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This site was last updated on February 15, 2008.

Pressure fuel systems

by Iskandar Taib
Previously published in the July, 1996 MACA Newsletter


Introduction

For Fast, FAI and ½-A Combat, and many forms of Super Slow Combat (indeed, wherever they are allowed), bladder fuel systems are used almost exclusively. They are cheap, simple, reliable and rugged. They will also feed fuel in the most violent of maneuvers.

However, not much has been written of them. Once in a while, one comes across an article in a magazine that explains how one goes about making a bladder fuel tank, but for the most part beginners are shown by example.

In the US, bladder tanks have traditionally been made out of pen bladders. These were used in old style fountain pens, and, as you can imagine, are pretty much nonexistent in this day and age (but see The Pen Sac Company, suppliers) In Europe, the "pacifier" tank was used (though before 1974 or so, everyone flew diesels and used metal tanks). The pacifier tank was, indeed, a baby pacifier with fuel tubing coming out at one end. I remember, when I first started flying Combat, combing the various drugstores and department stores, looking for cheap baby pacifiers. And when you could find them inexpensively, you had to take them apart, which for obvious reasons was not very easy. So, modern-day versions of the "pacifier" were not pacifiers at all, but were 1 or 2 cc latex pipette bulbs, which you had to get through a scientific supply house.

The pacifier tank and the pen bladders had one advantage - they are relatively "low pressure" systems, which makes setting needles relatively easy. However, they do have a tendency to burst, especially when they have been in use for a while.

An old-fashioned pen bladder

Bladder tubing

The modern-day equivalent to the pen bladder is latex tubing. Latex tubing comes in a large variety of diameters and wall thicknesses. It is inexpensive, and is comparatively durable (though burst bladders do occur now and then).

½-A bladders are naturally smaller than Fast bladders, so buy the appropriate size of latex tubing. The inside diameter determines the free inflated diameter, and the wall thickness determines the operating pressure. For Fast Combat, most people prefer tubing with a ¼" i.d. and a 1/16" wall. This is the typical high pressure bladder. Some people prefer a thinner walled bladder (usually ¼"x1/32" wall, or 3/16"x 1/32" wall), which puts out less pressure. This makes setting the needle a little easier. Thin walled bladders tend not to last very long, however, and do tend to break much more often.

1/8"x 1/32", or 3/16"x1/32" tubing is normally used for ½-A bladders.

Shelf life concerns

Rubber tends to deteriorate over time. Sunlight and fuel, and even ozone in the air are enemies of rubber, and latex is particularly vulnerable, since it is unvulcanized (or lightly vulcanized). The amber colored stuff is particularly hurt by UV radiation in sunlight. Black latex is better protected, but is apparently less resilient in general (I remember quite a few people using black latex tubing having problems with bursting bladders). The new, colored latex tubing is better protected against UV and does not have the problems with bursting that plagued the black tubing.

Assembling a bladder tank

A bladder tank is made up of 6 parts - the fuel tubing, something inside the fuel tubing to prevent it from collapsing, the bladder tubing, an end plug and a couple rubber bands (see diagram).

Use appropriate fuel tubing. Good fuel tubing will have a small enough diameter to tightly grip the needle valve, be stiff enough to be easily put on the needle valve, and flexible enough to be easily pinched off.

Some of the best tubing I've found was the thin-walled neoprene that Bear Manufacturing used to sell. It was soft enough to pinch off with your fingers, and would last practically forever. I keep reusing the same lengths again and again, after removing the bladder tube and washing in water and detergent. Unfortunately its not available anymore. Most neoprene tubing you find in hobby shops is way too thick-walled for this use. You can use silicone tubing, but it is very prone to tearing. If you have a burr on any metal tubing, or the fuel fitting, you'll likely end up with broken tubing.

I used to use brass or aluminum tubing inside the fuel tubing to keep it from collapsing when the bladder is assembled. These days I use aluminum pop rivets (remove the sharp "spike", though). If you can get them, ¼" to 1/8" tubing adapters can be found. The bladder fits into one end, and the fuel tubing into the other. I've seen them in auto parts stores, and Core House sells them. Pop rivets are a lot cheaper, though.

