The Vans RV-8 has 42 Gallons of fuel split between its two wing tanks.
The Vans website states 855 to 1030 miles range (75% vs 55% power) with a 160 HP engine. That equates to a burn rate of 9.7 to 7.2 gallons per hour. For simplicity, let’s just average that to 8 gph and 156kts. (My personal flying is around 150 kts but then again, I have 10hp less).
That’s a good range for anybody. However, there are a few reasons to want more – wanting to go faster; wanting to avoid landing when flying high with a nice tailwind, or cross country flying to a grass strip that has no fuel service. Oh … one more – buying gas when it’s cheap!
Thanks to the internet, I found most of the solution. Thus, what follows is my solution based on a description from Jon Thocker who stole his solution from Robert Gibbons, who probably brainstormed the setup with his buddies from Team Aerodynamix.
The centerpiece of the auxiliary tank is a circle track racing fuel cell. The hoses and various fittings are pretty typical with the exception of quick-disconnect fittings. The tank has both a fuel pickup and vent with roll over spill prevention.
Here is the complete list, formatted to be a sequence from the aircraft fuel selector, through to the tank, and then out to the external vent. You may have some of the parts in your “spare parts bin” but otherwise the total cost is somewhere between $400 and $500 (and closer to the larger number). You can click on any image for a larger version. You can also find a PDF if this document.
Fuel Selector Section:
There is an empty port on the standard Vans fuel selector. You will use this port for the auxiliary fuel system.
Next, you need to create a short fuel line from the 6AN to another 6AN. So you need coupling nuts, flare sleeves and some 3/8″ fuel line.
Note: if you want to hide most of the fuel line, then you will need more tubing here and less of the fuel hose later in this list.
The fuel line terminates through a bulkhead fitting and then the quick connect plug. Depending on how your current fuel selector is plumbed, you will need one of a 90 degree, 45 degree or straight fitting.
That completes the parts needed at the interface into the airplane’s fuel system.
The plug is removed from the unused port on the fuel selector. The 6AN to NPT adapter is installed in place of the plug. A short 3/8″ fuel like is created with coupling nuts and flare sleeves. The fuel line runs from the 6AN adapter to a 6AN bulkhead fitting which passes through the panel that already contains the fuel selector. Odds are good you will need a to make a washer or install a doubler plate to insure stiffness and to create enough thickness for the bulkhead fitting. In my installation, I added both a larger circular doubler below the panel and a smaller circular decorative plate on top.
Fuel Hose Section:
Next is the fuel hose and fitting to the tank. The fuel hose has a quick connect 90 degree fitting on one end and a 6AN 90 degree fitting on the other. (the quick connects have a very attractive gold anodized finish on the ends.) Optionally, you can install a cleanable in-line fuel filter.
I chose to buy the hose ‘by the foot’ since I standardized both the fuel and vent hoses on a single size rather than use a smaller size on the vent line. I purchased 8ft of -6 hose and used 5ft for the fuel side and 3ft for the vent side. It worked out perfectly.
The quick connect fittings are “valved fittings” which means fuel in the hose will not drain out the fitting when disconnected.
It’s really easy to get cut from the tiny wire braid. My solution was to place the hose nut in a socket wrench and twist it onto the braid. Then installed the threaded insert. Of course, had I been thinking, I’d have checked if Summit Racing had a video – which they do .. (http://www.youtube.com/watch?v=PeeflgGlenY)
Summit Racing Instruction Video – AN fitting on Braided Hose
I installed the quick connect fitting and then ran the hose to the tank – which was temporarily in place for checking layout. Then I marked the hose for the cut. I wrapped the hose with tape at the cut location and used a metal cutting blade in my bandsaw to make the cut. I cleaned out any debris after the cut and use a fine grit grinder stone to file away and stray braids. Then I installed the 90 degree AN hose fitting.
