"The only time you can have too much fuel is when you're on fire!"
Fuel Systems
This is my current fuel system (click to enlarge). It is a semi-returnless system and has performed very well so far.
This is my
previous system that was prone to vapor lock. (Click to
enlarge)
No More Header tank
The header tank had initially been conceived of as a way to simplify the plumbing of 6 fuel tanks, but it was prone to vapor problems. I was using various methods to deal with these until what started as a simple solution was anything but! I finally decided to scrap it altogether and I'm glad to be rid of it.
Semi-Returnless EFI system
I now use a semi-returnless system. This is not a true returnless system as used in late model cars, but due to my multiple tanks is the closest I was able to make it. I removed the aluminum fuel rail and replaced it with a steel fuel rail. The thermal conductivity of aluminum is many times greater than that of steel and due to the fuel rails close proximity to the turbocharger I wanted to reduce this heat absorption. All of the fuel lines in the engine bay are hi-temp/hi-press hoses to further reduce heat absorption. The return fuel flows through a small 3x3 aluminum fuel cooler that I built & installed in the duct inlet to the fuel pump/filter shroud and then it flows through the aftermarket fuel regulator that I mounted on the cool side of the firewall. The fuel then returns directly into the feed line back to the FI pumps. There is no longer a gascolator as I found it unnecessary and as a large aluminum heat sink it contributes to vapor. The outlet of the fuel cooler has a temp sensor and displays it on my engine monitor with alarms. As my leading edges are painted dark blue, I've often found that my fuel temps can be above 100F on a hot day before I even start the engine. When I first began to monitor this I was quite concerned about this, however I've found that once airborne the fuel temp will fall to ambient temps. I've installed an optical fuel sensor on the fuel line from each wing selector valve which will flash a large light on the panel as soon as air is detected in the fuel line. At cruise power settings this allows approximately 45 secs before the engine will begin to stumble, allowing plenty of time to switch tanks. I've found that the engine can be run until it dies and as soon as fuel tanks are switched and Facet pumps started to prime the line to the FI pump, the engine will restart immediately. The air in the fuel line will pass through the fuel injectors with no problem at all. So far I've not needed it but I'm considering installing a fuel purge solenoid valve with pushbutton activation to return fuel to a single tank, just in case a purge ever becomes necessary.
Why So Much Fuel?
All my life I have always been driven to explore and this plane is another tool to do that with. I knew that I wanted the plane to have the great endurance to not only go great distance, but also be able to hold full fuel without exceeding CG limits. Wingtip tanks are nothing new, however most other long distance flyers put a ferry tank in the backseat/cargo area. This works but give the plane an aft CG, as well as adding stress to the wing root. This also leaves you without a backseat/cargo area, even when tank is empty. I knew the best solution was to put more fuel in the wing - but how? I heard a rumor of an RV with outboard leading edge tanks, but I was unable to find out if it was true, so I decided to make it true. Details on how I built the OB fuel tanks are on the Wings page.
Update on the rumor
Since this was originally written, not only found this rumor to be true, I've actually flown in this plane. For more info click here.
The combination of wingtip, OB tanks and IB tanks creates a bit of extra work to manage all 6 tanks. If I was planning on using carburetion, then this wouldn't be to bad, but I will be using fuel injection so I need a place to return the fuel from the fuel rail pressure control valve. Duplex valves weren't an option. I don't like the option of returning fuel to one tank, then always returning to that tank. I also don't like the idea of always drawing out of one wing tank and transferring all other fuel to that tank. In the end I decided that a header tank would do the job. A 5 usg header tank still requires transferring fuel, but it also provides me with a useful 45 min reserve. It also allows for plenty of time to switch feed tanks in the event that someone unfamiliar with the fuel system is PIC. The header tank is far stronger than is probably necessary, but since it's in the cockpit this is a good thing, but not without a price.
Fuel Selector
The tank selection is made using 2 of Van's standard fuel selectors with the plug removed out of the 3rd position. Each selector can select from inboard main tank, outboard auxiliary tank or wingtip tank. Fuel from the selector feeds it's own Facet fuel transfer pump, which then pumps up to the header tank. There is a crossover valve between the sides that will allow either pump to draw from any one of the 6 tanks, by opening the crossover and using selecting the tank that you want & setting the other selector to "off". This setup is better than having one pump used for all fuel & one kept as a spare, as spare pumps tend to plug up and not work when you need them most.
Update
I am still using this system but I've since seen an 8 position fuel valve that Jeff Nielson had made, and have the CAD plans for it. I'm currently looking at buying a lathe/mill, which would make the construction of this valve a first project.
Header tank
I've now removed this header tank as it was prone to vapor problems, but for now I'll leave the following info here for those that are interested in how I progressed through the various stages of my project.
