Adding an Auxiliary Fuel Tank to an RV-8

When built according to the plans, the Van’s RV-8 has 42 gallons of fuel split between two wing tanks. This provides good range. However, there are a few reasons to want more fuel—going faster, avoiding fuel stops when there’s a nice tailwind, or cross-country flying to remote grass strips that don’t have fuel service. An auxiliary tank provides that extra range when it’s wanted and can be removed when it’s not.

After some quick research, I found most of the solution for an auxiliary fuel tank project that fits my airplane. What follows is my implementation, based on a brief description from Jon Thocker, who admits to having borrowed his solution from Robert Gibbons, who likely brainstormed the setup with his aerobatic teammates.

The centerpiece of this project is a circle track racing fuel cell. These tanks come in a wide range of shapes and capacities. They are polyethylene tanks with a fuel pickup and breather vent with rollover spill prevention. The variety makes it easy to choose a tank to fit most any aircraft or flying mission.

I made a conscious decision—based on weight and balance—that I would not fly with both a passenger and the auxiliary tank. This made choosing the location for the auxiliary tank easy: It would go in place of the passenger seat. It also meant the switch between seating and the tank needed to be simple, quick, and require no tools.

The “quick and easy” requirement was satisfied by using quick-disconnect fittings for both the fuel hose and vent hose. These fittings have an integrated spring value in the coupler, which prevents fuel in the hose from dripping when disconnected.

The process I followed is broken into a series of steps, each with its own parts list. We’ll start at the aircraft fuel selector, move to the tank, and then out to the external vent. But first we need to do some planning to determine the following:

• Where will the auxiliary tank be located when in use?
• Where will the fuel line connect into the aircraft fuel system?
• Where will the fuel vent exit the aircraft?

The answer to these questions will help with choosing the most suitable tank, the length of the hoses and tubing, and the placement of various assemblies.

Fuel Selector

Parts Needed:

• 1 AN816-6D male-male flared tube to pipe thread adapter
• 2 AN818-6D coupling nuts
• 2 AN819-6D coupling sleeves
• 1 length of 3/8-inch OD aluminum tube fuel line
• 1 AN832-6D male-male straight bulkhead union (alternately an AN837-6D 45-degree or AN833-6D 90-degree bulkhead elbow)
• 1 AN924-6D aluminum nut
• 1 Jiffy-tite 3000 Series quick-disconnect plug to 6AN female thread

There is an empty port on the standard Van’s fuel selector. I used this port for the auxiliary fuel system. If your fuel selector does not have an unused port, you’ll need an alternate solution for integrating the auxiliary tank fuel supply into your aircraft fuel system.

In my RV-8 the short fuel line terminates near the fuel selector through a bulkhead fitting and then the quick-disconnect plug. Depending on how the fuel selector is plumbed, you will need a 90-degree, 45-degree, or straight bulkhead fitting. Most Van’s Aircraft installations will use the straight fitting.

Locate the NPT plug in the unused port of the fuel selector and remove it. The plug will likely accept a hex key wrench. The 6AN-to-NPT adapter is installed in place of the plug.

Find a suitable location for the quick-disconnect plug. The plug threads directly onto the bulkhead fitting. In my RV-8, I chose a spot near the fuel selector. If you want to hide most of the fuel line, you should locate the quick-disconnect closer to the fuel cell. You’ll need more tubing here, but less of the fuel hose in a later step.

Plan out the route from the fuel port 6AN fitting to the location for the bulkhead fitting.

Cut a length of 3/8-inch aluminum tube to fit the planned route. Add coupling nuts and flare sleeves, and flare each end.

In my RV-8, the bulkhead fitting passes through the panel that already contains the fuel selector. You may need to make a washer or install an extra plate to create enough thickness for the bulkhead fitting. In my installation, I added both a larger circular washer below the panel and a smaller circular washer on top that was painted black. These spacers were cut from .063-inch aluminum.

Fuel Hose

Parts Needed:

• 1 Jiffy-tite 3000 Series quick-disconnect socket to 6AN hose end with 90-degree bend
• 1 length of #6 stainless steel braided hose
• 1 Summit Racing 6AN 90-degree female-thread hose end

Optional (for in-line fuel filter):

• 1 Russell Competition 6AN in-line fuel filter
• 2 Summit Racing 6AN straight female thread hose ends

The fuel hose runs from the quick-disconnect at the fuel selector to the auxiliary tank. Determine the location for the auxiliary tank. Use this measurement to determine how much fuel hose is needed.

