Radio control model plane engines have undergone a number of changes over the last two decades, enhancing both their performance and durability. We’ll trace those developments, as well as reviewing the advantages and disadvantages of different types of engines available to the rc modeler.
Currently, the most popular type of radio control aircraft engine is the glow plug engine, sometimes referred to in error as a nitro or gasoline engine. The actual fuel is neither nitro methane nor gasoline, although a small proportion of nitro methane is used in the mixture. Engines utilizing a glow plug have a simple ignition system that does not require a spark plug-hence no magneto, points or coil. However, glow plug engines must have a battery operated glow igniter in order to start the engine. The igniter causes the fuel in the glow plug to heat up enough to produce combustion. Once the engine is running the igniter is removed and the heat of the combustion keeps the fuel hot so the engine continues to run.
Similar to the bigger internal combustion engines, there are two-stroke and four-stroke model airplane engines, also referred to as two-cycle and four-cycle engines. The essential difference between the two is that a two-stroke fires per a single revolution of the crankshaft while a four-stroke fires once per two revolutions. Another difference between the two engines is their appearance. A four-stroke unit has internal valves which need to be opened and closed by external pushrods, though a two-stroke engine does not. Two-stroke rc airplane engines have been around longer than their four-stroke counterparts and produce more power for their size, as well as being much easier to maintain. Two-stroke units cost less due to their relative simplicity to produce. More rc training aircraft utilize two-stroke engines rather than four-stroke ones. Because two-stroke engines produce more power for their size, four-stroke engines will have higher engine size ratings for an identical sized plane. Four-stroke engines produce more torque at lower revolutions at a lower noise frequency, which more closely approximates the sound levels and patterns of a real aircraft.
Glow plug engines are also categorized by their method of compression. Ringed glow plug engines use an iron ring inserted around the aluminium piston that presses against the steel cylinder wall. This arrangement keeps the fuel/air mixture inside the compression chamber and oil out of it, as in an automobile engine. A more recent concept, the ABC engine, doesn’t use a ring, but achieves compression by means of a tapered sleeve inside the cylinder. The letters ABC denote the materials used; the piston is aluminium, the cylinder is brass, while the inside of the cylinder (sleeve) is chrome plated. The sleeve is gradually tapered inward towards the top of the compression chamber and expands outward as the engine heats up. The tolerances between sleeve and piston are so exact that a perfect seal is created when the engine is at running temperature. However, when the engine is cold, there is an imperfect seal between the sleeve and piston toward the bottom of the combustion chamber, making cold starts difficult at times. Though the ABC glow plug engines have a higher performance rating, there is little appreciable difference between it and the ringed glow plug engines according to most rc modelers.
Though the vast majority of rc aircraft engines are the glow plug design, there are other types worthy of mention. Gasoline or petrol rc engines have come into wider use in recent years. Though petrol engines have only been available in larger sizes, recent advances in technology have supported the production of smaller units, compatible in size to many of the glow plug types. Currently, both two-stroke and four-stroke engines are available for the radio control hobbyist, while a few years ago their choice was limited to a relatively few four-stroke units, at a much higher cost. The most recent petrol engines are using glow plug technology, making them both easier to start and maintain. These engines offer the advantage of glow plug technology along with lower fuel costs over nitro engines. While the initial cost of a petrol rc plane is higher than other types, this is more than offset by the fuel savings over the life of the rc model.
Diesel engines are another option for the rc modeler, though they are now rarely used. Such engines were manufactured decades ago, before the advent of glow plug technology and were the first engines utilized in radio control flying. Diesel engines do not use any form of plug for ignition but instead rely on the fuel/air mixture inside the combustion chamber to ignite from a process known as adiabatic heating, as the piston moves up and down. The compression inside the chamber may be increased or decreased by turning a threaded screw on top of the cylinder head. Increasing compression eases ignition of the fuel/air mixture, the fuel having a high ether content to insure ease of ignition. During the last fifteen years, battery operated rc aircraft have increased drastically, due largely to the introduction of lithium polymer or LIPO batteries. LIPO batteries are essentially a gel mixture of the two elements fitted into the rc plane. Both the power and endurance provided by these batteries are a quantum leap over previous battery technology, often providing near gasoline power for the rc model. The primary advantage of LIPO batteries is their relative power for a much lighter weight than their gasoline counterparts. They are an ideal choice for the beginning rc modeler, since they are easier to control by offering adequate power instead of the overwhelming power sometimes experienced with gasoline or nitro fuels. While LIPO batteries have become more efficient during the last five years, they still lack the endurance of glow plug or petrol engines.