Radio controlled models may be powered by various means including electric motors, 2-stroke glow engines, 4-stroke glow engines/petrol engines or even minature gas turbine engines. Each have there own advantages and disadvantages and all are widely used by thousands of modellers world-wide. The choice of power plant will be based on the type and size of aircraft you must power, the size of your aicraft, the noise limitations of your flying area and of course how much you want to spend.
Electric Motors are the newest form of power for model aircraft but probably the most popular for model cars. They are also ideal for small speed boats and scale boats of all sizes. With the vast improvement in NiCad battery technology (reduced size and greater capacity) over the past few years, electric motors have really become a viable means of power.
Electric motors to power small planes, 1/12 & 1/10 scale on and off road cars, and small speed boats are all very similar in construction. They usually have a ferrite core motor and operate at high RPMs. The “Mabuchi 540” is a popular example of this type of power plant and many manufacturers supply a variation on this motor. Model aircraft also use higher performance motors with Cobalt Magnets. These motors are more expensive but provide more power. Large scale boats will quite often use heavier, high torque, lower RPM motors for power.
Power for these motors is usually provided by a series of NiCad cells made up into packs. Six and seven cell packs are most common while larger Cobalt aircraft motors may require up to 28 or more cells. These are light and can be recharged quickly in about 15 to 30 minutes. Their run time is short, however, usually in the 4 to 10 minute range. In scale boats, weight is not nearly as critical and is quite often required. In these models, 6 and 12 volt lead acid and sealed batteries are used. These are also rechargeable (overnight) and with the higher capacity will give much longer run times in the order of an hour.
Petrol Powered Engines
Petrol powered engines (usually using unleaded) are becoming more popular in powering larger model cars, boats and planes. They don’t usually come in small sizes suitable for medium and smaller size models but for the large models they provide good power with a reasonable cost and inexpensive to run. A spark plug provides the fire and no external power is required in starting. Fuel is usually a gas and oil mixture. Boat and car versions are usually equipped with a recoil pull starter for easy starting.
Model glow engines come in two forms 2-stroke and 4-stroke. Both use “glow fuel” which is a methanol based fuel with castor and/or synthetic oil as a lubricant. The major difference in the two types is the way fuel is delivered to and exhaust is removed from the engine during operation.
In a 2-stroke engine the fuel/air mixture (as metered by the carburettor) is forced into the combustion chamber during the down stroke of the piston. During the upstroke the mixture is compressed and when the piston reaches the top of its stroke, the glow plug ignites the compressed gases, forcing the piston down. On the way down exhaust gases escape through the exhaust port while the fuel mixture enters the chamber. The entire power cycle takes place in 2 strokes of the piston.
In a 4-stroke engine the fuel/air mixture (as metered by the carburettor) is brought into the combustion chamber during the downstroke of the piston through a valve operated by the crankshaft. On the upstroke the valve closes and the mixture is compressed. When the piston reaches the top of its stroke, the glow plug ignites forcing the piston down. On the next upstroke of the piston a second valve opens and allows the exhaust gases to escape. The fuel mixture then again enters on the down stroke. The entire power cycle takes place in 4 strokes of the piston.
The glow plug is common to both 2-cycle and 4-cycle engines and is made up of a tiny little coil of platinum wire. To start the engine, an electric current, supplied by a 1.5 volt battery, must run through the coil to heat it. The engine is then turned over to make the engine start running. Once the combustion cycle has started, the coil in the glow plug can retain heat between firing and the electric current is no longer necessary.
The mixture for both types of glow engines is usually adjusted by two needle valves on the carburettor. One needle valve adjusts mixture for idle and low speed operation while the other is for high speed mixture adjustment. Engines for control line models do not usually have carburettors and operate only at full throttle. A needle valve is mounted at the air intake and provides mixture adjustment for high speed only.
Throttle control for R/C engines is usually accomplished via a rotating barrel in the carburettor. This barrel controls the amount of fuel/air mixture going to the combustion chamber and is activated by a small arm mounted on the side of the carburettor.
Two-cycle engines are the most common model aircraft power plant. They are simple, light, easy to operate and easy to maintain, and are generally inexpensive. They operate at a high RPM with a high pitched sound. Four-cycle engines are growing in popularity and produce a lower, more scale- like sound. They produce their power at lower RPM than two-cycle engines. Because of their valves, they have a higher part count and thus are usually more expensive than two-cycle engines. They may also require a bit more maintenance and adjustment than their two-cycle counterparts but they are not difficult to operate and maintain and they sure sound great!
Gas Turbine Engines
These are real jet engines in minature. They can be used to power jet models at very high speeds (200 mph +). They are definately not for the beginner and so will not be covered in any great details here.