Thanks to a project supported by the German federal state of Bavaria, we were able to run a successful program of interdisciplinary research and development at a high level. By the completion of the 3 year project all the objectives defined at the outset were achieved: Project partners ISA from the German Armed Forces University (Universitaet der Bundeswehr) in Munich, MTU and IfTA GmbH were able, with the controller developed by IfTA GmbH to substantially increase the stable operating range of ISA"s own turbofan engine Larzac unit with the help of active compressor control.

This project was carried out in the context of an aerospace research project funded by the Bavaria federal government. Project partners included IfTA GmbH, MTU and the Jet Propulsion Institute ISA (Institut fuer Strahlantriebe) at the German Armed Forces University (Universitaet der Bundeswehr) in Munich who initiated the project. In this context, the scope for active control of compressor instabilities on an aerospace gas turbine was investigated. The aim of this research project was to displace the surge line of the compressor in the LARZAC 04 C5 turbofan engine unit at ISA (Institut fuer Strahlantriebe) with the help of active measures, i.e. the use of a controller, in such a way as to increase the size of the stable operating range and to improve the efficiency of the compressor. Following initial successful control tests with other research groups, using only dedicated test compressors, this work constitutes the first application involving a real and complete aircraft engine.

In this project, IfTA GmbH supplied all the controller hardware and software and supported the university with signal recording (metrology, recording) and evaluation (analysis, algorithms). Moreover, in the course of this project, further development and new development took place on hardware and software with a view to optimising the control function.

The application range of a compressor is restricted by what is known as the surge line, expressed during the phenomenon known as "surging" by a reversed flow across the entire cross section of the compressor. As a weaker and therefore less hazardous "preliminary stage" of this phenomenon, partial flow problems can move around the circumference of the compressor, also known as rotating stall.

The basic concept underlying active control of a compressor is to detect surge precursors or rotating stall problems and then to use a controller and an actuator to influence the system in such a way that the compressor does not encounter these undesirable operating conditions. A favourable solution has proven to be modulated air injection in the tip area of the blades on the first compressor stage, because this is where instabilities first occur.

Based on a modified form of the AIC system applied for active instability control of combustion oscillations on land-based gas turbines, various control strategies were devised during this project which were optimized for different compressor problems. With the help of these strategies and a specially adapted set of actuators, the research project, which lasted for three years, was able to deliver successful verification for the functional capability of active compressor stabilization. In overall terms, it worked to actively displace the surge line over the complete compressor speed range, and thus the feasibility of an industrial application of this innovative technology was proven. In comparison to constant air injection, active stabilization proved to be more effective, i.e. the same stabilization effect was achieved with less air and, at the same level of air consumption, the stable operating range was extended to the range achievable with constant air injection.

The innovative and research-related content of this collaboration was reflected by joint publications at international symposia.