Jets in Miniature
Model jet planes depend on high-precision nickel alloy castings
Nickel magazine, Sep. 00 -- For many, flying propeller-driven model planes has been a
rewarding pasttime. Flying jet-powered model planes is a more recent activity.
It is also more demanding for several reasons, one being the need for miniature jet engines.
Whereas propeller-driven planes make use of small electric motors or piston engines, which are readily available, the dedicated fans of jet model planes have undertaken to build the engines themselves.
To do so requires considerable technical knowledge, manual skills and tenacity in pursuing a target. Among the more difficult components of small jet engines are the hot parts of the gas turbine, which work at around 800°C and at speeds of 120,000 revolutions per minute.
The forces acting on the turbine wheels are comparable to those of large jet engines. For reasons of safety, the hot parts have to be of proven quality and, at the same time, resist the environment inside the gas turbine. Since the development of the first gas turbines, high nickel-containing alloys have been the material of choice, and the small parts for these model jet engines are no exception.
However, commercially available parts have proved diappointing'a situation which prompted Dieter Albisser and Bernhard Lüscher, two Swiss enthusiasts of model planes, to take matters into their own hands. After a lengthy development period, they were successful.
One piece of equipment they designed and built is an injection moulding machine for the production of the wax models that are used in casting the actual parts. After the casting moulds are assembled with the wax models inside them, they are heated so that the wax can be removed, leaving the necessary voids in the mould for the liquid alloy to fill. The quality of the wax models is an important element in the casting process. This is the same process used to produce components in full-sized engines.
For the stators (the system of stationary airfoils in the gas turbine), Albisser and Lüscher used alloy 1.4856 of preEN 10095, consisting of a minimum of 58% nickel, 20 to 23% chromium, 1% cobalt and 8 to 10% molybdenum. For the turbine wheels, they used N07713, which contains 68% nickel, 13% chromium, 4.5% molybdenum, 2.5% iron, 2% niobium, 6% aluminum and 1% titanium and is cast under vacuum.
The castings are now being produced by Zollernsche Hüttenwerke, Siegmaringen, Germany, a highly experienced producer of cast high-precision superalloy parts.
Each part is checked over its entire surface for cracks and is X-rayed. Each melt is also checked, for chemical composition and mechanical properties.
Typical performance data for model jet engines are as follows:
Mass: about 1 kg
Thrust: about 80 Newtons
Fuel consumption: 1 litre of kerosene every four minutes
Electronic control and microprocessor.
By A. E. Bauer, a Switzerland-based consultant to the Nickel Development Institute.
Photo: BERNHARD LÜSCHER DIETER ALBISSER
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