When cutting stainless steel braided hose, cover the section of hose to be cut with a couple layers of tape. This protects the braid and minimizes fraying. Mark the cut location. Cut the hose as straight as possible using a die grinder with a cutoff wheel, hacksaw, or band saw with a metal cutting blade. Clean out any debris after the cut, and use a fine-grit grinder stone to file away any stray braids.

Working with stainless steel braided hose can be a little tricky—it’s easy to get cut from the cut end of the wire braid. My solution was to place the hose nut in a socket wrench and twist it onto the braid. Then install the threaded insert to secure the fitting on the end of the braided hose. There are demonstration videos on the internet, found by searching “braided hose assembly with AN fittings.”

Start by attaching the quick-disconnect socket fitting to the fuel hose end. If installing the optional in-line fuel filter, measure approximately 12 inches from the quick-disconnect socket. Wrap the hose with tape an inch or two to each side of the cut location and cut as before. Install a 6AN hose end on each of the cuts, then install the fuel filter.

Temporarily place the auxiliary tank where it will be located in flight. Run the hose from the quick-disconnect plug to the tank. Mark the hose length and cut.

Finish by installing the 90-degree AN hose fitting using the same technique as with the quick-disconnect socket.

I chose the rear passenger seat area for the auxiliary tank. The fuel hose runs from the quick-disconnect at the fuel selector to the top of the auxiliary tank. The RV-8 is a tandem configuration, so the fuel hose runs along the floor on the left side of the cockpit. The fuel hose has the quick-disconnect 90-degree socket fitting on one end and a 6AN 90-degree fitting on the other. In the RV-8, the 90-degree quick-disconnect socket keeps the hose low and close to the floor. If your quick-disconnect plug is horizontal, then a straight quick-disconnect socket may work better.

Fuel Tank

Parts Needed:

• 1 RCI circle track fuel cell (size and shape to fit the airplane and flying mission)
• 2 Summit Racing 8AN female to 6AN male reducers

The tank is a polyethylene circle track racing fuel cell. I chose a 22-gallon tank because most of my flights will be 400-500 nautical miles each way into fields without fuel. This tank measures 26x18x15 inches.

The tank has two 8AN fittings. One is the fuel line and one is a rollover vent line.

A trick to determine which connector is for fuel and which is a vent is to cut a piece of safety wire about 12 inches longer than the tank is deep. Create a 90-degree bend about 6 inches long at one end. Carefully insert the bent end approximately 6 inches into the filler opening of the tank and slowly rotate it until it hits a pickup tube. Since the vent is at the top of the tank and the fuel pickup is at the bottom, this pickup tube must be the fuel side. With the bent wire still in the filler opening, slowly lower the wire into the tank while keeping it in contact with the pickup tube. This will identify which corner of the tank is the pickup point.

Orient the tank so the pickup corner is at the lowest point when the tank is in flight. Attach the 8AN-to-6AN reducers to the tank and tighten. These will not be removed.

Vent Hose

Parts Needed:

• 1 Summit Racing 6AN swivel 90-degree female-thread hose end
• 1 length of #6 stainless steel racing hose (measured from auxiliary tank to the vent line disconnect plug)
• 1 Jiffy-tite 2000 Series quick-disconnect socket to 6AN hose end with 90-degree bend

The vent hose runs from the tank to the quick-disconnect plug for the vent line. The vent hose has a 6AN 90-degree fitting on one end and a quick-disconnect socket 90-degree fitting on the other.

The vent hose uses a Jiffy-tite 2000 series quick-disconnect, rather than the 3000 series used for the fuel line. The different series connectors are different physical sizes and cannot be swapped. This prevents any confusion between the vent hose and the fuel hose.

The vent hose is assembled with the same steps as the fuel hose. Attach the 6AN 90-degree hose end fitting onto the braided hose. Measure the distance to the vent line quick-disconnect plug. Cut the hose to length and install the hose end quick-disconnect socket.