The header tank is made from E-glass & Derakane - vinyl ester resin. It is used extensively in the pulp industry where it is exposed to very corrosive chemicals & physical abuse & holds up to it very well. I decided that it would be the lightest, toughest material to build my header tank from. It is well baffled with small openings between compartments to provide a stable fuel level for the controller and to prevent sucking air into the outlet. Baffles also add tremendous strength. I wanted to ensure that the tank would survive any impact that I could survive, to minimize chance of fire, so I ended up beefing it up a little more than planned. Finished empty weight including plumbing is 7 lbs., but I am very pleased with it's apparent strength (I don't plan to crash test it).
Update to Aluminum
I've left the preceding paragraph in place as I feel that it does have value to any one who doesn't have the ability to TIG weld aluminum, but would like to use fiberglass. I also left the pictures on the sidebar.
Due to a design change, I decided I wanted to increase the strength of the tank, as well as change some of the features of it.
I decided that aluminum would be best suited for this. I used .125, 5052 sheet aluminum. I was able to get this cheap from a local boat yard that builds high performance river boats. It is the same grade as recommended for fuel tanks.
The new tank has a few new features as well;
It still has a sump with a water drain on the bottom, with a fuel outlet located just above. It also has a second outlet in the form of a flop tube. I know this adds weight & I don't even have an inverted oil system (gotta invent one for the rotary) and I know, I know the RV-9 ain't supposed to fly upside down, but neither is a C-150 and it will do it until the engine coughs.
I omitted the fuel level sensor ports, instead I added 2 capacitive fuel probes. Other than that the construction is basically the same as the fiberglass tank.
I had intended to take pictures of the inside of the tank before closing it, just to document the interior construction for the web site & the inspector, but I had counted on using the camera that I keep on my bench at work, since this is where I was doing the work. It hasn't been used in awhile and it turned out to have dead batteries. I have a box full of most types of batteries for instrumentation equipment, but unfortunately this required those goofy stubby camera batteries. Since I was working late and it was the off-shift, I couldn't access a company camera. I defiantly wasn't going to hold up production for the sake of a picture, so you will have to do without. (Mr. Inspector). I will do a CAD of this tank when time permits.
I feel that this tank has a very high degree of survivability, but it doesn't come cheap. Final weight of this tanks is 11lbs. Not very good when you consider that I began this update with the idea to reduce the 7lb. weight of the fiberglass tank. But I feel that this is worth the price since it contains fuel within the cockpit.
As a side note;
When I removed the fiberglass tank, I decided to do some non-scientific destructive testing. This consisted of me jumping on it with my heavy work boots for @ 5 minutes. I was eventually able to crack a seam open, but it would not collapse.
Wingtip tanks were fairly straight forward. Baffles were made to fit in tip and to seal off leading and trailing edges of tip. Leading edge was left open to allow access to nav/strobe lights. Tank water drain was placed near the rear spar, while fuel filler cap was placed just aft of the main spar. Aluminum strips were imbedded into the edges to strengthen for nutplate attachments.
Click here for a written procedure for converting wingtip tanks
All tank vent lines are tied together on each side and the header tank is tied into both sides at the top of the fuselage "loop". In the unlikely event of overflow of the header tank during fuel transfer all fuel is drawn back to the tank that is feeding the pumps, through the vent lines.
FWF
#1 fuel injection pump will draw from the sump outlet on the header tank, through a gascolator, then pump through a filter and into the fuel rail. #2 fuel injection pump will draw from a flop tube in the header tank then pump through a filter, and on to the fuel rail. Fuel rail pressure is regulated by a pressure control valve which will return excess fuel to header tank.
May 31, 2005
Fuel tank selectors
Each selector valve can select between inboard, outboard, or wingtip tanks to feed separate pumps. Crossover pump is only used in event of a pump failure.
Fuel selectors connected to plumbing & pumps.
Selector/pump assembly between seats
Building console over fuel selector/pumps
Fuel console.
More info on this can be found on the Panel & Console page
Header tank is well baffled for strength as well as slosh prevention.
Inlets, recirc & vent is plumbed in. Dip in vent line is to allow
for a rib.
Tank is upside down, all fuel plumbing is through the bottom. Sump has water drain off lowest point, while fuel outlet protrudes 1" to top of sump.
Tank right side up & painted. (I know it didn't need paint, but fiberglass looks so damn ugly without it)
Ports on side are for infrared sensors to provide Hi/Lo level to transfer pump controller
Top side of aluminum tank
Bottom side of aluminum tank. Sump is on the upper center.
Aluminum header tank in place. Bulkhead support bracket can be seen, crossing center of tank.
Drain line from the header tank exits the fuselage behind the firewall, beside the muffler housing.
Wingtips being converted to fuel tanks. Leading & trailing edges carry no fuel, middle 3 sections only. Inside of wingtip is coated with Derakane - a chemical resistant resin.
Aluminum strip is imbedded in the fiberglass to add strength for nutplate attachment