RADIO CONTROL PYLON RACING
Duane Gall, 1267 S. Beeler Ct., Denver CO 80231; E-mail: [email protected]
HEY, TANKS! "Bladder tanks" have made their appearance on the racing circuit within the last couple of years, and have become nearly universal in that short time.
Even if you're not a racer and are just stopping by here on your way someplace else, read on. The following information may save you a headache or two in your flying.
These new bladder tanks are not the traditional high-pressure tanks used in Control Line (CL) Combat—the ones made from a fountain-pen bladder or a length of surgical tubing, forcibly expanded to hold a couple ounces of fuel. They are conventional hard-shell tanks, containing an internal bag or membrane to keep the air and fuel from coming in contact with each other.
The bladder itself generates no pressure; the only pressure comes from connecting the air pickup to the muffler, which most of us do anyway. You could even run one without muffler pressure if you wanted.
Advantages
- Vibration is not a critical issue. One no longer needs to be as careful to mount the tank "just so," and surround it with foam padding to prevent the fuel supply from foaming and the resulting lean running of the engine.
- Tank placement is not as critical. Fuel draw is improved enough that the tank can be mounted at the aircraft's center of gravity, if desired.
- The engine doesn't cough or surge during a hard pylon turn—especially near the end of a run, when a conventional tank contains more air than fuel, and the pickup tube is likely to grab a breath of air while the fuel is sloshing around it.
These tanks are typically the Tetra brand from Performance Specialties (Box 3146, Gardnerville NV 89410; Tel.: (702) 265-7523), or the Bubble-Jett brand from Jett Engineering (Suite J, 6110 Milwee St., Houston TX 77092; Tel.: (713) 680-8113). Prices are in the $15–20 range.
The tightwads among us may object to spending the extra money, but the selection of sizes and shapes of bladder tanks is not as broad as the selection of conventional tanks.
My excuse is that I prefer to fly midwing Quarter 40 designs, such as the Nemesis. To fit one of the commercially available tanks into those, one must cut away part of the wing center-section; otherwise, there isn't enough room fore and aft. In an airplane that typically pulls 20–30 Gs in a turn, I prefer not to chop away pieces of the wing, if I can avoid doing so.
What to do? Make your own bladder tank setup
I used a Sullivan Slant Oval style six-ounce tank (SO-6) with stock components. It fits in my airplane, and is available in any hobby shop.
The first thing to do is get rid of the brass tubing that comes with the tank; brass corrodes in fuel. Unless you plan to replace the tubing (or the whole tank) every season, plastic is a better choice.
The plastic tubing that comes with the tank is all right, but the best to use is the inner Du-Bro nylon pushrod material (frosty clear, 1/8-inch outside diameter). It is thicker and stronger than the plastic tubing that comes with the tank and will not be affected by fuel. The tubing may be used as a pickup tube when connected directly to the bladder, but it is too stiff to use as a muffler line unless you heat it first and shape it. Better to use a softer silicone or fuel tubing for the muffler connection.
The next step is to remove the filler-neck assembly from the Sullivan tank. Inside the filler-neck area is a rubber grommet that holds the fittings in place; pry this grommet out and discard the brass fittings. Cut the stock pickup tube to get the bladder out of the tank shell, and then remove the old bladder.
The bladder supplied by Sullivan is a good material, but is a little small in capacity for my needs. If you want more capacity, you can substitute a slightly larger bladder from another manufacturer or fashion one from compatible material.
I built a simple stopper for the filler-neck using plastic tubing fittings (see illustration). The inner tubing is the Du-Bro material. Make sure the stopper is airtight; test the assembly with a syringe to see whether or not it is really airtight. If it leaks, re-seal the fittings with glue or use a different fitting.
When reassembling, put the bladder back into the shell, insert the stopper, and secure it. The completed custom bladder tank holds 5½ ounces of fuel. Note that the bladder almost fills the volume of the tank.
The Du-Bro plastic tubing has a marvelous quality: if held over a candle flame for a moment, the end will flare itself into a perfect little nipple that clings tight to flexible fuel tubing.
A photo shows the Sullivan stopper with the plastic tube fittings installed, and some sample connectors that I made out of the plastic tubing.