In my RV-8, the quick-disconnect plug for the vent is installed horizontally in a bulkhead in the aft baggage compartment. The 90-degree quick-disconnect socket directs the vent line down and keeps it out of the way along the side of the baggage compartment. Depending on your aircraft installation, a straight disconnect may be more appropriate. Plan the placement of your vent hose quick-disconnect plug to determine which fitting works best for you.

Vent Line

Parts Needed:

• 1 Jiffy-tite 2000 Series quick-disconnect plug to 6AN female thread
• 1 AN833-6D male-male 90-degree bulkhead elbow
• 1 AN924-6D aluminum nut
• 1 Summit Racing 6AN female to 4AN male reducer
• 3 Summit Racing 4AN tube nuts (black)
• 3 Summit Racing 4AN tube sleeves (black)
• 1 length of -inch aluminum tube (inside the fuselage)
• 1 AN833-4D male-male 90-degree bulkhead elbow
• 1 AN924-4D aluminum nut
• 1 4-inch length of -inch aluminum tube (gentle 90-degree bend for outside the fuselage)

The vent line starts with a quick-disconnect plug and uses aluminum tubing to run to the bottom of the airplane where it exits with a forward facing vent tube. Since the RV-8 wing tank vents were -inch aluminum tube, I chose to have the auxiliary tank use the same tubing and installed it using 4AN fittings.

For simplicity, the auxiliary tank vent hose uses the same 6AN hose as the fuel hose. The vent line in the fuselage uses -inch aluminum tube, matching the vent tubes for the wing tanks. This requires reducing the 6AN vent hose to 4AN for the aluminum tube.

I wanted to avoid having the vent line pass through any removable panels to simplify the annual condition inspection. This meant passing through the bulkhead at the back of the baggage compartment. Fortunately, there was an unused hole (two actually) in the bulkhead.

The 6AN bulkhead fitting requires one or more spacers to create a thick enough surface to tighten the bulkhead nut. After tightening the bulkhead fitting in place, attach the quick-disconnect plug facing the axillary tank location. On the opposite side of the bulkhead fitting, attach and tighten the 6AN-to-4AN reducer.

Locate the point where the vent tube will exit the fuselage. The exit point will most likely be some place on the bottom of the fuselage. This is the location for the 4AN 90-degree bulkhead fitting. Drill a hole for the long end of the 4AN 90-degree bulkhead fitting.

Insert the long end of the bulkhead fitting into the hole so the fitting is pointed straight out of the fuselage. This will orient the bulkhead fitting to keep the vent line low and close to the floor of the fuselage. The 4AN bulkhead fitting requires one or more spacers to create a thick enough surface to tighten the bulkhead nut. Install one spacer on the outside of the fuselage, then install the bulkhead nut.

Plan out the route of the aluminum tube vent line from the 4AN reducer to the bulkhead fitting exiting the fuselage. The vent line should finish by running along the inside skin of the fuselage into a 90-degree bulkhead fitting, which provides the turn to exit the fuselage. Use rubber grommets when passing through intermediate holes to protect the tubing.

Cut a length of -inch tube to fit the planned route. Add coupling nuts and flare sleeves, and flare each end.

Install and route the vent tube. Attach it to the 4AN reducer at one end and the 90-degree 4AN bulkhead fitting at the other. Tighten all of the fittings.

Cut 4 inches of -inch aluminum tube and form a large-radius 90-degree bend. Add a coupling nut and flare sleeve, and flare one end. Attach the tube to the exposed end of the bulkhead fitting on the exterior of the fuselage with the open end of the tube facing forward.

With all of the assemblies complete, hoses fabricated, and tubing installed, trace the complete route from the fuel selector to the exterior vent, and check that all the fittings are tight.

Tank Platform

The tank installation will be specific to each aircraft. The key goal of the tank platform is to provide a secure location so the tank cannot move in any axis.

In my RV-8, the tank installs in the passenger area, as far forward as possible, placing it at the same CG as the rear passenger seat. This means the tank straddles the flap actuator bar.

The ramp is constructed of -inch panel plywood and 2×4-inch stock. The stock is cut with a 4.5-degree bevel on the top to fit the slope of the RV-8 aft seat area.

