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02-23-2013 | 05:58 AM

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hugger-4641

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From: McKenzie, TN

RE: Introduction to Glow Engines

Ok, I've had a couple people ask for the tuning article that follows my previous article, so here it is. Open to all criticism and construction! Doesn't mean I'll implement every suggestion in my original article, but I definitely want to hear other's opinions.

Tuning a Glow Engine. By: Gerald Hughes (Jerry) aka Hugger-4641.<o

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>This article is intended to cover basic principles and methods of tuning a .25 to 1.60 sized two cycle or four cycle glow engine. It is recommended that you understand the basic principles of how a glow engine operates before attempting to tune one. There is an article titled “Introduction to Glow Engines” that precedes this article and explains the basic components and principals of common glow engines. There are also other resources available to help understand these concepts, including the manual that came with your engine. This article addresses the most common glow engines used by beginners in the hobby, which are not generally the highest priced or highest performance engines available. Some engines have special carburetors and tuning procedures that may be slightly different from the general methods that will be mentioned here. If you can obtain the manual for the engine you have it is recommended that you do.<o

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>The basic information and methods in this article can be applied to both two cycle and four cycle glow engines. Let’s start by mentioning a few things that need to be correct before attempting to tune an engine, these are:<o

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>fuel<o></o>glow plug<o></o>fuel lines and tank<o></o>throttle linkage<o></o>needle valve settings<o

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>Fuel- Engine manufactures usually recommend an optimal fuel mix for each engine. The brand may not be important, but the ratio of Methanol, Nitro-Methane, and lubricating oil is important. In the absence of a manufacturer’s information, a fuel with 10% to 15% nitro content and 15% to 20% oil content is usually a good place to start. The oil can be castor, synthetic, or a blend of the two. I personally prefer a blend of synthetic and castor oil since they both offer different benefits to the engine. Many engines can run on a lower or higher Nitro content or even no Nitro at all, but if the engine is designed for certain type of fuel, tuning can be a headache if not impossible when using the wrong fuel. Most entry level engines like O.S., Magnum, Asp, Thunder Tiger, K & B, GMS, and Saito, just to name a few, will run well on 10% to 15% nitro. Higher performance engines like Rossi, Jet, and Enya, just to name a few, may be designed for a higher or lower Nitro content. If so, then the manufacturer’s recommendation should be followed. Fuel should be stored in a cool, dark, storage area, and should be sealed to prevent moisture from the air being absorbed by the alcohol in the fuel. Prolonged exposure to light and air will degrade and contaminate fuel and make tuning difficult, possibly to the point of a total inability to produce combustion in an engine. If you have any doubts about the condition of your fuel, save yourself some headache and buy a new bottle of fuel. <o

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>Glow plug- Engine manufacturers will also usually recommend a glow plug or a range of plugs that work best in the engine. Glow plugs are generally classified by heat range from “hot” to “cold”. An O.S. A-3 (now called a #6) is considered a “hot” plug, where as a # 8 is considered a “cold” plug. Cold plugs will usually give a longer service life and will tolerate a “leaner” fuel mixture better. A hotter plug will tolerate a “richer” fuel mixture better, but will fail very quickly if the mixture is too lean. When running a glow engine, it is better to err on the side of the mixture being too rich, so where tuning is concerned, especially during break in, a hotter plug is usually more forgiving. I usually start with an O.S. # A-3 on two cycle engines (A3 is now called a #6). Most four stroke engines will use an “F” plug, which looks just like any other plug, but has a slightly different filament due to the differences in combustion cycle of a four stroke engine. <o

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>In addition to different heat ranges of plugs, there are also two different “styles” or lengths commonly used. Some engines may require a “short” style plug, where others may work with either a “long” or “short” plug. If a manufacturer specifies a “short” plug, it is likely that damage can be done to the piston by using a long style plug. Most of the engines already mentioned here will accept either a long or short plug, but this is another reason you should obtain the manual for your engine so you will know for sure. <o

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>There is another common type of plug called an “idle bar” plug, which has a small bar across the end of the plug that partially shields the Platinum filament. Idle bar plugs are not usually recommended by manufacturers for the previously mentioned engines, so we will not concern ourselves with them at this time.<o

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>Fuel lines and tanks- This is a common area for problems and mistakes. Let’s assume that your fuel lines and tank are new and/or in good condition. If they are not new, it is inexpensive to replace them if you have any doubts about their condition. Cracks, discoloration, stiffness, corrosion, and dry-rotting are obvious indications that tubing, metal tubes, and/or the tank need replacement. <o

