SPORTPLANE BUILDER

Here they are . . . and not necessarily in the order of their importance: 1. The fuel tank(s) 2. Fuel tank vent(s) 3. Finger screen 4. Gascolator 5. Fuel quantity indicator 6. Fuel selector valve 7. Suitable fuel lines and fittings 8. Primer 9. Fuel tank tests 10. The fuel flow test These are the bare minimum needs for a safe and efficient fuel pump type system installation for your homebuilt. However, incorporating all these features in fabricating your fuel system will not automatically ensure its reliability . . . not unless each is properly fabricated, plumbed (installed), and functioning. The discussion that follows can be helpful in developing the fuel system for your own project. If nothing more, you can use it as a comparative checklist to ensure that you have not overlooked anything important.

Tony Bingelis

up a few test samples early in the project and store them submerged in fuel for at least a few months. The vinylester resin (used with the Glasair kit) seems to be quite impervious to 100LL aviation fuel . . . as are some polyester and epoxy resins. However, all these may not fair as well when exposed to auto gas containing alcohol type additives. I only have one recommendation to make. Avoid using auto fuels containing methanol, or any other exotic aromatic additives, as they will in time ruin most any installed fiberglass tank, plastic fuel line, or carburetor float. How about sloshing tanks? Sometimes a poor fiberglass layup can leave tiny pin holes, which may develop into fuel seepage or leak sources. For example, fuel could penetrate

these tiny surface imperfections and wick its way through the cloth weave and exit the tank's external surface as a leak some distance away from its point of entry. Such leak sources and fuel seepages in fiberglass tanks are difficult to trace. For this reason, many builders believe that a fiberglass tank should be sealed with a sloshing compound. Actually, just as many builders believe it's a good idea to slosh metal tanks for the same purpose . . . to seal tiny pin holes. NOTE - The newer Randolph #912 Sloshing Sealer is reportedly the sealant to use if the use of auto fuel containing alcohol is ever anticipated . . . but maybe you ought to check that out for yourself. Metal Tanks - In my estimation a metal tank is the lightest, most reli-

n

The Fuel Tank(s)

It seems that most high performance composite kits feature fiberglass fuel tanks built into the wings. Often a small fiberglass header tank is also built into the fuselage. On the other hand, other less expensive lightplane kits featuring mixed construction usually offer optional ready-made fuselage or wing tanks fabricated of fiberglass. In contrast, the projects built from plans are, in many instances, older designs and the fuel tank's depicted area shown to be made of aluminum. Fiberglass Tank - If your tanks are to be fabricated of fiberglass, be absolutely sure that the resin (epoxy,

SQUARE SHAFT

' L.--HANDLE CAN BE I • INSTALLED FOUR

' i'

DIFFERENT WAYS.

polyester or vinylester) you intend to

use is one that will not be adversely affected by aviation fuel or auto gas. Many resins, especially epoxy resins, are not compatible with fuel. I know from personal experience, and have learned from the experiences of others, that some resins, if used, may in time result in delaminated seams, soft spots and fuel tank leaks. To be absolutely sure this won't happen to you, you should personally check the resin's compatibility with the fuel you will be using. Make 72 APRIL 1992

HANDLE CAN ONLY BE

INSTALLED ONE WAY

FIGURE I.

FUEL SELECTOR VALVE DILEMMA (WHICH WAY IS LEAST CONFUSING ?)

able and long-life tank you can install in a homebuilt. An aluminum tank can be a riveted one sealed with a tank sealant such as Pro Seal (PRC trademark) - or it can be a welded aluminum tank. A welded tank should have plenty of welding rod added to the seam as it is being fabricated. That kind of weld may not be pretty but it won't crack and fail from vibration as may a welded tank seam where the edges are simply fused together. Beware! If the welded tank seams are narrow and pretty, the tank is a likely candidate for a short service life. Fuel Tank Test Avoid the risk of installing a tank that leaks. Testing a tank for leaks is relatively simple and may be done several different ways. Usually the tank is pressurized with air and soapy water is applied to find the escaping air bubbles (leaks). CAUTION - Do not use more than a pound or so of air pressure in testing for leaks. Some sources say not more than 3.5 psi. This is too much and will bulge and distort some metal tanks. Anyhow, since an accurate inexpensive low pressure gauge is hard to find, the procedure could be risky. Here is a safer way to check for fuel tank leaks: a. Plug and seal all fuel tank openings. The filler cap will be the most difficult to seal. b. Next, slip an ordinary balloon (use your favorite color) over one of the tank fittings. c. Adapt a bicycle valve stem to screw into one of the tank fittings and introduce air carefully into the tank. Watch the balloon. When it has expanded to about 12 inches, stop. d. If after a few hours the balloon is still inflated, you have no leaks. e. If it has gone down, reinflate it and start swabbing the tank with soapy water until you find the escaping air bubbles signifying the presence of a leak.

