Hi Dennis
This article is pinned at the top, If you ever want to read it it will be there
Plugs? Castor? Synthetic? Percentage?
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I have been asked many times to help explain why we have so many things that can affect the run quality of Stunt engines. I will touch on a few of them , and hopefully help to eliminate some of these problems . Among them are tanks and fuel systems, glo plugs , fuel ,and overheating
What are things that make for a great, or bad engine run. We see these things most every weekend, and it is a very big point of frustration to many modelers. We all want our engines to run right, and it is nice when it goes through the pattern smoothly, coming on and off, exactly when and where you want it. Unfortunately, a lot of times, they growl , belch, shut off ,seem to have a mind of their own ,and are a total pain to deal with. One of the biggest causes of this that I have seen is improper fuel. Fuel is one of the most critical aspects in running model motors. Use the right fuel and you will probably notice nothing; the wrong fuel will have you grumbling, or worse, will have your motor screaming, belching and running with absolutely no consistency whatsoever.
Most fuels on the market today use a synthetic base and are blended for the R/C sport flier. These are typically very low on oil content, usually in the 12% through 15% range. This is never acceptable for our use in C/L Stunt. There are many reasons but the most important is the fact that we normally do not run our engines in a peaked two cycle, but rather a broad range of four cycle and rich two cycling. Any time you run with the motor set to come on and off in the maneuvers (like a typical 4-2 break) you are not only asking the fuel to lubricate the motor, it also has to cool the engine. The only way you can run in a 4-2 is to heat and cool the parts in the combustion chamber very rapidly. This makes the oil content critical, because it’s the unburned oil that helps carry away the heat.
Years ago, most fuels had only one oil ,castor. This is still a very good oil with many good but some bad points. Some of its good points; it carries heat out of the motor and gives a good plating action on all surfaces, especially when they’re hot. It also has tendencies to move toward hot surfaces, helping to protect them. A few of its bad points; it burns and sticks to the piston sides and the ring groove and all other parts that are hot enough, and will carbonize the chamber. It will stick rings in their grooves, freeze wrist pins and build up ridges on sleeves. This causes excess friction and heat and will ruin your motor in time.
The alternative to castor is synthetic oil and almost all fuels have these in them; the vast majority has all synthetic. Virtually all fuel manufactures use one type of synthetic; these are normally polyalkylene glycol based oils. They are mostly made up of alcohol started linear polymers , of oxypropylene groups. These are made by several companies and are available in a large range of molecular weights and viscosities.
This group of oils is the modern version of the old Ucon oils and also have good and bad points. Some of the goods points; they are very good lubes without containing any wax; they have outstanding load carrying capacity, film strength, anti-wear properties, are resistant to sludge formation, and will help keep your engine clean. The bad points are they give no rust protection by themselves, they don’t plate hot surfaces as well as castor and they burn at high heats.
As you can see, both oils have advantages and disadvantages to them; it’s for these reasons that they work much better blending together than they could ever work alone. Throughout many years of flying ,testing and other research have proven this to me beyond any doubt; plus you can see this for yourself. Recently, a friend of mine had a motor that would go into the pattern and lean out and act very inconsistently. The only change that was made was to substitute one tank of my fuel in the model. The results were drastically different; the motor now ran very smoothly, going into a two cycle instantly when the nose was raised and back into a four cycle instantly when the plane was leveled. This was tried back and forth both fuels; his and mine. The results were the same every time. I see this type of thing happen much too often, and it is extremely frustrating for Flyers to deal with. They often blame these fuel problems on cooling, cowlings, motors ,fuel filters, and unfortunately some don’t have a clue how to recognize or solve this problem. This is a frustration that you can live without!
I would like to tell you there is one Stunt fuel formula to run in all motors, I said I would like to tell you that…unfortunately this is not the case, and will never be as long as we have such a wide range of motors and running styles. What I will tell you is a good formula for the most common types of engines. Make sure you pick a fuel supplier who will give you consistent fuel day to day ,and will blend fuel for your motor needs or has fuel to match your needs. Stay away from any supplier who will not tell you the oil percentage, or who say one type works for all motors. I see this much to often also, It is unfortunate, but a lot of fuel manufactures will try to fool you about the oil and nitro percentage. One trick is to measure the oil by weight and all other ingredients by volume. Doing so, they can claim that the fuel is for example 18 % oil , when in reality it is only 14.9 % oil content. Using weight one for 1 ingredient and volume on the others does not equal 100 % . Other things are changing oil types, going to cheaper Nitro’s, and adding in other types of Nitro parrafins.