The platform has a series of supports made of strips of wood running the width of a thin sheet of plywood. The forward most strip runs just behind the rear seat crotch strap anchor point. The next two strips flank the flap tube.

The ramp slopes 4.5 degrees from the rear seat’s crotch strap mount to the back of the baggage compartment bottom. The slope provides the necessary clearance for the strap mount and flap tube. It also gently slopes the tank in flight so the fuel pickup is at the lowest point.

I created a jury stick with all of the locations and measurements. The jury stick simplifies the layout and glue-up of the support strips on the platform panel.

The ramp has perimeter blocking to keep the tank from sliding fore or aft and left or right. It is primed and painted light gray.

Installation

Because I chose to use the rear seat location for my auxiliary tank, the first step in using the tank is to remove the rear seat. In an RV-8, the seat cushion is held in place with Velcro. The seat back is held in place by removable hinge pins. It takes less than 2 minutes to remove the seat and seat back.

Install the fuel hose 6AN 90-degree hose end fitting to the 6AN reducer on the fuel port of the auxiliary tank. The fitting will not be removed, so make it tight.

Install the vent hose 6AN 90-degree hose end fitting to the 6AN reducer on the vent port of the auxiliary tank. Again, the fitting will not be removed, so be sure it’s tight.

Installing the auxiliary tank starts by placing the tank platform in the aircraft. Next, the tank is placed on the platform and secured with a strap.

The fuel hose is run in position to the quick-disconnect. Holding the spring-loaded retaining collar up, press the quick-disconnect socket onto the quick-disconnect plug, releasing the retaining collar.

The vent hose is installed similar to the fuel hose. Gently tug each quick-disconnect to ensure they have locked in place.

Before removing the tank, make sure it is empty or mostly so. A 22-gallon auxiliary tank weighs approximately 150 pounds when full, making it difficult to handle.

Remove the vent hose by pressing in on the quick-disconnect socket while pulling back on the spring-loaded retaining collar. This will release the connector and it will pull off. The fuel hose is removed similarly to the vent hose, but leave both hoses connected to the tank. When the auxiliary tank is not in use, I have a little rubber ducky that covers the fuel quick-disconnect plug.

Flight Test Procedure

As with any alteration to an experimental aircraft, it’s important to establish a test procedure to determine safe operations and any effects the modification may have.

Leak Test: With only the vent line quick-disconnect connected, apply a small amount of air pressure to the vent line at the bottom of the fuselage. Confirm that both the vent line and fuel line hold pressure. Place a blue, lint-free absorbent towel under the fuel selector. With the fuel selector on the auxiliary tank, apply a small amount of air pressure to the vent tube on the exterior of the fuselage. Hold this pressure for 1 minute. Check the absorbent towel for any signs of fuel.

Leave the absorbent towel under the fuel selector for the remaining test procedures and inspect at the end of each test.

Ground Operation Test: With both the vent and fuel quick-disconnects attached, engage the boost pump and switch between left, right, and the auxiliary tank at 10-15 second intervals. Turn off the boost pump and check the absorbent towel for any signs of fuel.

Ground Run Test: Start on a wing tank. Turn on the boost pump. Move the fuel selector to the auxiliary tank position. Initially, the boost pump will indicate air. Within a few seconds, the boost pump sound will change as fuel flows from the auxiliary tank. Observe the engine fuel-pressure measurement. Switch the boost pump off and continue to observe the engine fuel pressure measurement. If all conditions are nominal, perform taxi tests to evaluate the performance of the auxiliary tank during ground maneuvers. Shut down the aircraft and inspect the absorbent towel for any signs of fuel.

In the case of a tailwheel aircraft, the auxiliary tank may be low and sufficiently aft of the engine to provide inadequate fuel flow. If this is the case, the POH should indicate use of the auxiliary tank is restricted to cruise or level flight only.

Flight Operations Test: Take off on a wing tank and climb to 4000 feet over the local airport. Switch on the boost pump, then switch to the auxiliary tank. Monitor fuel flow, fuel pressure, and engine rpm. After 15 seconds, switch off the boost pump. Periodically verify fuel flow, fuel pressure, and engine rpm. Perform a series of gentle turns and gradually increase to steep turns. Switch back to a wing tank and wait 2 minutes before starting descent-to-land procedures.