Which way is OFF? Before you install the fuel selector valve - blow into each port and label the tank serviced in each position. Beware if the valve stem is square and

the handle can be installed in any of 4 positions . . . see Figure 1.

of the flight attitude. You should realize that most fuel system schematic drawings do not show how the vent lines are fabricated and positioned to keep the fuel from overflowing or siphoning out. Orient the vent's overboard inlet to face into the slipstream. This provides beneficial ram air pressure which helps move the fuel to the carburetor. Although such a ram air proviso is much more important in a gravity flow system than in a pump driven fuel system, it is, nevertheless, worth installing. A clogged or obstructed fuel tank vent in a gravity flow system will

eventually cause fuel starvation because the tank will not feed . . . it's very much like pressing your finger over the end of a drinking straw. The liquid in the straw will not flow (drain out) until the obstruction (your finger) is lifted. A fuel system fed by an engine driven fuel pump is just as vulnerable. Here's why. A blocked vent line causes a negative pressure to develop in the tank as the engine fuel pump continues to "suck away." The result? The tank walls begin to suck in and collapse . . . then the engine fails due to fuel starvation. For this reason it is most important

Fuel Tank Vents Every fuel tank must be vented. Don't forget this important detail when fabricating your fuel tanks. Install a 1/4" diameter 5052 aluminum tube vent line in the tank, running it along the top (inside) of the tank from the inlet end to the opposite side. The vent inlet end should be connected to an external line and routed overboard in such a manner that fuel will not siphon or leak out, regardless

This cooling shroud provides easy access to the gascolator. The cool air is ducted from the rear baffle and into the top of the metal shroud to vapor lock potential. SPORT AVIATION 73

VENT

AUXILIARY PUMP (ELECTRIC)

CARBURETOR

FIGURE 2. FUEL SYSTEM FILTERS (SCHEMATIC)

to protect your fuel tank vent openings against itinerate bugs or any sort of nest building flying pests. Don't think for a minute that the

strong fuel smell will keep your fuel tank vents from becoming insectaries. Those darn bugs don't seem to mind the smell of fuel fumes, so

don't let that flawed thought lull you into complacency. What to do? Protect each fuel tank vent opening with a small piece of aluminum wire screen glued over the vent opening with 5 minute epoxy . . . or, maybe, with Pro Seal.

Finger Screen A finger screen is called that because it looks like a small finger fabricated from a brass wire screen and soldered to a threaded brass bushing which is normally screwed into the bottom of the fuel tank. Its purpose, of course, is to keep foreign

particles from entering the fuel line. The finger screen is the first of 3 screen filters normally found in the typical aircraft fuel system. Why so many filter screens? Well, it is hard to believe but debris somehow manages to find its way into the fuel system too often. This can happen even after a new fuel tank has been carefully inspected, blown out, vacuumed and otherFuel tank welds should be made with plenty of welding rod added. Merely fusing the edges may result in a nice looking narrow bead but it may soon crack in service. 74 APRIL 1992

wise thoroughly cleaned. Sometimes it seems as though a scrap of something or other lurks for

months in a corner of the tank awaiting the most inopportune moment to enter your fuel system. Fortunately, an installed finger screen will do a good job of keeping the fuel line open regardless of the size or origin of the particles. A finger screen is most effective when screwed vertically into the sump bottom of the tank from the outside. This also permits it to be easily removed for inspection and cleaning. Any smaller foreign particles slipping past the finger screen will be trapped by the gascolator screen which is located further downstream . . . it is made of a finer mesh. Beyond the gascolator screen is the one in the carburetor. This is the last chance to screen out whatever crud the finger screen and the gascolator screen allows to pass through. The Gascolator As already mentioned, one function of the gascolator is to filter out the smaller foreign particles which may find their way into the fuel system, and to serve as a fuel system drain. Ideally, the gascolator also functions as a water trap for the fuel system. To this end, the gascolator is normally mounted at the lowest point in the fuel system where water, if present, will naturally settle. One of the pilot's time honored duties is to check the gascolator for water by draining a bit of fuel out through the quick drain valve (normally screwed into the bottom of the gascolator) as part of the preflight. However, blindly following this routine may be a useless exercise for some aircraft. For example, in a low wing taildragger fitted with wing tanks, the gascolator, unfortunately, is seldom the lowest point in the fuel system. In this type of installation, the wing tanks must be fitted with individual quick drains to provide a means for draining any water which might find its way into the tanks through condensation. Look your installation over and see what your situation is in this regard. Anyway, do provide the necessary drain points for safety's sake. Engines don't function too well on water cocktails.

remaining. Note that this does not necessarily mean you have to have fuel gauges. The fuel quantity may be shown by any sort of visual indicator - just so long as you can tell how much fuel remains. In short, you do have to have a fuel quantity indicator, but it does not have to be a gauge mounted in your instrument panel. One final observation. Using one gauge to monitor two tanks may save money on gauges but may cost you an airplane. Fuel Selector Valve I guess a simple shut off valve is really a fuel selector too. After all, it does allow you to select either the ON position, or the OFF position. One thing is certain. Each tank should have a means to shut off the fuel in an emergency. Here are a few other tank selector related things to keep in mind. Those three-position brass fuel selector valves are heavy, so don't mount yours on the bottom of the tank. After a period of time, the frequent switching coupled with its weight can strain the tank outlet so much that a crack might develop around the tank sump boss and cause a fuel leak.

