Internal Combustion Model Airplane Engines

Although today, with the technological advancement of model airplane power plants, especially on the electric motors which now uses Lithium Polymer batteries, we should also learn the internal combustion type. Because without those miniature internal combustion engines, radio controlled airplanes and helicopters will not become a reality because there were no available electric motors powerful enough and batteries light enough to sustain flight. There are three main types of model airplane engines – the glow engines, gas engines and the diesel engines. Although the gas or ignition type is popular on big birds (1/4 scale models), we will concentrate on the two standard size model engines. Glow engines are by far the more numerous and popular, faster revving and generally more powerful size for size. They are suitable for powering all types of model aircraft and are produced in three general categories – standard engines for general use; racing engines for competition models; and RC engines for radio controlled aircraft. Standard engines may also be used for radio controlled models fitted with a throttle control or RC carburettor. This applies particularly to smaller engine sizes. Larger RC engines are generally specially designed to produce maximum power at more moderate revolutions than either standard or racing engines.

Glow engines have three particular disadvantages, although these are outweighed by the simplicity of operation and general flexibility of the type. First they need a special type of ignition plug, known as a glow plug, which can burn out and need replacement. They also need a battery connected to the plug for starting (and if the battery is ‘flat’ the engine will not start!), and special alcohol-base fuels which are a little more costly than diesel fuels and also attack paints and cellulose dope finishes. For this reason, model aircraft powered by glow engines must be finished in fuel proof dope, or given a final coat of special fuel proof paint.

Diesels are self-contained engines that they need only a supply of suitable fuel to run. They are generally heavier and more robust than glow engines, so they are usually longer lasting. They vibrate more and are less responsive to throttle control than glow engines, so are not generally recommended for powering radio controlled models. They cannot run as fast as a racing glow engine, so are less competitive in this respect. On the other hand they can be excellent power units for small and modest size free fight ‘sports’ models and certain types of control-line models. Diesels are produced in a much more restored range of sizes than glow engines. Very small diesels (smaller than 0.5cc) are difficult (and costly) to produce and can prove tricky to start and adjust. At the other end of the scale, diesels larger than about 3.5cc generally prove disappointing in performance. Thus diesel production is virtually limited to a size range from 0.5 to 3.5cc with the 1cc and 1.5cc sizes being by far the most popular.

Engine sizes – and how they are specified.

Engine sizes are specified by displacement or the interior volume swept by the piston in making its stroke up and down the cylinder (swept volume). In the case of diesels, displacement is always quoted in cubic centimetres or cc (mainly because the model diesel originated in Continental Europe). In the case of glow engines, displacement are (nearly) always quoted in cubic inches (because this type originated in the United States). Manufacturers produce glow engines in a more or less standard range of sizes, originally representing logical steps in power output. These are 0.049 cubic inch (also known as 1/2A); 0.09 cubic inch (also known as Class A); 0.19 cubic inch, 0.29cubic inch, 0.35cubic inch, 0.49 cubic inch and 0.60 cubic inch. Quite often the cubic inch is dropped and just the figures quoted 049, 09, 19, etc.

Some manufacturers produce additional sizes, e.g. smaller than 0.049 cubic inch for powering tiny models, and intermediate sizes to cater for a particular size or type of model, such as 0.40 cubic inch for RC models. The need for intermediate sizes is rather more commercial than realistic however. Classification of porting nearly all present day model airplane engine are of similar layout, the main differences being in the method of inducting the fuel/air mixture into the crankcase and then transferring it to the top of the cylinder. Induction is controlled by a rotary valve, either a hole-opening into a hollow section of the crankshaft, or a disc with a hole, driven by the crankshaft. In either case, sheathe hole in the crankshaft (or disc) comes opposite the end of the carburettor tube, the intake port is opened and then closed by subsequent rotation of the crankshaft. The circumferential length of this hole determines the intake timing.

When induction takes place in front of the cylinder (through the crankshaft), the layout is known as front rotary (readily distinguished by the carburettor, or strictly speaking, the intake tube) coming in front of the cylinder. With rear rotary engines the intake tube attached directly to the back of the crankcase. Transfer of fuel and air mixture drawn into the crankcase to the top of the cylinder is controlled by piston movement opening the top of a transfer port (or ports) formed in the side of the cylinder. This porting may be conventional, or specially arranged to give a ‘boost’ to the charge to fill the cylinder head in the most effective manner. The latter is a relatively new development with model airplane engine and is known as schnuerle porting. It is now a feature of many high performance glow engines.