With the increased popularity of electric propulsion, glow fuel has become less widely available. If you’re having trouble finding glow fuel maybe you want to mix your own. The three ingredients are methanol, oil, and nitromethane. Glow engines will run without nitromethane, but with nitromethane in the fuel your engine will produce slightly more power, and the needle valve will have a wider range and will therefore be easier to adjust. Most pilots like 5% to 15% nitro. 5% is plenty for me. Lots of guys stick with 0% just to save some money and keep it simple.
If you’re going to mix your own fuel you’ll have to figure out how much and what kind of oil to use. In the old days castor oil was the standard lubricant for glow engines. Nowadays there are some high quality synthetic lubricants that are used in commercially available fuel. Castor and synthetic have different lubricating properties and different thermal breakdown temperatures, but they both work well for the intended purposes. Each type has a couple of pros and cons. If your engine overheats from a lean run, when synthetic oil is burning up and evaporating, castor is still lubricating. When castor oil finally reaches its critical temperature it turns to goo and slows the engine down, thus preventing damage. Score one point for castor oil. But the disadvantage is that an engine lubricated with castor oil will soon be covered with brown gunk, which isn’t pretty. It’s sticky, it attracts debris, and if you leave it for a week or more the throttle and piston may get stuck. If stored for a year, it will almost certainly get stuck. An engine lubricated with synthetic will stay clean and pretty, so score a point for synthetic. It should also be noted that it’s not difficult to clean up a castor-coated engine, and a stuck engine will generally come loose with a little bit of heat. The vast majority of commercial glow fuel in the USA has a mix of castor and synthetic.
To determine which type of oil is best for your home made fuel you probably want to think about what kind of engines you are running. An engine with a cast iron piston in a steel sleeve will benefit from castor oil in the fuel, and more oil overall. This category includes Cox 049 engines and a lot of older engines made in the 1950s, 1960s and 1970s. There are some later engines as well with this metallurgy, including some from Enya, the early OS FP engines, and a few others. When in doubt, stick a magnet on the side of the cylinder and it will tell you if there is iron in there. When these engines were new the manufacturers tended to recommend 20% oil or thereabouts, and the recommendation was most often for castor oil, or at least a mix.
Another factor that affects oil requirement is the type of crankshaft bearings in your engine. Engines with bushings tend to do better with more oil, whereas those with ball bearings can get by with a bit less.
If you have newer engines, or old innovative engines that set the modern standard, it is most common to run them on 17 or 18 per cent oil, usually synthetic or a castor/synthetic blend. Some people use as little as 14 or 15 per cent oil, but if you’re going to do this you need to be good at adjusting the needle valve so it never goes lean and starves the engine for oil, and it’s a good idea to make sure that part of your oil is castor.
If you decide that you want to use castor, you can buy small bottles of it from Sig, or you can buy large or small amounts of castor oil from a website called Bulk Apothecary. If you want to use synthetic oil, there are quite a few good types and brands out there, but I do not consider myself an expert so I will refrain from naming oils that I have not personally used. I will say that Klotz Super Techniplate has a blend of castor and synthetic and has served very well in my engines.
Nitromethane can be found in small jugs online from various sources with various levels of shipping costs from reasonable to outrageous. One good way to get hold of nitromethane is to buy the 5 gallon barrel of 50-50 mix of methanol and nitromethane from VP Fuels if you have a local VP dealer. This is also a great place to get pure methanol. Even if you don’t have a VP dealer nearby, if there is a drag strip or a hot rod fuel shop you can almost always buy methanol. Just watch out for unknown “nitro” products. Racing fuel shops have products labeled with the magic N word that are actually nitro-something-else. You have to see the whole word, and then you know it’s actually nitromethane.
It is customary to quote glow fuel components by volume. A gallon (128 fluid ounces) of fuel with 20% castor oil and 10% nitromethane has 25.6 ounces of oil, 12.8 ounces of nitro, and 89.6 ounces of alcohol. Note that these are not ounces by weight, they are fluid ounces. The obvious way to mix a gallon of fuel is to pour each component into a measuring cup or cylinder with markings on the side, and pour them into a gallon jug. But if you want a trick to make it easier in the future, measure one ingredient and pour it into the jug, then make a permanent mark on the jug using a sharp object to scratch the plastic, marking the level of this ingredient. Then add another ingredient and mark the level, etc. After the first batch you’ll never use (or wash) a measuring cup again. If you’re making more than one gallon at a time, pour the finished gallon of fuel into a different jug and use the marked jug again to make another.
Here’s my favorite way to measure fuel, which is even easier and more accurate. Use a scale. Because fuel components are measured by volume, the volume has to be converted to weight by referring to the density of each component. I’ll save you some searching by sharing the information I found.
- Various oils have slightly different densities, but you can say .95 g/ml for castor or synthetic and be close enough for our purposes.
- Methanol is .79 g/ml
- Nitromethane is 1.14 g/ml
- 50-50 mix of methanol and nitromethane is .96 g/ml
Let’s say you want to make a gallon of fuel just like the stuff you usually find at the hobby store, with 5% nitro and 18% oil. The first thing you need to do is sit down at your computer and open a google window. Type “fluid ounce milliliter” into the window and a unit converter will pop up on your screen. 5% of a gallon is 6.4 fluid ounces. (128 x .05 = 6.4) On the google page, when I type 6.4 into the first box, the second box tells me that this is 189.271 ml. At 1.14 grams per ml, this is 215.77 grams of nitromethane. (189.271 x 1.14 = 215.77) My scale measures to the nearest 5 grams, so I’ll just call it 215 grams.
