|This article provided by Jim Ewing, Great Hobbies.|
Radio controlled models may be powered by various means including
electric motors, 2-cycle glow engines, 4-cycle glow engines, and gas
engines. Each have there own advantages and disadvantages and all are
widely used by thousands of modellers world-wide. Your choice will have to
be based on the type of aircraft you must power, the size of your aircraft,
the noise limitations of your flying area, the sound you wish your model
to have, 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.
Gas Powered Engines
Gas powered engines (using gasoline) are becoming very 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-cycle and 4-cycle. 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-cycle 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-cycle 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!
Where Should I Start?
For straight simplicity, ease of maintenance, overall performance and lower cost, we generally recommend that the beginner start with a two-cycle engine for their model aircraft. Choose an engine that is in the upper portion of the displacement range recommended for the model. You can always throttle back and you have reserve power if you need it to get out of a spot.
Ball Bearings or Bushings?
You can get engines with either ball bearing supported crankshafts or with just bushings. Ball bearing engines usually have a bit better performance, run smoother, and last longer with proper maintenance but are more expensive than those with bushings.
Ringed or ABC?
The piston and cylinders for model engines are generally constructed in one of two methods; ringed or ABC. Ringed engines have been employed on model engines for years and were the main method of construction until recently. An aluminum or iron piston with a ring moves in an iron sleeve. The ring provides the compression when operating. Ringed engines are easy to flip start, generate good power, are inexpensive to restore compression after long usage by simply replacing a ring, and are generally slightly cheaper. Ringed engines require a generous break-in period where the motor is run very rich to provide lots of lubrication while the ring fits itself to the cylinder. They are also more easily damaged if the engine is run too lean.
More recently a method of construction has been developed called ABC (aluminum, brass, chrome) where an aluminum piston runs in a chrome plated brass sleeve. The piston and cylinder are matched at the factory to give a perfect fit and provide optimum compression. ABC engines start easily by hand, give more power than their ringed counterparts, have a good lifespan, and are less susceptible to damage with a lean run. They are slightly more expensive to buy and very expensive to restore compression if required as the entire piston/cylinder assembly must be replaced. No extended break-in is required for an ABC type engine.
Engine TermsThe following are a few terms that relate to engines that you may not be familiar with:
Schnuerle Porting — A standard 2-cycle engine usually has one fuel inlet port on the side of the cylinder opposite the exhaust outlet. On a Schnuerle ported engine there are several fuel inlet ports on three sides of the cylinder allowing more fuel to flow to the combustion chamber. This gives somewhat more power than with standard porting. A Schnuerle ported engine is usually slightly more expensive due to higher manufacturing costs involved.
Long or Short Glow Plugs — There are two lengths of glow plugs available. The short ones are generally used on engines of .15 cu in displacement and smaller. The long plugs are used on all engines larger than .15. Please follow the manufacturers recommendations.
Idle Bar Glow Plugs — An Idle Bar Glow Plug has a metal bar across the bottom of the plug which prevents raw fuel from dousing heat from the element during idle.
Long Stroke — The stroke of an engine refers to the distance the piston travels from top to bottom. In a Long Stroke engine this distance is a bit longer than on the standard engine making the engine a bit stronger in torque and operation lower RPM. Quite often an engine is "Long Stroke" if the stroke distance is greater than the diameter of the piston.
Propellers — The size of an aircraft propeller is described by two numbers; the diameter times the pitch in inches. For example, a 10 x 6 propeller is a prop of 10" in diameter and having 6" of pitch. The diameter is simply the length of the prop. The pitch is described as the distance the propeller will move ahead in a perfect or solid medium at 100% efficiency in one revolution. That is to say, if you were to rotate your 10 x 6 propeller exactly once, your plane would move ahead 6", assuming this could be done with 100% efficiency and no slipage in the air.
Different sizes of motors require different size propellers to keep their operating RPMs in an optimum range. The following link is a prop chart (72K JPG) from Top Flite for two and four stroke engines. Please note that even the same size prop by two different manufacturers may perform differently. Experimentation is perhaps the best way to come up with the best performing propeller in your particular plane.
Engine Thread Shaft Sizes — It is important in knowing the shaft size of your engine when ordering certain spinners, spinner adapter nuts, safety spinners and prop hubs.
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