To make bends, carefully heat the material over a candle flame until it loses its "frosty" look and gets very soft. Then hold it in the desired position until it cools. It stores a lot of heat, so be patient.
Only the fuel-pickup tube extends into the tank; the air pickup ends flush with the inside of the stopper.
The end of the machine screw that squeezes the stopper is blunted with a piece of the smallest available fuel tubing. This is to avoid puncturing the bladder when it expands inside the tank.
A "fish mouth" shape is cut into the end of the fuel-pickup tube, and an additional notch is cut into the side of the tube—about 3/16" from the end. This is so the bladder can't block off the end of the pickup tube completely, until the tank is virtually empty and the bladder is compressed all the way around the tube.
Failure modes and fail-safes
The Achilles' heel of bladder tanks is that you can pop the bladder if you get too rambunctious with the syringe when filling or defueling. A pinhole leak is tolerable, and a large tear will put you out of business until you perform major surgery.
If you want a sort of fail-safe arrangement at the cost of a little extra weight, cut a fish mouth in the brass "clunk" that comes with the tank, and use it as the end of the pickup tube. If the bladder tears, you will not have a useless bladder tank with the pickup tube extending straight back and prepared to draw air as soon as the fuel is half gone, but a conventional clunk tank with a torn bladder floating around in it, and the fuel pickup attempting to follow the fuel wherever gravity or G-forces push them. It's not an ideal situation, but it could get you through the next heat.
Final assembly details
To continue construction, you must supply 3/8-inch-diameter heat-shrink tubing (Du-Bro shown, available at hobby shops) and a baby bottle insert (eight-ounce Gerber™ brand shown—100 for approximately $3.50 at most grocery stores).
Cut roughly 3/8-inch of heat-shrink, and slide it over the fuel-pickup tube, all the way to the stopper. Gather the neck of the baby bottle insert around the tube (do not wrap or twist it; just gather it), collar it temporarily with a twist tie or rubber band, and push it all the way to the stopper, inside the heat-shrink.
Gingerly warm the heat-shrink with a candle flame, being careful not to burn or melt the baby-bottle insert. Maybe you can manage to soften the insert material just enough to smooth out the wrinkles and pleats.
Pressure-test the assembly with a syringe, as shown.
If you don't own a syringe suitable for this purpose, I highly recommend the Jett-Tanker (shown), available from Jett Engineering for roughly $30. It comes with a machined-aluminum plunger, calibrated in 1/4-ounce increments, and holds enough fuel for two test flights.
The purpose of pressure-testing is not to see whether you can pop the bladder; I know you can do that. It's to see if the assembly is really airtight.
If there is leaking around the connection to the stopper, get one of the little wire wraps that come with the Sullivan tank, wind it once around the outside of the heat-shrink, and twist the ends with pliers until it tightens enough to cause a noticeable indentation in the heat-shrink. Then clip the twisted end, and tuck it up against the stopper, out of the way of the bladder.
Keep the whole connection as short, stout, and close to the stopper as you can.
The assembly in the middle photo eventually failed because of the 1/4-inch or so of unsupported plastic tubing between the stopper and the heat-shrink. As the bladder expanded inside the tank, it forced the tubing to bend, fatigue, and finally split. I rebuilt the assembly with a shorter connection, and it's fine.
Connect the bladder to your fueling syringe, and fill it with just enough fuel to allow it to pass through the opening in the outer shell of the tank. If the bladder needs some more while the stopper is still not all the way "there," fill a little more so the parts that need to expand into the far corners of the tank will have a head start.
You will pop the bladder if you stick it in the tank empty and immediately try to ram six ounces of fuel into it.
A photo shows the completed custom bladder tank. Note how the bladder almost completely fills the available volume. This tank—or more properly, the bladder inside this tank—holds exactly 5½ ounces of fuel.
As you might guess from those forces, it does want to squeeze out a bit of fuel at first. But after the initial engine start, the only role of the bladder is to keep the fuel encapsulated.
When half-empty, the bladder flops around in the tank, like a blob of Jell-O™. Air comes in to fill the void left by the departing fuel, but it comes in between the bladder and the outer shell. The only thing the engine "sees" is pure, unadulterated, liquid fuel.
Now, why didn't I think of that a long time ago? M*A
Transcribed from original scans by AI. Minor OCR errors may remain.
