Chapter 5 : Airplane Powerplant

The aircraft powerplant consists of all the accessories needed by the aircraft to produce thrust. It consists of the engine, propeller, fuel tank, spinner, fuel tubing and other accessories that are directly related for the powerplant to function (see Fig.18). The model aircraft commonly utilizes the internal combustion engine and electric motors for thrust. Let us first tackle the internal combustion type (or I.C.E.).

Figure 18 : RC aircraft powerplant components for internal combustion engine (I.C.E.)

Usually the model aircraft use three different types of internal combustion engines with regards to the fuel they use.

1) Glow engine - engine that uses methanol for fuel and castor oil for lubricant. The igniter is called the glow plug, shaped like a miniature spark plug but uses coil for combusting the fuel and air mixture (see Fig.19). Power source for the glow plug is 1.5volts NiCad (Nickel Cadmium) battery or dry cell with high ampere. Usually 75% methanol and 25% castor oil is the fuel mixture. Castor oil readily mixes with methanol unlike the petroleum based motor oil. But for more speed, nitromethane is added depending on the type of aircraft flyer desire (e.g. pylon racing, sport flying or pattern flyer). This type of engine can either be a four-stroke or a two-stroke engine (see Fig.20a & Fig.20b).

Figure 19 : Typical two cycle glow engine

2) Diesel engine - This type engine use kerosene for fuel ( jet engine fuel ) and with
some additive for easy combustion. It combust fuel and air mixture by means of compression in the combustion chamber unlike the glow engine that utilize glow plug to combust the fuel and air mixture. It has more torque than the glow engine so it can swing larger propellers.

3) Gas engine - Gas engines are commonly used by larger RC airplanes because this type has larger displacement. It uses ordinary unleaded gasoline and petroleum based motor oil. You can compare its size with an ordinary handheld chainsaw.

If you are not very familiar with internal combustion engine you may ask, what is the difference between four strokes and two-stroke engine? A four-stroke engine (see Fig. 20b) combust the fuel and air mixture in the combustion chamber more efficiently compared to a two-stroke engine (see Fig. 20a). It is because the two cycle engine as illustrated in Figure 20a, the fuel and air mixture from the crankcase goes to the combustion chamber and the burned mixture are exhausted at the same time so some of the raw fuel is mixed to the burned fuel and exhausted. Fuel is wasted because the inefficient combustion. Unlike the four cycle which the: 1) Intake, 2) Compression, 3) Combustion & 4) Exhaust are four separate cycles which are more efficient because the fuel is completely burned and exhausted before taking in fuel and air mixture.

Figure 20a : Event cycle of a two stroke engine

Figure 20b : Event cycle of a four stroke engine

Figure 21: Electric powerplant components

In the past flying electric was not very popular because of the weight penalty of the batteries. But today, there are may who prefer flying with electric motors because of the technological breakthrough. Compared with internal combustion engine, starting is easy, no mess because of the castor oil emitted by the engine, quiet & no need for fuel. The only limitation I perceive is the shorter flying time and it is not as powerful as the internal combustion engine. We don't need brute force to fly so I think as long as it is strong enough to carry its own weight; there is no problem. Having extra battery packs can solve the limited flying time by charging it alternately after the first one is exhausted.

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