Tips & Techniques

Bladder Pressure Fuel Systems

Should you decide to try a bladder pressure system on your 1/2A engine, you will need to gather some equipment. You also need to insure the needle valve in your engine will shutoff nearly all fuel flow to the engine. Otherwise, you may not be able to reduce the flow enough to obtain the proper setting. Most bladders provide enough pressure that the needle will be turned in about half way from the suction feed setting. The threads on the needle need to be fine enough to allow precise needle settings. 100 threads per inch or more is desirable. You will need a syringe to fill the bladder as you must generate some pressure to expand and fill the bladder with fuel.

After the bladder is filled, you must clamp the fuel line to retain the fuel in the bladder until the engine is started. Hemostats look professional and sharp, but a homemade clothes pin type clamp actually works better in competition situations. Bladder assemblies are made from surgical tubing, fuel line, brass tubing, and a small nylon cable tie. Other hardware is popular with many users, but the listed materials will build a proven setup. The simplest bladder for 1/2A is made from 1/8" (KT20) thin wall latex tubing. To make one tie a knot in one end of a piece of tubing. Cut the tubing long enough to go from the far end of the bladder compartment to the needle valve. That's it. You now have a bladder.Compartments for these bladders need to be longer than for old fashioned fat bladders.

To make an old fashioned fat bladder, cut a section of small fuel tubing long enough to extend from the bladder compartment to the needle valve. Insert a 1/2" length of 3/32nd" outside diameter brass tubing in one end of the fuel tubing. For bladder tubing, I use (KT21 or KT21a) 3/16" OD, 1/32" wall surgical tubing for 1/2A bladders. Regardless of what tubing you select, I recommend you use material that provides fairly low pressure. This usually means thin wall bladder tubing. Heavy wall tubing produces too much pressure and makes needle settings needlessly sensitive. Tie a knot in one end of the surgical tubing. Cut the tubing with the knot in the end from the rest of the roll so you end up with a bladder about two or three inches long. Slip the bladder over the brass tubing/ fuel line assembly. The bladder material should completely cover the brass tubing portion of the fuel line. The tie is used to keep the bladder on the fuel line. You can use soft copper wire, or a small rubber band in place of the tie wrap. Your bladder is now ready for testing and use.

Where and how to contain and carry the bladder in the airframe deserves some attention. In combat planes the container of choice for many years was a section of cardboard model rocket tube. Rocket tubing comes in various sizes, has a smooth finish inside, and is lightweight. Somewhere along the line we discovered that a simple opening cut in the foam wing panel and lined and covered with plastic packing tape worked fine. Generally, the requirements for a suitable compartment are: A smooth, non abrasive inside finish that will not puncture the bladder, a fuel proof finish or a configuration that will contain the fuel and protect the airframe from fuel in the event of a bladder failure, and in most applications, light weight. It is not critical that the bladder be mounted near the engine as the fuel will be forced to the needle in a reliable manner with little regard as to where you carry the bladder. It is important that the compartment be large enough that the bladder is not in a bind against the sides when the bladder is fully fueled. When you have the engine, plane, and bladder properly configured the remaining adventure is learning "The Drill." "The Drill" is the technique and procedure necessary for starting and adjusting an engine using a bladder system. The procedure is not difficult, if it were then it would not be suitable for combat where simplicity and consistency are rewarded. However, the procedures are different and attention to detail is more important than with a suction system. What follows is "The Drill" I use for 1/2A combat. The same procedures with different numbers will work for larger engines.

Other methods and techniques will get you airborne, and folks have their preferences on how to work with a bladder. But this way works for me and many others.

1. Fill your syringe with an appropriate fuel. Cleanliness is a must. Filter your fuel at least once before it gets to the needle valve assembly. I filter my fuel when I put it in the can in my pit box. I also have an filter in my fuel can that filters the fuel as it comes out of the can. Some flyers also run a filter between the bladder and the engine.

2. Fill the bladder with 3/4 to 1 ounce of fuel. Most disposal syringes are marked and so it is easy to meter out a precise amount of fuel. Put your clamp on the fuel line to keep the fuel trapped in the bladder and attach the end of the fuel line to the engine.

3. Get the needle valve setting in the ballpark by holding the airplane/engine nose down so that fuel coming out of the needle valve will be directed out of the engine. Momentarily release the fuel line clamp so fuel will flow to the needle. Take note of how much fuel is coming through the needle valve and running out of the engine. The proper amount for our 1/2a is a series of drips such that you have a line of drops about 3/4" apart. Again pinch the fuel line and stop the flow.

4. Rotate the propeller to close the exhaust port. Prime the engine in the closed exhaust port ever so slightly with one small drop. If you get more than a small drop in the exhaust port, dump it out and start over. Flip the prop over several times.

