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Although practical model jet engines have been around since the early 1980's it has only been in the past few years that home construction has become viable. Careful design has made simple home construction possible using basic tools and a small centre lathe.
Much of the current development has happened in Europe with one of the simplest designs so far to come from Germanys Kurt Schreckling. Thomas Kamps also of Germany has produced a design that uses a commercial turbocharger compressor in his construction which speeds up the building. Both of these designs have been published in books in both German and English.
Model jet engines are very quiet when compared with the conventional glow plug two and four stroke engines as used by the majority of model aircraft fliers. These engines are, however, inefficient in the terms of fuel consumption, but the fuels they do use such as petrol-diesel mixtures or propane are a lot cheaper and more readily available compared to the methanol, castor oil and nitro-methane mixes used by glow-plug engines.
A history of micro gas turbines is available on RCuniverse.com.
Models powered by model gas turbines can use scale size air inlets unlike some ducted fan powered models. This is a definite advantage as many jets do have relatively small air intakes. The engines also have a very high power to weight ratio so model aircraft fitted with them can have high levels of performance if needed. It should be noted that these engines take time to spool up and they still generate thrust when throttled back. Therefore it is imperative that a high level of pilot ability is needed if you hope that your model will survive.
Jet engines in their simplest form, the turbojet, are extremely straightforward in operation. A turbojet consists of a compressor which is driven by a shaft connected to a turbine with a combustion chamber or chambers linking the two. Air is sucked into the engine it is compressed after which it is passed to the combustion chamber where the fuel is burned.
This burning raises the air temperature, which in turn causes it to expand out into the turbine stage which then powers the compressor, (which then starts the cycle all over again,) the residual energy in the form of expanding gas is exhausted out of the back of the engine. This rearward flow of gas produces a substantial force which then propels the jet forward.
Full size jet engines use axial flow compressors in the majority of cases as these have many technical advantages such as reduced frontal areas and high pressure ratios. Early full size engines used centrifugal compressors as these were well understood at the time, are simple in construction and have a high pressure ratio so only a single stage is needed.
Model engines usually use centrifugal compressors with car or lorry turbochargers often providing the compression stage. These turbochargers can be obtained as spare parts or salvaged from breaker's yard. Kurt Schreckling with his design has chosen another method where the compressor stage is constructed from plywood. This can be done quite easily although it does take a little time make.
Model engines are small and the time available to heat up the air passing through the combustion chamber is very short. Successful combustion has taken a lot of development effort especially for liquid fuels to be used safely and efficiently. more details about combustion chambers...
The turbine stage gets very hot so special materials are needed. This is not to much of a problem as the temperatures involved are a lot less than full size engines and the material required is usually available locally. Single stage turbines are used and home built engines normally use the axial flow type. It is possible to use the radial type of turbine such as those from turbochargers but the radial type is difficult to make.
A lathe is essential for construction although the amount of turning required is not huge. Maybe your local engineering college runs a night course, or you could develop a friendship with one of the students. The shaft connecting the compressor to the turbine will need the use of the lathe together with the compressor housing and turbine parts. If wood is to be used to form the compressor it will still require the use of a lathe.
Some welding and brazing will also need to be done, so a small welding facility is required. A pillar drill and normal hand tools will just about complete the tool requirement. It should be noted that the high temperature steels used will wear out cutting tools very quickly, especially the cheaper ones! A good supply of high quality hacksaw blades is a must.
All of the materials required for the build are relatively easy to come by. Items such as ball-races for the shaft are off the shelf items and compressor wheels from turbochargers are not difficult to locate.
The high temperature steel required is worth as scrap a lot more than normal mild steels so scrap metal merchants tend to keep it separate from the mild steel. Hence these merchants should be your first port of call when looking for out of the normal materials.
Several books have been published on model jet engine construction, many will be available in your local library. It always pays to develop a friendship with the librarian as their talents in obtaining obscure information are often overlooked.
The Gas Turbine Builders Association has hundreds of members world-wide exists to encourage and support the safe design, construction and operation of small gas turbines.
The association publishes a regular news letter which details the latest developments in the field of gas turbine construction. Many articles have been written on improvements, construction and other interesting aspects of gas turbines.
See also: Engine.