So what percentage do you try? For motors like Fox .35s, OS Max 35s or the old McCoy’s and K&B’s, use a fuel with 26 to 28% oil content; preferably half castor and half synthetic, up to 75% castor is OK. These motors have very small bearing surfaces, and are subject to much wear and heat, most are all plain bushing motors and most have unbushed rods. They need a lot of oil to help cool the engines. Since these motors run hot, they need extra oil to keep them lubed,clean, and to carry out heat . If you have one of these that is in very good shape but, is just starting to get some brown or black varnish plating on it, the synthetic mix will clean it up for you, resulting in increased life. Do not use the synthetic blend in an old motor that has a lot of time on it with all castor fuels; the synthetic will remove the castor varnish off the piston and sleeve and will in some cases, leave you with the worn-out motor that had to start with. Also always try to NOT use prop shaft extensions with these engine, as it adds a lot of wear on the crankshaft bearing.
For motors with larger bushings and bushed rods like to OS FP , Magnum GP series, Tower, and Brodak’s a 22-25% half-and-half oil mixture works the best. For S.T. .46 51,and .60s and most all ball bearings Stunt motors, a 23% half blend works best. Again the Synthetic blend will help keep the engine clean, and insure long life. If you use all castor in these types, it can stick the ring in the groove , resulting in poor compression and shortened engine life. If you have a ringed engine that castor has gummed up badly, most times running the synthetic blend will free the stuck ring, and the engine will return compression and power for you.
The tuned pipe motors like a little more synthetic and I recommend a 15% synthetic, 7% castor blend or a 20% half and half with 1 ounce of Aero-1 fuel supplement. Although many use 1\2 – 1\2 with great success. This works very well in the Precision Aero , OPS and Max VF engines, Super Tigre Thunder Tiger, AERO TIGER and most all of these type engines..
Four Strokes engines also like the blend, I have found that a 15 % synthetic – 3% castor blend works well for them, normal oil percentage is 18 to 20 %. This will vary some from engine to engine, but is a good starting point. Most like 10 to 20 % nitro, going up to 25% to 30% in the hot summer weather. Aero-1 Fuel additive can help 4 strokes tremendously, as these engines are lubricated mostly by “blow by” and can run very hot. Fuel and tanks are also very critical for 4 stroke operation. Make sure you have a tank that delivers fuel easily to the engine, as four strokes don’t seem to like having to pull fuel from the tank. Use as short a fuel tank as possible and keep it close to the engine. A lot of people use muffler pressure or pumps to help feed the 4 stroke engines. I have used OS VF pumps, Perry vibration pumps and Perry pressure pumps with my test on 4 strokes. I would suggest,as we do with 2 strokes, to use a Sullivan “Crap trap” fuel filter. They hold a lot of junk ,and have a very good double cone design, that pushes the debris away to the sides and almost never stop up. If you get a stopped up filter on a four stroke ( or 2 stroke for that matter) you can burn the engine up in one flight
When you use motors for the first time, you should also make sure you have the motor properly broken in. This will range from six tanks of fuel for one engine to almost two gallons for others. OS, for example, says two hours running time for their motors. A good break-in procedure is to use the same fuel as you will for your Stunt run ,and try to do your break-in on a bench; this is a lot better and an easier way to do a proper break-in. A diameter, one inch smaller than you plan to run at, at a 3 or 4 pitch, should be the prop to use. This will let the motor turn many revolutions more per motor run time. Start out in a very sloppy four-cycle for cast iron lapped piston and most ringed motors, slowly progressing to the fastest it will run in a four-cycle, then put it in a short two-cycle burst for short times. After the correct amount of time it should be able to run in a two-cycle without heating up and going leaner. Using 3 to 6 ounces per run with 5 to 10 minutes cool down time in between.
For ABC, AAC ,ABL, ABC-R and ABN motors, start out in a very fast four-cycle and about every 45 seconds; pinch the fuel tube to kick the motor into a momentary two-cycle. These types of engines normally take more break-in time than do their iron lapped piston cousins . If you can run the motor in a fast four-cycle and without touching the needle, pinch the tubing to lean the motor into a two-cycle for 20 seconds or so, then it should go right back to a four. After breaking in the engine with a few tanks of fuel , you can start using the needle to cycle back and forth from 2 to 4 cycle. When it is broke in you should be able to hold a 2 cycle for 30 seconds or so, and come back to a 4 quickly by turning the needle richer . If not, it probably needs more running time.
Plugs can also be a major cause of trouble, and poor runs. When you first crank the plane, notice if it goes rich and sags slightly when the battery is removed; if so, the plug is normally too cold. This is critical to getting a proper Stunt run.
Most plugs are designed to provide a colder range than we want in C\L aerobatic engines , and you should try to get the right range for the motor. Many days of testing and much time and expense buying almost every plug on the market has yielded these results ;Thunder Bolt R\C long, T Bolt #3 , T Bolt 4 stroke, Glo Devil RC #300 long, Enya 3 & 4, Fireball RC long, the Hobby Shack RC long, SIC RC long a few of the OS hotter plugs and some of the FOX long and Miracle plugs are best plugs for our use. In almost all instances, use a long plus, as they will be substantially hotter than the shorts, plus they are deeper in the combustion chamber and this tends to keep things hotter and keeps the plug elements cleaner.