Fuel Tank Section:
The tank is a circle track racing fuel polyethylene tank with a polyethylene bladder. The original write up used a 16 gallon tank while my system uses a 22 gallon tank. In both cases, the width x depth of the actual tank measures somewhat less than is specified. Jon measured the 16 gallon tank and found it to be about 23″ x 17.5″ rather than the specified 25″ x 19″. In my case, the 22 gallon tank is 25″ x 16.25″ rather than the specified 26″ x 18″.
The tank is standard with 8AN fittings. Since I used the same hose for both fuel and vent, I needed two 8AN -6AN reducers. If you use a smaller vent hose then you will need one 8AN-6AN and one 8AN-4AN.
One thing puzzled me. The tank has two 8AN fittings. One is the fuel and one is a roll over vent. They are unmarked. There is no on-line documentation. In the website pictures, both are black or both are silver. My tank came with one fitting in silver and one in black. I ended us using a small piece of 0.021 safety wire to gently probe the two fittings. The black one has a ball in it so I concluded it was the vent.
WARNING: This next section includes misinformation from the manufacturuer
Later, I received and email reply from the manufacturer. It confirmed what I had figured out but then it pointed out a mistake with my intended installation. All of my pictures I found showed the tank backwards from the way the manufacturer described its installation. Here is the message from the representative…
The fittings face the front of the car with the cap towards the rear. The pickup should be the fitting on the right (passenger) side. You can clarify this for certainty by turning the cell upside down and checking
for the ball bearing in the other (vent) fitting. We place removable stickers to identify these fittings but it sounds like your cell did not get labeled.
With both fittings facing toward the front of the car with the cap-side facing the rear, the fitting on the right (passenger) should travel to the right (passenger) rear corner, which would be the cap-side corner on the same side as the aforementioned fitting.
I installed the tank according to the email descriptions and contrary to Jon Thocker’s pictures. When I did my ground tests with minimal fuel, it would not pickup – even with the boost pump on. I attributed it to the deck angle of the RV-8 on the ground. However, the test flight procedures indicated low fuel pressure and sporadic fuel flow.
I put the plane back in the hangar and came up with a couple tests to determine exactly where the fuel pickup was located within the tank.
With the fuel cell oriented so the fill cap is toward from front and the fuel and vent lines are aft, the fuel fitting is on the left and the pickup is toward the left rear corner.
Vent Hose Section:
Next is the vent hose and fitting from the tank. The vent hose has a 6AN 90 degree fitting on one end and a quick connect 90 degree fitting on the other.
Jon’s instructions use a smaller hose so his setup uses 4AN fittings. An advantage of Jon’s system is that you can not confuse the fuel line and the vent line. I solved that possibility by using a different “flow” rating for the quick connect which makes it incompatible with the one use for the fuel hose. I used a 3000 series quick connect for fuel and a 2000 series for vent. In the end, I don’t think this saved me any money. The connector was a little more expensive.
I went with a 90 degree quick connect because I wanted to reduce stresses on the quick connect fitting and assumed a hose running down was better than one sticking out in the open. You need to plan the placement of your quick connect plug to determine which fitting works best for you.
Vent Line Section:
The vent hose terminates with a quick disconnect. From there, the vent line runs down to the bottom of the airplane where it exits with a forward facing vent tube. Since I had used -6 hose, it meant my vent line starts with an 6AN fitting and uses a reducer to 4AN. All of the vent line is done with 1/4″ aluminum tubing.
I wanted the disconnect to be clear of any removable panels. This was to simplify the annual condition inspection. This meant I would need to pas through the bulkhead at the back of the baggage compartment. I didn’t want to weaken it in anyway. Fortunately, there was an unused pass-thru hole (two actually).
I had to fabricate a washer/spacer and then Installed the 6AN bulkhead elbow. I then added the reducer. This allowed me to transition to 1/4″ tubing. I ran it down the back side of the baggage compartment bulkhead and then, below the longeron, I brought it forward through another vacant hole in the bulkhead. I used a rubber grommet in this hole to protect the tubing. I then routed the line to the bottom of the airplane where I connected to the 4AN bulkhead elbow which passes through the bottom skin. Under the airplane I added another home made spacer and the bulkhead nut. I finished off the vent line with a curved tube with faces forward like the tank vents for the wing tanks.
