Monday, April 6, 2009

Types of RC Jet Engine

The most impressive technology that the Remote control industry has yet created is RC Jet engines. The look and the sound of Remote Controlled Jets enable them to be an amazing sight at the flying field. In this post we will look at the differences between a real full scale jet engine and an rc jet engine. Remember that we are not talking about electric rc airplanes or electric ducted fan models electric rc jets those are found in hobby stores out there but we are only taking into account the real jet engines which are scaled to be used in radio controlled or rc jets and burn kerosene or jet A-1. Many a times EDF jets (electric ducted fan jets) are confused with real gas rc jets, though EDFs are great models and can perform advanced aerobatics.

You must know that rc jets having gas turbo engines are the most complicated and expensive radio controlled airplanes available before you go buy one. A beginner should clean their hands on the slower kind of beginner rc airplanes before getting an rc jet as to fly one you will require a huge budget and many hours of flying experience to be a successful rc jet flyer.

How radio controlled Jet Engines Work:

In order to have an understanding of radio controlled jet engines, we are required to have an insight look at the real full scale jet engine used by passenger carrying jets. The definition of a jet engine is that it is a device which operates in a medium of fluid which is air, and discharges it at a higher rate, to achieve propulsion. This is simple Newton’s third law of motion which is “every action has an equal and opposite reaction”. In other words radio controlled jet engine expels air at a higher rate than it sucked in, hence getting propulsion. This is the basic principle on which all jets and radio controlled jet engines work.

Now take a little deeper look; since we know that volume and temperature are directly proportional to each other, the heated air will have more volume as it expands. That is why when the temperature of a given volume of air in a chamber increases, its pressure increases as well. When we release this heated, pressurized air, it exits at a greater speed then the speed it entered the chamber, as a result the engine gets the thrust to move forward.

Now let’s discuss the types:

Pulse Type:

This one is a simple kind of a radio controlled gas engine, consists of a tube, a fuel and igniter. The tube has a shape which makes the inner flow of air more in velocity and less in pressure. An radio controlled or rc pulse jet engine works by igniting air and fuel mixture at a very high frequency of air bursting in from the inlet. The air coming in from the inlet has more velocity and less pressure but when inlet closes and fuel gets ignited in this air, the air gets hot to escape from the outlet which is a converging opening. This makes the escaping air high velocity. The escaping air from the chamber leaves a low pressure, which upon opening up of the inlet makes the air to rush in. This whole cycle continues to keep the rc pulse jet running.

This process is described in the diagram below:

Though radio controlled pulse jet are very simple in design but are very loud and not very efficient. Mostly hobbyists use them because they are simpler in design and do not have moving parts. They are built in small scale to be used. They are not much used in full scale jet airplanes.

Turbo RC jet:

In this type of radio controlled jet engine, turbines are used to compress the air, adding fuel and then igniting it to get the thrust. A turbine is actually a compressor which consists of a set of many blades which when spin, move the air through each other and towards the closed end, this builds up the pressure and compress the air.

This figure shows the working cycle of a Turbo RC Jet Engine:

A typical cycle of a turbo is:

1. Air is compressed when it enters the turbo chamber.
2. This compressed air is then enters the combustion chamber where it is mixed with fuel.
3. Upon ignition, the compressed hot air leaves the chamber at a high speed, giving the turbo rc jet engine propulsion required.
4. Interestingly, the hot air leaving the chamber is routed through a turbine which drives the compressor which was used to compress the air in the first place.

Turbo rc jet engines are very efficient as it is evident from its working cycle where the energy produced is also initiating the process. Though these are very efficient but are not in subsonic speeds, turbo radio controlled jet engines require higher operating speed than sound to give their optimum performance. That is why turbo engines are not used in the subsonic operations.

Turbo Fan RC Jet Engines:

As we discussed the working process of a turbo radio controlled jet engine, turbo fan also works on almost the same principle. The cycle gets a bit different where not all the air intake is taken through the combustion chamber but some of the air is made to exit not burned. Some cool air is mixed with the air fuel mixture to increase the fuel efficiency, reducing the speed of exhaust.

This figure illustrates working cycle.

How Model RC Jet Engine Works:

Until recent years, Model radio controlled jets were not true Turbojet engines as previously, the look of the rc jet airplane was spoiled by electric motors or gas engines with a propeller, Pulse engines or Ducted fan airplane types. Now we have real turbo rc jet turbine engines for our beloved radio controlled airplane flying hobby.

The little difference rather an advantage for our radio controlled jet engine is the type of turbine. The small rc jet engine uses the centrifugal flow turbine as compared to the axial flow turbine used in bigger passenger jet planes. The difference between the two types of turbojet turbine is the stages of compression. In a centrifugal flow rc turbojet turbine, the air when passes through the rotating impeller it is thrown outwards after entering into the turbojet turbine. The air is compressed when it hits the inner walls of the turbojet turbine chamber after that the air is passed to the turbojet combustion chamber. This greatly compressed air has an increased temperature and in turn pressure which upon ignition gives immense energy.

The Combustion Process:

As like real jets, the fuel which is used in all types of rc jet engines is Jet-A1 commonly known as kerosene. To mix this fuel with the hot pressurized air, the fuel is entered in the turbojet combustion chamber in form of a fine mist. A small glow plug just as used in the 2 or 4 stroke engines is there in the combustion chamber to ignite the mixture of hot air and fuel. The fuel air mixture, upon ignition explodes within the turbo radio chamber. The shape of the chamber is such that it compels the hot gases rearward the turbine and in turn increases the velocity of hot air through the turbine. This all makes the pressure of the hot gasses to build up at the narrowing cone pipe to exhaust with a great thrust from there.

This figure below shows the basic working of a centrifugal rc jet engine:

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