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>Fuel lines are easily confused and switched if one is not paying attention. The pressure line from the muffler should attach to a piece of metal tubing that passes through the bung to the inside of the tank and bends upward terminating as close as possible to the top of the tank. The “bung” is a term that is commonly used to describe both the opening in the tank as well as the rubber seal that fits in the opening and through which the fuel lines also pass. This seal must be tight and free of leaks. The clunk and fuel “pick up” line are inside the tank and connect to a straight piece of metal tubing that also passes through the bung . The clunk should be within ¼” of the rear of the tank but not touching the tank. The clunk should swing freely from the bottom of the tank to the top so as to allow fuel to be picked up in any orientation the plane might be in. From the metal tubing the pickup line and clunk are attached to, fuel tubing should be connected to the High Speed needle valve. This may be a remotely mounted needle valve, usually located on the firewall or the back plate of the engine, or the needle valve may be attached directly to the carburetor. If the needle valve is a remotely mounted assembly, then another fuel line will exit the needle valve assembly and connect to the carburetor.<o

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>Installation of the fuel tank is often restricted by the design of the plane. As a general rule, the fuel tank and engine should be mounted so that the center of the bung is in line with the carb inlet. If the bung is lower than the carb inlet, fuel may not draw from the tank properly, especially as the fuel level in the tank is drawn down. If the bung is higher than the inlet, fuel may want to siphon on its own and usually results in flooding the engine. This can often cause “fluid lock” also called “hydra lock”, which occurs when the cylinder and/or the crankcase gets full of excess fuel which cannot be dispelled, thus causing the engine to “lock”. “Hydra-lock” can usually be alleviated by removing the glow plug and spinning the engine until the excess fuel is pushed out. However, the condition that caused the “hydra-lock” should be corrected if possible. Planes with engines mounted in the inverted position often suffer from this problem. If it is possible to move the engine or the tank to correct the alignment problem, then it should be done, but often the design of the plane and engine cowl limits this option. Scale and Pattern type planes are a typical example of this. Mounting the engine inverted allows the engine head to be hidden inside the cowl, which reduces drag and improves performance as well as being more cosmetically pleasing to eye. And besides, real planes don’t have a big engine head sticking up in front of the windshield, right? However, the benefits of mounting the engine inverted sometimes come at the expense of the carb inlet being lower than the fuel tank bung. The problem with flooding and hydra-lock usually don’t occur once the engine is running, but often happen during fueling and startup. Fortunately, most trainers and “novice” level planes are configured to allow proper positioning of the fuel tank and carb, sometimes at the expense of cosmetics.<o

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>Throttle linkage- While the engine may be able to run with an improperly adjusted linkage, it will prevent many problems if you take the time to get this right. Several undesirable things can result from an incorrect linkage such as poor throttle control, inability to idle, inability to reach full throttle, inability to shut off the engine, and binding of the push rod, which can cause servo failure or drain the flight battery too quickly. Details will vary depending on the type of linkage and the type of plane, but it is important to get the push rod or whatever linkage you have to move freely in both directions. The articulation between the servo arm and the throttle arm is very important. You will notice most throttle arms have more than one hole to connect the linkage and also the servo arm will have multiple holes. The linkage needs to be configured so that the motion of the servo arm and the throttle arm match as closely as possible. This will usually mean installing the linkage in the throttle arm first and then finding a hole in the servo arm that matches the length of the throttle arm. I usually remove the servo arm from the servo, power up the electronics and radio, and move the throttle stick and trim lever to full throttle. I always have a voltmeter of some kind installed during set up of any linkage so I can tell if the servo is under too much strain at any given position. I move the carb barrel so it is full open and temporarily install a short piece of dowel rod or ink pen top to keep the barrel open. If the linkage was able to be installed in the throttle arm without removing it, then it should already be set properly, if not, then I move the throttle arm to the proper position and lock it down. Next I re-install the servo arm in the proper position and adjust the linkage to the proper length. When done correctly, there should be no strain on the servo at any point in the travel from full throttle to shut off. When the throttle stick is moved to idle and the trim lever is centered, the barrel should close, but there should still be a slight crescent opening in the barrel, just about big enough to stick a tooth pick into. This enables the engine to idle. When the trim lever is now pulled all the way back, the barrel should close the rest of the way. If all is done properly, the engine should idle at 2000 – 3000 rpm when the trim lever is centered, and the engine should die when the trim lever is pulled all the way back. If it is impossible to perfectly match the servo arm travel with throttle arm, I will adjust so any binding that must occur happens only at the “shut off” position and not at the full throttle position.<o

