then the short rib is cut down to look like this:

There is enough rib left to mount the platenuts for the Duckworks reflector plate (the platenuts were mounted to the outboard rib at this time as well):

The reflector plate itself is cut down to 7 7/16", with 1/4" more cut off the outboard end than the inboard end (if I had it to do over, I probably would have made the reflector plate symetrical). Next, the hole in the leading edge for mounting the lens must be marked. The up and down (or fore and aft) measurement is set as per the Duckworks instructions, but the center of the hole is aligned over the center of the reflector plate. Here, the hole is complete:

The remainder of the landing light installation will be completed after the leading edge is mounted to the wing.

At this point, the basic auxilliary tank set up is complete. Except as noted above, the tank is built, sealed, and plumbed very similarly to the regular main tank.

Here is the auxilliary tank with the stiffeners riveted and sealed into place. Note that rivets have been sealed into the unused holes (where the leading edge ribs would have been in a standard RV):

Here the auxilliary tank ribs have all been riveted and sealed into place, and all interior tank details have been completed. The tank is now ready for the rear baffle and final sealing:

And finally, the auxilliary tank mounted on the wing skeleton:

A detail of the fuel vent NACA scoop and plumbing:

Wing spagetti! This is the plumbing between the main tank vent and between the tanks and the auxilliary fuel pump (viewed through the wing access cover between the tanks):

This completes the construction of the right auxilliary wing tank. The left one was built exactly the same way.

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