For the end plug, a lot of people use fired .22 LR shell casings. Unfortunately, brass tends to react with nitromethane and will turn green. It then releases lots of verdigris, which can do nasty things to your needle valve settings. Chromed shells (like those you get with CCI Stingers) might be better, or better yet, the aluminum CCI Blazer cases used for, oh, .25 ACP. Bud Bodzioch uses plugs made out of pieces of very large O-rings. Some people simply tie a knot in the end of the bladder, but that works best with small tubing sizes (such as used with ½-A) and wastes tubing. Perhaps the best end plugs would be aluminum aircraft flush rivets, but I haven't been able to find a good source of these. In the picture above, we're using a .22 pellet.

I use rubber bands to hold the bladder together, and there are quite a few who use wire. Some people use small cable ties, but I've never been able to get those to work.

Bladder pinch-offs

It is of supreme importance that there be a means to shut off fuel flow when the motor isn't running, or when you need to turn off the motor in a hurry. Over the years I've seen this accomplished in many ways. Some people use hemostats, bulldog clips and other means to squeeze the fuel line shut. This has drawbacks - the needed tool may not be within easy reach when you need it most, they tend to get lost, etc. etc. There is also the danger that the tool may slip and end up in a running prop, with disastrous consequences.

Small clamps that stay attached to the airplane or on the tubing are a lot safer and handier. Simplest to obtain is the large size Perfect line clip (the dumbell-shaped variety). Thread the fuel tubing through one of the big round ends. To pinch off the line, slide the clip over the tubing so that the narrow part of the clip holds the tubing shut. Some actually cut off half the clip (you only need one half of it) or bend the unused portion at a right angle to the other half to get better leverage.

A similar clip can be made out of a cotter pin glued to the engine mounting lugs.

I have personally been using polycarbonate IV tubing clamps. Not only can they be used to shut off fuel flow when the motor isn't running, they can also be used to shut off fuel momentarily during the starting process.

Bladder housings

Bladders have to be attached or housed in the model somewhere. The traditional method was to use a "rocket tube". "Rocket tubes" are lengths of cardboard tube embedded inside the leading edge of (usually) the outboard wing. The most common tube used was the Estes BT-55 (1.134" o.d.) for Fast Combat and BT-50 (0.959" o.d.) for ½-A. Estes charges way too much for these, but there are alternative sources. Usually, you paint the inside of the tubes with polyurethane, then sand it when dry. Then the end caps (usually 1/8" balsa) are glued on. This is a real pain, so alternative bladder housings are often used.

The simplest is an oval hole cut into the foam wing, coated on the inside with silicone caulk, and covered with plastic. The plastic covering is usually held on with a bead of epoxy. After trying this for a while, I noticed two things. First, its often hard to get the covering to stay attached. Second, some bladders tend to develop aneurysms when used in this kind of housing. Bladders last longer when constrained to somewhat less than their full, free expanded diameters.

Ed Bryzs uses the so called "Bryzs Bladder" method. The bladder has a wire loop on the fuel tube end and a large rubber band attached to the plug end. The wire is tied to an anchor (usually Ed uses a motor mount lug, as he uses a radial mount for his motors) and the rubber band is stretched to attach to a peg near the rear of the wing. The bladder is completely exposed to the elements. While this works very well, a burst bladder can mean a lot of fuel sprayed all over the place.

Some people use golf tubes. Though relatively inexpensive and light, they are somewhat smaller in diameter compared to the rocket tubes and require a longer length to accommodate the same amount of fuel. The end caps are the ends of Kodak film canisters welded to the tube using a soldering iron.

Lately, I've been using tubes made out of drafting mylar. Roll the mylar (acetate would actually be cheaper, and better, perhaps) on a form - a 1 ¼" dowel, perhaps (I use Aerotech 29mm high-power rocket motor mount tubing, which yields a tube with an o.d. of about 1.25"). About 2 layers is adequate. Use some thin cyanoacrylate under the last inch or so of mylar to seal the edge. Then wrap some filament tape around the tube in a spiral. This will keep the tube from coming apart if the glue fails. Remove the tube from the form, and use thin CyA to seal the inside edge. The caps are then glued on (I use square scraps of 1/8" balsa, applied to the ends using liberal amounts of CyA. After the glue sets, I trim the cap to shape with a knife and a sanding block). I make the hole through which the bladder fits using a conical grinding stone on my Dremel. The smaller drain hole (underneath, towards the far end) is made with a smaller grinding tool, again on the Dremel.