A lot of times the plug problems show up as rich inside maneuvers and leaner outsides; this happens because gravity and centrifugal force ,forces the oil-fuel charge down on the element on insides, thus cooling the coil and pulls it away on the outside maneuvers, letting it naturally go leaner. I have seen this problem instantly cured by simply changing plugs. Please do not be afraid to put in a new glo plug , or try different types of plugs .
All of this assumes you have your tank height perfect (you did adjust your tank height, didn’t you), you’re right side up and inverted lap times are the same. This is important; don’t skip this step. If your using a profile sometimes you will need to have the tank center higher than the engine center. The 3 \16 to 3 \8 range will do for most fox 35s . Others will run on center line ,or just off of it. Another case of run problems are tanks or fuel delivery systems, which includes the tank, fuel tubing ,fuel filter, and anything else connected to the fuel system. When these problems arise in most cases, the engine changes speeds in flight, either faster or slower , and is generally inconsistent in the needle setting. This is almost always blamed as an “engine problem” when in fact it almost always turns out to be a tank problem, or fuel delivery system problem. I find most every time I see this , it is one component of the fuel system that is at fault. Either a hole in the fuel tubing, junk in the filter, a hole in the tank, a tank with an internal crack in the pick up or feed line . The next most common problem of this is water in the fuel. Water will give a very inconsistent needle setting, and will change at random back and forth from lean to rich. There are a few other things that cause problems with fuel delivery, muffler or pipe pressure will, most times magnify any little leak or problem you have and make things much worse than they were. A few other causes are an engine with a leaking backplate gasket, or an improperly cooled engine. A basic rule of thumb is to have a good intake area, with double the size of the exhaust area. Make sure you model (if fully cowled) has the air blowing all the way across and past the engine before the air flow exits the cowling. If your plane goes lean in maneuvers and comes back to a four-cycle slowly, it can be running too hot, you most likely need more oil, or less back pressure from the muffler. I have seen a lot of fuel with water in it (methanol attracts water) and this will cause erratic runs and needle settings. Always use fresh fuel and don’t be afraid to try another fuel if you think this is the problem.
Never try to put a brand new engine in a plane and try to break it in, trim and fly at the same time. I have seen this too many times with disastrous results. It is very hard to richen a too lean needle when the plane goes lean in flight. Keep good care of your equipment and it will usually take care of you; abuse it and it will most times let you down.
Now, a little more about fuels .For you guys who absolutely gotta buy the bargain R/C sport fuel…No amount of persuading will convince you otherwise; you at the very least need to add a healthy dose of castor oil. You can roughly figure 1.3 ounces will raise the oil content one percent (i.e. 13 ounces of oil to make 15% oil fuel into 25% oil fuel). This is not recommended and at the very best will usually be a guess, but it is much better than not adding anything at all, and I know people that do this all the time and get it to work for them .Example using Fire Power Cool 15%, pour off 13 ounces and add 13 ounces of Castor, this will be close to 11% nitro, and 24 to 25 % oil , this would make an OK fuel for plain bearing FP type motors, add a little more oil, and you have a fuel that you could run in your Foxes. The final thing I would like to say is to make sure you use an after-run oil between sessions and when you store the motors. This is another must do, because of the nature of the fuel we use. Then nitromethane or any nitropariffins burn, they leave behind, in your motor, acid and water, this, along with the water carried in partly by the alcohol, gets together and eats your bearing and other parts. Good quality after run oil is easy to get; don’t skip this step. If you can’t find a good after run at your local hobby shop, there are many available that are made by several companies…then try Prather’s. They make a good one and so does RJL and Aero Products. Do not use motor oil, Marvel Mystery oil; this is not after run oil.
Marvel makes excellent oil that can be used and as an after run oil and it is available from most auto parts stores and is called Marvel Air Tool oil. As a matter of fact, most air tool oils can be used as an after run oil; they are designed to fight corrosion in metal air tools and this is exactly what we are looking for. Another good place to get these types of oils are the large home supply stores like Home Depots and Builders Square type stores. Look in the department where they carry air compressor and paint guns. There are many brands of these oils so you see you have no excuse not to use them.
As for fuels, there are many good companies out there that will supply you with a good usable Stunt fuel. You will need to search them out in your area. . If you are using some of these suppliers, call them up. I’m sure most of them will oblige you. The model magazine are full of 800 numbers for fuel suppliers and the ones that I have mentioned come highly recommended; however, this is by no means all of them.
SIG for example has a very high quality fuel that is stocked by dealer all over the country; their Champion is 20% blended oil and with extra castor or oil supplements, such as AERO-1, makes an excellent Stunt fuel. Sig and I believe Power Master also make 20 or 25 % all castor fuels, as well as good 4 stroke fuel. Power Master is now making fuel for control line aerobatic planes, as are many other companies. There are more companies making good C\L stunt fuel such as S&W and others. I just don’t know them all and have not used their fuels in a while Keep in mind things will vary slightly, so don’t be afraid to try something new, or your buddy’s fuel if you suspect you have a fuel problem.
Randy Smith
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