It is O.K. however to mount a simple shut off valve in the tank sump boss because it will most likely be used infrequently to shut off the fuel in an emergency or for maintenance. Mount your selector valve, if you need one, where you can easily see and reach it. WARNING - Builders have a propensity for forgetting which tank position is which, and which position is OFF. They know exactly how it is to be hooked up until after it has been installed and then they can't seem to remember. Verify each position of the valve by blowing through each port before you install the valve. Mark each position on the valve so you won't forget. Use bits of masking tape if necessary. That should solve the dilemma, shouldn't it? Oh, yeah? How about the handle (pointer)? Is yours one where the handle or pointer can be put on in any of four positions or can it only be installed one way? (See Figure 1) Then, too, some selector handles can confuse the pilot because either end can pass as the pointer. Paint the pointer, or otherwise identify it, to eliminate any doubt as to which end is which. Finally, don't use a "both" position in your fuel system if an engine driven fuel pump is required. If one tank

The Fuel Quantity Indicator The rules require each aircraft to have a fuel quantity indicator which, in effect, shows the amount of fuel

This fiberglass tank lasted 3 1 / 2 years until the new owner used auto fuel containing methanol. Before long the tank softened, became discolored, and began to leak. It was replaced with a welded aluminum tank. SPORT AVIATION 75

tem. They all have the 45 degree flare and are not compatible with the 37-1/2 degree AN fittings. Mixing the two

types will guarantee some dandy fuel leaks. You can use 3/8" diameter aluminum lines (nothing smaller)

arately.

Obtain the specific fuel consumption figures from the engine manufacturer's operators manual for your engine. As a safety factor,

allow at least 150% more than the actual takeoff fuel consumption for the engine.

anywhere inside the

wings and fuselage . . . up to the firewall. All fuel lines ahead of the firewall must be flexible because the engine moves around on its shock mounts. The minimum recommended size for fuel What would you do if the integral fuel tanks in your

composite began to deteriorate due to the use of auto fuel containing alcohol type additives? In some you are using.

as AN-6 (dash 6). Accordingly, the fuel

runs dry ahead of the other, fuel pressure may be lost. Here is food for thought. Do not switch tanks just before takeoff. You just might turn the valve accidentally to the OFF position. You may not

even realize what you have done because the engine will keep running on the gascolator fuel until you have taken o f f . . . suddenly you find yourself in a glider, nose high . . . and not a thermal in sight. The Primer An aircraft engine is almost impossible to start in cold weather without the help of a primer system. This is true even of the engine equipped with a carburetor that has an accelerator pump. In the case of Lycomings, the shot of fuel injected into the carburetor, when you move the throttle, is squirted into the engine at a point far

below where the spark plugs can get

at it. Since vaporization of the fuel is

reduced in cold weather, you have a

starting problem. You should try to hook up primer lines to at least two cylinders. Most builders use 1/8" copper lines and brass automotive compression fittings to make the installation. Use Suitable Fuel Lines and Fittings Don't make the mistake of using automotive fittings in your fuel sys76 APRIL 1992

Custom

Built

Sport

Aircraft

Handbook, Section 17, Fuel Systems. Source: EAA; toll free order number is 1-800/322-2412. +

lines is 3/8" soft (5052)

aluminum tubing. The equivalent size flexible hose (Aeroquip 303, for example) is identified

designs the adjacent structure would also deteriorate and begin to fail. Be sure you know what kind of fuel

Recommended additional reading:

lines from the gascolator (on the firewall) to the engine driven pump, and from the pump to the carburetor must be flexible hoses. These hoses should be fitted with standard AN fittings and connected to

standard AN aircraft fittings. Slip-on type hoses clamped to fittings have no place in an aircraft. They are a hazard to life and limb. To lessen the risk of vapor lock, protect your hoses from engine heat by fitting them with firesleeves.

If you wish to contact the author of this column for additional information, please send a SASE to Tony Bingelis, 8509 Greenflint Ln., Austin, TX 78759.

Fuel Flow Test A pump type fuel system operating on an engine driven fuel pump must

have a backup pump of some sort. .. usually an electric pump of suitable capacity. (A fuel pressure gauge is considered essential, too.) Even so, don't even consider flying your homebuilt before you conduct a simple fuel flow test. Its purpose is to see if your fuel system will provide enough fuel during the full throttle nose high climb attitude encountered during takeoff.

With the aircraft positioned in a tail

low (climb) attitude, disconnect the fuel line at the carburetor inlet, turn on your auxiliary (booster pump), and check the time in minutes required for a gallon to run out. Assuming a takeoff fuel consumption of 14 gallons per hour, the flow time must be less than 5 minutes (60 mins./14 gals, equals 4.28 minutes). Check the flow for each tank sep-

BOOKS BY TONY

The following books by Tony Bingelis are available from the EAA Aviation Foundation, EAA Aviation Center, Box 3086, Oshkosh, Wl 54903-3086, 1-800/843-3612, in Wl 1-800/236-4800, in Canada 414/4264800. Major credit cards accepted.

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