To cut the hole in the wing core (my tubes actually go behind the spar rather than in front of it) I use a tool made from a length of drainpipe. They sell sections of pipe meant to fit under a sink. They are 1 ¼" o.d. Sharpen the inside edge, using a grinding stone on a Dremel tool. If you keep the edge sharp, it will cut better. Some time ago I made a jig (its got two pieces of wood maybe 2 inches apart, with 1 ¼" holes the appropriate height off the surface) to guide the tool. Wipe some talcum powder on and inside the drainpipe tool, and you'll find it cuts much easier and straighter, with less effort. Push it in with a twisting motion.

If the plug of foam does not want to come free when you pull the tool out, use a hobby knife to cut through the outside of the wing in the appropriate place to release it.

How to use bladder tanks

The first step is filling the bladder. Most of us use 2 ounce syringes. You can get these from SIG at inflated prices, from a friendly neighborhood large animal vet, or from a Farm and Fleet. Phil Cartier sells special tips that fit at the end of the syringe that makes inserting the tip into the fuel tubing easier. Alternatively, you can press-fit lengths of 3/32" brass tube into the tips (this works best with "Monoject" brand syringes).

Notice the little sheet-metal screw inserted above. This will prevent the plunger from coming out of the end. This can save lots of time when fumbling through a pit stop with fuel-covered fingers. It could also save your eyes.

The end of the syringe is a male Luer connector. In this case, it has a locking ring. The little adapter tip is a female Luer to 1/8" barb adapter. You can get Luer parts in nylon from Small Parts.

Bladders have to be "trained" to fill from the far end. The easiest way is to fill the bladder with air, gripping around it with your fingers, except for the last inch or so near the far end. The bladder will inflate in this section. Clamp off the fuel tubing, and leave the bladder inflated like this for about 2-3 minutes. A well-trained bladder and proper filling techniques can make all the difference between having 4 ounces of fuel on board or 2.

Wear safety goggles, or glasses. With bladders, there is always the danger of squirting fuel.

Insert the bladder as far as it will go into the rocket tube. The end of the bladder should be within a half inch of the end of the rocket tube. This sometimes means that almost the entire length of the fuel tubing will disappear inside the airplane. That's OK, because the bladder will grow in length as you fill it and the fuel tubing will reemerge. Attach a filled syringe to the fuel tubing. Start by sucking out what air is in the bladder. This will eliminate air bubbles. Fill the bladder with one syringeful of fuel. Clamp off the tubing. Fill another syringe, and insert the tip into the fuel tubing. Be ready - the fuel will try to force the tubing off the tip, and the plunger out the back of the syringe. If either happens, expect to get drenched with fuel, which can be a messy, if not excruciatingly painful experience. Unclip the fuel line, and depress the plunger to add additional fuel. Do not overfill the bladder - overfilling causes "false pressure". Your engine will go lean once some fuel is used up and the false pressure is gone. You can more or less learn to sense false pressure through the force exerted on the plunger.

Clamp off the fuel line, remove the syringe and attach the end to the needle valve. If you do not already have a needle setting, do the dribble test. Hold the plane slightly nose-down and unclamp the fuel line. Watch the fuel coming out of the venturi. Adjust the needle valve so that the right amount comes out. This, of course, varies with the size of the motor. A .36 Combat Special will require the fuel to fall from the venturi in an unbroken stream for about 2 inches. After a while you will get good at this. Clamp the fuel line off again.

Prime the motor. Attach the glow-clip. Remove the fuel clamp, but squeeze off fuel flow using thumb and finger. Crank the motor, and when it runs on the prime, stop pinching off the fuel. The motor should keep running if the needle is set fairly close. If the motor runs very rich and is in danger of quitting, pinch off the fuel several times momentarily as you adjust the needle valve. If the motor runs lean, open the needle valve. You will find that the needle valve will be a lot more sensitive than when suction tanks are used. Usually, only 3-4 clicks separate a too-lean from a too-rich setting.

The right needle setting on the ground isn't always the optimum setting in the air - many motors will unload in the air, causing a perfect needle setting on the ground to be too lean in the air. When I first started flying Combat (around 1984 or so) everyone was setting motors very rich. These days most people will launch with a very slightly rich needle. Perhaps it was the props in general use at the time. In any case, the degree of the problem will depend on the motor, the fuel, the propeller and the model.

If the motor quits (the model crashes, the plug blows, whatever) shut off the fuel flow unless you want a massively flooded engine.