18% of a gallon is 23.04 fluid ounces of oil. Going back to the google window, this is converted to 681.374 ml. At .95 grams per milliliter, this becomes 647.3 grams of oil. On my scale that’s 645 or 650, either of which is close enough.
The remaining 77% of this gallon of fuel is methanol. Multiplying 128 ounces by .77 yields 98.56 fluid ounces of alcohol, which google tells me is 2914.7671 ml. Multiply that number by the density of methanol (.79 g/ml) and you get 2302.67 grams of methanol, or according to my scale, 2300 grams.
Figure out the formula for each fuel blend you want to make and record them in a computer file or on a piece of paper so you don’t have to do the math over and over.
Now here’s the easy way to do the mixing. You’ll need a small funnel, a rag, a paperclip, an empty fuel jug, and your fuel components. Set the scale on a low bench or table at about waist level. I am fortunate to have a huge restaurant sink, so I put the scale down in one of the basins. Put a small rag on the scale and set your empty jug on it. The rag will catch any accidental drips. Straighten the paper clip and bend it to make a big V. Hang the V on the top of the jug with one side in and one side out, then put the funnel on the jug. The paper clip allows air to escape around the funnel instead of coming back up through the funnel and creating burps and bubbles. Now turn the scale on and hit the Tare button so it reads zero. Pour oil into the funnel until the scale says 650 grams. Here’s the best part: You don’t have to wash the funnel because the weight is accounted for and it’s still in the funnel! Hit the tare button to zero the scale, and pour nitromethane into the funnel until the scale says 215 grams. Zero the scale again and pour methanol into the funnel until the scale says 2300 grams. Make sure you use the alcohol to wash the previous ingredients from the funnel into the jug as you pour. Now you have a clean funnel and one gallon of glow fuel. Put the cap on and shake it vigorously until it’s all mixed.
Now let’s assume that you’ve been to the swap meet and found a gallon of 20% nitro fuel for cheap. You want to dilute it with methanol and oil to make three gallons of 6.67% nitro fuel. Assuming that the original fuel has 18% oil again, and that you want 18% in your final product, you just need to split the fuel into three jugs and add the correct amount of alcohol and oil. First, put an empty jug and funnel on the scale and zero it. Pour the entire gallon of fuel into the empty jug. Now you know exactly how much the fuel weighs. Divide by three and pour this much fuel into an empty jug, then again into the other empty jug. Now each of the three jugs contains 1/3 of the fuel. You will be making 2/3 of a gallon of 0% nitro fuel to add to each jug. 2/3 of 18% is 12%, so you can do the math to find out how much oil to put in the fuel. Or you can go to the numbers quoted above for a full gallon and multiply by 2/3. In any case the final answer is 431.5 grams (rounded off to 430). Because methanol is 82% of this fuel, and you’re making 2/3 gallon, the total methanol content comes out to 1635 grams. Get one of your jugs that holds 1/3 of the 20% nitro fuel, add the oil, then the methanol, then repeat the process with each of the other two jugs. Now you have three gallons of 6.67% nitro glow fuel. It’s worth noting that if you want to do a little bit more math you can split your jug of 20% to make two gallons of 5% and one of 10%, or any other combination you want.
If you start with the 50-50 mix of methanol and nitro from VP, use double the volume for whatever nitro percentage you want. If you want 10% nitro, then you’ll need 20% of a gallon of 50-50 mix, which is 25.6 fluid ounces. Use the density data for 50-50 mix to find how many grams that is. When it’s time to add the methanol, remember that you already put 12.8 ounces in when you added the 50-50 mix.
Maybe your most economical option for obtaining nitro is to buy a gallon of really high nitro fuel at the hobby store, and dilute it with alcohol and oil sourced elsewhere. Or maybe you buy a lot of weird fuel at swap meets. I happen to have a gallon of 30% nitro Cool Power helicopter fuel, and two gallons of 15% Byron fuel that I got at the swap meet. My plan for this stuff is to dilute it with 0% Omega, because I can get a good discount at the hobby store if I buy it by the case. I also have my own alcohol and oil. The heli fuel has 23% oil, the Byron has 18% oil, and the Omega from the store will have 17%. If I mix the three jugs of swap meet fuel, eight gallons of 0% Omega, plus 116.5 ounces of alcohol and 11.5 ounces of oil, the final product will be 12 gallons of fuel with 5% nitro and 17% oil.
You may notice that I don’t care about the difference in the type of lubricants between one type of fuel and another. The helicopter fuel from the swap meet is all synthetic, whereas Byron and Omega have a castor/synthetic blend. As a mild mannered sport flier I find that I get good performance no matter what kind of oil I use. I want at least some castor oil in the mix, but other than that I don’t worry about it. If you are into pylon racing or 3D aerobatics, or if you have a tightly cowled engine, maybe you should be more concerned than I am.
With just a little bit of arithmetic the possibilities are endless. I apologize if this article is long and boring, but I thought I should go through all of the details for those who are math impaired.