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

Blogging - The new telephone call?

First Lazair Nouveou Blog

So I'm very new to this blogging thing.

This site will be a tool to communicate news and info about Lazair Nouveou and the journey of resurrecting the famous Lazair ultralight.

Where to start - where to start......

The Lazair project is moving at an incredibly slow pace. Although some things are happening, any visible progress is invisible. We have obtained all the tooling, what there was of it, from Michigan (October 2010), and we have all the necessary rights and licenses in place to sell under the Lazair name (HUGE sales importance). We have been capturing the Lazair design in Solidworks (parametric 3D CAD software), and are updating a few items to take advantage of modern manufacturing techniques and materials. The end product will appear to be exactly like the Elite or Series III Lazairs, however there are several major differences; strengthened wing spars (for 350lb pilot), different main landing gear wheels and nosewheel with better brakes, relocated fuel tank (from behind seat to inside leading edge D-cells), Hirth F-36 engines, BRS recovery parachute, and rigid polyurethane foam for the wing ribs. It will be named the "Mark IV".

Design Issues
Ultraflight released very few drawings for parts (only tubes) to the public, there is some debate as to how much of the design was captured. In making Solidworks models of the Lazair parts everything is pretty straight forward and simple to model, although very time consuming to reverse engineer, except for the wing ribs. I asked Dale how he designed the rib profiles, his answer was he hand lofted them using parts of other ribs he was familiar with and developing the lines until they were pleasing to his eye. Very creative and yet another indicator of his natural ability as an aircraft designer. However this becomes a nightmare when trying to model the ribs in 3D CAD. So I had a couple of choices; I could simply model something that looked OK but wasnt really representative of the real airfoils, and therefore couldnt be used as a base for modern CNC manufacturing techniques to make templates, I could send the ribs out and have them Z-scanned and a point cloud for each developed and make models from that (really cool technology, but very expensive), or I could trace out each rib and graphically extract coordinates and get some useable data to start from (the old school method). I chose the latter course ( I never knew how important those kindergarten tracing lessons were going to be). I created large graph paper and layed each rib down and traced the profile. I then took a 6 inch scale and hand computed each coordinate in half inch increments. A labor of love to be certain, but when finished, a few months later, I had something that no one else on earth has, not even Dale, Lazair rib coordinates. I loaded the coordinates into my CAD software and "wah-lah" I had 3D models of the ribs. After some adjusting and smoothing for measurement inaccuracies I now have beautiful rib models that I can send out and get templates made (molds actually, explanation below).

Why move the fuel tank, and why that drives new rib material and the need for accurate rib models.
The decision to relocate the fuel tank is twofold. Firstly the traditional location is a pain in the back to refuel (literally), you either have to remove the fuel tank (which means lifting it back into the holder when its full of fuel ~30lbs), or contorting your self and holding a gas can at an awkward angle to refill, both of which are extremely hard on your back. Some folks developed special hoses and other things to make the job easier, but this means you have to take that stuff with you if you plan on refueling off your home field. The other reason is to reduce the total lift height from the fuel tank to the carburetors. Although a good pumper diaphragm carburetor will lift the fuel, any small leakage in the fuel line, primer bulb, or joint in the fuel line will cause fuel starvation. It was a significant problem as evidenced by the Lettair Service bulletins. Relocating the fuel tank to the leading edges makes it much easier to access and refuel and drastically reduces the lift height from nearly 24" to about 6" max. But what happens when gasoline is spilled into the leading edge when refueling? This is a major concern and has many facets to overcome. The largest problem to overcome is the fact that gasoline will melt polystyrene (i.e. the blue foam the original ribs were made from), and it could have and adverse effect on the tapes used to hold the covering material on. The design changes to overcome these obstacle are rigid polyurethane foam for the rib material, this is an excellent substitute for polystyrene, and scuppers near the fuel inlet to capture and absorb any spilled gas before it gets to the tapes. Polyurethane foam has the same density as polystyrene and is rigid. Polystyrene is semi-rigid but has very low compression strength. Polyurethane foam is inert to all chemicals, but is not UV resistant (easily overcome with a coat of ordinary inexpensive latex housepaint). Fabricating ribs from 2 lb density polyurethane foam was a bit of a head scratcher to overcome. Polyurethane foam boards in 1" thickness are available, but not locally and they are quite expensive. Looking around for a solution found expanding foam (often referred to as A/B foam) and is quite readily available and affordable from the marine industry. So a plan to cast the ribs from expanding foam was hatched, but to cast something you need a mold. I couldve just got a 1" thick board and made female molds by simply tracing the originl rib profiles, but that just didnt cut in my world. I wanted ribs that were accurate to the original Lazair ribs, but were repeatable if I ever needed to make new molds. So right now shiny new HDPE molds are being CNC cut from the hard work that was put into capturing the rib profiles in CAD and new polyurethane Lazair ribs will be available soon.

Whats the hold up? When will we have new Lazairs rolling off the assembly line?
This is the question I am asked all the time when someone gets wind of what I'm up to. And the answer is.....I wish I knew. I have basically everything needed to make new Lazair parts and whole ultralights except for a place to work in. My little town of Brooklet, Georgia and the larger Statesboro area has few offerings of spaces large enough to set up shop in, and what is available is priced far out of my meager budget. I am still vigilant looking for shop space, but until something comes along I will keep plugging away in the garage.

There will be more to come on the Blogspot page as I get used to this means of communication, check back from time to time. Until then email me at Fltofancy@gmail.com.

Later,
Gene