I used black for the final bulkhead elbow and the nuts to keep with the exterior color or the aircraft (standard blue anodized would have looked dumb).
Tank Ramp Section:
The tank is 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 solution is to build a ramp that slopes 4.5 degrees from the back of the baggage compartment bottom to just forward of rear seat’s 5-point harness crotch strap mount.
By placing the forward most ramp leg just aft of the crotch strap mount, the ramp can not move forward or aft.
The two middle legs of the ramp are just either side of the flap control bar.
The ramp also has perimeter blocking to keep the tank fro sliding fore or aft, left or right.
The ramp is constructed of 1/4″ panel plywood with 2xX stock cut with a 4.5 degree bevel on the top. I created a jury stick with all of the locations and measurements. The ramp is primed and painted a light gray.
The installation is in two parts. First, the rear seat is removed. This is pretty quick. My rear seat bottom is Velcro’d and just pulls out. If the hinge pins for the back are snug or sticking, I use a small pair of vice-grips to grab the ‘handle’ and pull. The second part is easy as well but one step may be a challenge depending on how long your arms are. In my case, my arm is just barely long enough. I place the tank ramp in the airplane and insure the two lap segments of the seatbelt are fully extended and out of the way. I them place the tank into the perimeter frame on the tank ramp. Here is the tricky part – I kneel on the right wing walk and extend my arm to the very back of the baggage compartment to attach the vent hose quick disconnect to the plug. With that accomplished, I attach the quick disconnect on the fuel hose to its plug. Finally, I use the rear lap belt to secure the tank. I tuck the rear shoulder harnesses backward into the baggage compartment. (Truthfully, I stuff them in an old sock and then bungee them back onto themselves in the top of the baggage compartment.)
The test procedures started with a leak test. With only the vent line quick disconnect connected, I applied a small amount of air pressure to the vent line at the bottom of the fuselage. This confirmed that both the vent line and fuel line were holding pressure.
The next test was a ground operation test. I first placed a blue, lint free absorbent paper towel under the fuel selector. With both the vent and fuel quick disconnects attached, the airplane was started on a wing tank. The boost pump was turned on and then the fuel selector was switched to the AUX position. Initially, the boost pump indicated air – which was expected. Once the AUX fuel hose was flushed with fuel, the boost pump was switched off. The airplane was taxied for 4 minutes using only the AUX tank. The fuel selector was switched back to a wing tank and after 1 minute, the airplane was shut down. I inspected the paper towel for any sign of liquid.
The third test was in-flight operations. After taking off on a wing tank and climbing to 4,000′ over the local airport, I switched on the boost pump and then switch to the AUX tank. After 15 seconds, I switched off the boost pump. I then proceeded with gentle turns and eventually steep turns. The total test time was approximately 15 minutes. I then switched back to a wing tank and waited 1 minute before starting decent to land procedures.
The final test was a CG and load test. I operator from a 1,940 ft grass runway. My airplane has a cruise pitched propeller on a 150HP engine making it optimized for cross country flying and not for maximum performance at take-off distance and climb. I make three take offs and landings with the tank empty. Then filled the tank completely and repeated the test. To measure ground roll distances, I placed colored flags every 100′ along the runway and placed a small video camera under the wing. The camera looked cord-wise so both the landing gear and the flags were observed. The wind was calm and the density altitude was measured at 420′. Under these conditions, the full AUX tank adds approximately 10-15% to the ground roll distance.
The Google Earth image shows a yellow circle at 1,000 nm and a green circle at 500 nm. These represent an approximate range traveling one-way or round trip respectively while maintaining greater than VFR minimums. The referent circles do not take into account wind or other factors.