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>Needle Valve Settings- Ok, now we’re down to the objective! Assuming everything else previously mentioned has been done correctly and you also read the first article, you should now have an engine that is safely secured and running. If it is idling or running at mid to low rpm, we’ll assume for the moment that the low speed needle is ok, or close enough for now, so let’s look at the high speed needle first. There are two methods for adjustment I commonly use depending on the circumstances. The better of these two is to use a tachometer to find the maximum rpm and then richen the mixture for a loss of 400 to 600 rpm. There are many inexpensive tachometers out there. I use a “Glo Bee” (about $20) because that’s what my LHS had in stock. Set throttle to full throttle, and assuming you started with the needle set at 2 ½ turns out, as mentioned in the previous article, slowly turn the needle valve in (clockwise) one click at a time until you find the maximum rpm. It is important to remember that turning clockwise closes the needle valve and “leans” the fuel mixture, and turning counter clock wise opens the valve and “richens” the mixture. (Be very careful around the propeller, take your time and move very slowly and cautiously) As you are turning the needle valve in, you should see the rpm rise until maximum rpm is reached, then it will start to drop off again. Do not let the engine run for more than a second at this setting, quickly open the needle valve back up several clicks rich until the rpm drops 400 to 600rpm below what the observed maximum was. <o

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>With the second method you must use your ear as your “tachometer”. The process is similar, turn the needle valve in until you find maximum rpm then richen back up a few clicks. Here’s where it gets a little different, you must have access to the fuel line between the carb and the tank. Pinch the fuel line briefly and notice if the engine speeds up briefly then slows back down to its previous rate. If so, great, keep turning the needle valve in a click at a time and then pinching until you find the point at which the engine no longer speeds up when pinched. Quickly open the valve back up to a point where the engine does speed up again when pinched. Then open one more click just to be sure. (always better to be too rich than too lean!) It is also important to note that if you have a remotely mounted needle valve, there is often a slight delay in reaction between pinching the line and the engine speeding up, if this is the case, pinch the line a couple times in succession to verify the results.<o

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>If you have a trusted assistant, you can improve this process by holding the plane vertical while making these adjustments. (keep the prop above your head!) This simulates what will happen in the air when the plane is climbing and the fuel draw is a little more strained. Also, when a plane is flying, there is more air being forced past the intake, which causes the engine to “lean” out on its own. This is why you always want to err on the side of being a little too rich as opposed to being lean. <o

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>If you are getting a good transition and good high speed results, but just can’t get a high or low enough idle, there may be a throttle stop screw that needs to be adjusted. If you have the manual, check to see if it shows an idle stop screw. This screw is usually perpendicular to the carb barrel, and simply adjusts how far the barrel can be closed, which determines how fast the idle will be. <o

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>If you cannot get a good transition from idle to full throttle and back to idle without the engine quitting or sputtering, setting the low speed needle (or air bleed) may be needed. This can be done using the same process as the high speed needle, except you don’t go to full throttle. Leave the throttle at idle, and use the pinch test or a tachometer to find the best setting. One big difference is that it may be impossible to move the low speed screw with the engine running. The adjustment screw is usually very small and may be recessed into the carb barrel. If you have a very long screw driver with a small enough tip, you may be able to do it without getting into the prop, but one slip will be a serious risk, so you may have to keep shutting the engine off and adjusting the screw in small increments. It is important to note that if you do have to make adjustments to the low speed setting, you will most likely need to go back and re-adjust the high speed. <o

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>There are a couple in-flight tests you can do to verify your success. Obviously, you should have a good idle and a good transition from idle to full throttle. If you can do a prolonged vertical climb and fly inverted, any problems with your needle valve settings will likely show up during one of these two maneuvers. Always make a short test flight and then land and check the temp of your engine. It may sound perfect and still be running too lean while in the air. Fly one or two circuits in a normal pattern, then land the plane. Wait about one minute after shutting the engine off. If the muffler is still too hot for you to touch, you probably need to richen the mixture another click or two. If you continue to fly at too lean a setting, the engine will probably overheat and quit before you are ready, either due to the fuel vaporizing or the glow plug burning out. This can also damage the engine cylinder, pistons, and rings. If all goes well with the first flight, then proceed with a vertical climb and inverted flight test. Check the engine temp again when you land. If you have to richen more than two or three clicks, you were probably not as accurate as you needed to be with your initial needle valve setting procedure. As they say; “practice makes perfect”, don’t be afraid of trial and error, just try to limit the errors by learning as much as you can on the front end!<o

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>There is also no shame in asking for help. If you’ve followed all the instructions here and are not successful, there may be a problem that will require an experienced person to diagnose. Finding a club and getting personal help is always a good plan. I don’t know when or where you are reading this article, but at the time I am writing it, RCuniverse.com is a great place to get help online. Pictures and/or videos of your efforts and problems are always helpful when trying to get online help. Many things that are hard to describe or easily missed are glaringly obvious in a picture or video!<o

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>I hope this has helped you with your glow engine endeavors. At some point in the near future, I intend to follow up with an article on basic RC air frame and control surface set up. Maybe soon after that, an article on basic flight training for RC planes. Until then, happy flying to you, and feel free to contact me personally on RCuniverse if I can help in any way!<o

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