5. Hook up the glow plug clip. Flip the prop smartly to start the engine. When it starts the engine is going to run on the exhaust prime for a moment. During that moment you must release the clamp, or your finger pinch, and allow fuel to flow to the engine.

6. If the engine is running too rich, pinch the fuel line momentarily to keep it running. Alternately pinching and releasing will usually allow you to keep the engine running while you adjust the needle valve. Use extra caution during this time to avoid the propeller.

7. Disconnect the glow clip for final, fine adjustment of the needle valve. Set the needle so the engine is slightly off peak on the rich side. When the engine unloads in the air, it should be at full power. 8. If the engine quits at any time, pinch off the fuel line to prevent excessive fuel from entering the engine.

Tips and Information for the VA .049 MkII

1/2a flying is great fun. In no other engine class do you stand to get so much airborne time for so few bucks. No other class of gas powered model is as likely to bounce rather than break when the ground smites the aircraft. However, when compared to larger equipment, 1/2a engines and planes are more subject to the whims of the weather and nature and less forgiving of lax practices or sloppy techniques. Clean fuel, hot starting batteries (with the Nelson NL02 (HD) and NL05 (Hot) plug use 1.2 to 1.5 volts; for the Flat Element Nelson plugs (NL03 and NL06) closer to 2.0 volts is better), fresh glow plugs, and attention to detail are rewarded.

As with many ABC engines, most VA MKIIs may seem ready to go after only one or two runs. However, do the new engine a favor by putting half a dozen two minute ground runs on the engine before launching off into the clear blue. Needle settings for these runs should be just on the rich side of peak RPM and with a small prop. Sloppy rich runs will not allow the engine to heat up properly and extra stress is put on the rod and crank.

For better front end lubrication, and easier starting, especially on a new engine, put a drop of light oil just behind the prop drive washer before you start flipping. I use a black Grish 5x3 cut to 4" and fuel with 30% castor oil and 30% nitro for break-in. Adequate head clearance is magic when consistency is an issue. The engine is delivered with .006" to .010" clearance above the piston at TDC. Those numbers have worked well in many situations. However, if you are blowing plugs, the head is probably too close for your fuel / weather condition and a head shim will need to be added. Start with an additional .002" or .004" shim.

Before installing a new glow plug, smear a small amount of Permatex anti-seize compound (part number - 133a at auto shops) on the plug threads. This will help keep the steel plug from seizing in the aluminum head button. If the plug seizes the head button will slip and turn under the retaining ring when you attempt to remove the plug. If you can not get the plug out you may have to remove the retaining ring and head button from the engine and hold the button while you remove the plug. If the engine ever feels as though there is a compression leak, check carefully for leaks around the plug and the head retainer. Light oil or WD-40 will usually bubble if there is a leak. If there is a leak, make sure your plug is a Nelson Standard (no machined cuts on the flats of the hex) and also tighten the plug. If a black "liquid" shows up in the exhaust oil there is probably a problem. Stop running the engine and investigate. The cause may be a piston that has backed off on the wrist pin carrier.

Electric starters are intended for automobiles, pickup trucks, and high dollar model engines with ball bearings, beefy connecting rods and large combustion chambers. A VA MKII does not fit in those categories. However, if you feel you must use a starter... make sure the engine is not flooded or you may bend the rod or break the crank.

Top performance depends on using the proper propeller. For C/L combat the black Cox 5X3 cut to 4.25" works far better than it looks like it should. For Free Flight and R/C the APC 5.5X2 and 6X2 also work well. For Racing try the APC 4.2X4. When available, the KT15, KT16, KT17, KT11 and KT12 props are sometimes magic. Some 1/2a props of the nylon/plastic cast variety have crooked, out of true hubs. If the spinner / prop bolt is wobbling, true up or discard that prop.

Often, small engines are run many minutes compared to larger engines and each minute represents more cycles of the moving parts than an oversized / overweight engine sees. To get as much service as possible out of each engine, make sure you're running good fuel with castor oil. Compared to a .36 or .40, these little fellows use so little fuel that there is no reason to use fuel that may have inadequate oil. Use fuel with 20% to 25% castor oil. In the VA, castor oil based fuel will make more power and promote durability.

Keep your fuel clean and tightly capped. I filter mine when I put it in my pitbox fuel can and again as I draw it from the can. I use a liberal amount of either Marvel Air Tool Oil or Resilone for after-run oil following each flying session. I have found the initial start at the next session to be easier if I flush the oil out before cranking. I do this by removing the plug and adding a good deal of fuel to the case through the intake and to the cylinder through the plug opening. I then invert the engine, dump the fuel out the plug hole and the flip / spin the prop until fuel and oil stops spitting out and then replace the plug and proceed with a normal start.