Fuel tanks - And their complications

I've been working on the fuel tanks and their installation on the Mark IV Prototype recently and thought I might share some of the thoughts and doctrine that went into their design and some practical challenges in what might be considered a relatively simple system. The decision to relocate the fuel tank(s) into the wing leading edges is discussed in the previous blog so I won’t go over that again. This blog is more about how to make that decision a reality.



Gasoline, and more particularly current day ethanol mixed gasoline, is a tricky chemical concoction to handle and even more so in an aircraft application, as opposed to a marine or automotive installation, it eats pretty much anything it touches especially rubber. Airplanes have a history of causing peculiar things to happen to both the gas and the tanks it is stored in, not to mention the compounding problem of what happens if you have fuel starvation. The initial problem to solve is how to store the gas on board, a gas tank of course. This is not a real problem if you have a budget to have custom rotomolds made for rotomolding nice polyethylene tanks, not an option at this point in the program. Where to get an affordable gas resistant material in the shape of a cylinder to fit inside the circular openings of the leading edge ribs? Literally a couple of years and hundreds of hours of research later the local hardware store had the answer all along. Polyvinyl Chloride pipe is gasoline AND ethanol resistant, I can make fuel tanks from PVC pipe! Inexpensive, lightweight, and it works. OK so I was skeptical about PVC standing up to gasoline just like you, so I did a practical test. I bought a small PVC fitting and immersed it in a container of 90/10 gasohol (same stuff you get at the pump everywhere) with 2-stroke oil mix. I let it sit for 30 days on continuous immersion. When the fitting was removed there was no, REPEAT NO discernable degradation in the material. It was not soft, or swollen, or hard, or brittle. As a matter of fact I wiped it off with a towel and noted only a very minor tint from the 2 stroke oil and after letting it dry overnight the tint faded and there wasn't even a hint of gasoline odor. I still wasn't convinced. I made a test tank with all the baffles and cemented joints and brass fittings and yada, yada, yada. I filled the tank about half full and let it sit for 30 days. At the end of the test I drained the fuel, no noticeable change in the gas; it ran fine in my weed eater. I cut the tank open and there was no degradation of the joints or any other PVC parts. I think this is a good material to make fuel tanks with. Still it will be recommended to drain the tanks for extended storage.



The next concern is installation. The leading edge ribs are very thin and will surely cut into the tanks from vibration if left unprotected, and there is the complication of how do you feed two engines off two tanks simultaneously and continually when one tank could un-port in an uncoordinated or prolonged turn. Protecting the tanks is easily accomplished with some corrugated split conduit available everywhere and some judicial clamping.

PVC fuel tank installation in leading edge ribs, tubing seen is not completely installed or plumbed, but illustrates plumbing routing. Small line on top is fuel tank vent, lower is main fuel line run to engine position

Close up of fuel tank installation showing conduit protection from sheetmetal ribs

 The plumbing problem however, persisted for quite some time. I didn't want to complicate the operation by installing valves to feed both engines off one tank at a time, this has caused many a fuel starvation problem in "real" airplanes and still causes problems today on airplanes with multiple tanks. I didn't want to isolate the tanks one to each engine, I wanted both engines to have access to both tanks all the time. After a bit of brain picking I came up with a solution, and a clever one I think. Hydrodynamics says that if two tanks are plumbed together and exit via one route the tank with the highest hydrodynamic pressure (head) will always feed (empty) first. If we consider the Lazair in a turn (bank) one tank will be higher than the other (i.e. one wing high, one wing low with respect to the center line of the craft) therefore that higher tank will feed, and in the case of an uncoordinated turn the lower tank will un-port, that is fuel will run away from the outlet. But the higher tank will also feed into the lower tank, an unwanted situation. How to control the cross feed into the lower tank while supplying both engines with adequate fuel? Check valves to the rescue!! Check valves like I need are unavailable; believe me I looked high and low. But sometimes the most obvious answers can’t be seen until they hit you right across the head, or fall into your shopping cart at Wal-Mart. Inline Marine fuel bulbs that are used to prime the outboard engine are PERFECT check valves for this installation. They come in the right tube sizes (3/8”), they are alcohol resistant (an arguable point because the ones I use on my boat get hard after about a year or so), and they will provide a means to prime the engines (probably not necessary with the pumper carbs and the gas so close). So the fuel is routed thusly: both tanks output to a fuel bulb and then to a common tee, this tee is routed to another tee that feeds both engines. A diagram would really be helpful right about now.

Fuel bulb shown for reference only, not hooked up yet, no fittings installed in tank yet

That solves the fuel delivery issues, and the beauty of the system is it is self-leveling. The highest tank will always feed until both tanks are equal, and the tanks will swap out automatically as the ultralight banks and levels in flight.

The only problem to overcome now is un-porting and filling issues. Un-porting is handled by internal baffles in the tanks. I considered fuel tank foam, like they use in race cars, although an elegant an ultimately effective solution, alas again too expensive.  I designed baffles from .020” thick aluminum sheet metal that is sandwiched in place inside the tank with two rings made from thick walled PVC tubing with a small section removed and cemented into place as to make a retainer of sorts, and holes in proper places to allow fuel to run out the bottom and air to vent at the top. There are three baffles in each tank, one on the outboard third, one about halfway and one near the inboard tank outlet. The thought is that each section will hold a certain amount of fuel for a certain amount of time until it drains into the adjoining section.

Inside of fuel tank showing metal baffle, retaining ring and holes for fuel flow and venting

So the high tank is always feeding to the drain port and to the engines and the low tank will have some fuel retained in the inboard and middle sections closest to the drain port so as to not allow all of the fuel to run to the low end. This also reduces the tendency for either wing to become out of balance because fuel has run to the outboard end of the tank, as in a prolonged bank (i.e. turns around a point). The tank fill ports are still somewhat a work in progress. I have several options to consider, and perhaps the best one is again derived from the marine world. Boat shops have many, very nice deck fill options including a black plastic deck fill and a polished stainless steel deck fill, either of which would make excellent exterior fuel filler necks, but one is a bit cheap looking and the other, although reasonably affordable, weighs a ton.

Possible tank filler neck

Possible tank filler neck

I am also looking at an internal fill option that would hide the filler neck inside the leading edge and would be accessed via a hatch on the leading edge. What would the market prefer? Comments welcomed.     

More Next Time,

Gene

Fltofancy@gmail.com