The basic design of a jet engine is more straightforward than you might think. Air is sucked in at the front and then compressed and mixed with kerosene. The mixture is ignited, and the combustion gases propel the aircraft forward. Of course, the whole thing is much more complex in reality for a variety of reasons, not least because most of the compressed air does not even pass through the combustion chamber, but flows past the outside of the core engine and generates the thrust.
High speeds in the engine interior put considerable strain on the rotating components in the compressor and turbine. To keep vibration stresses low, particularly at the junction of disk and blade, is a major design challenge for development engineers. They must ensure as far as possible that the blades’ natural frequencies never lie within the operating range—in other words, at the speeds that normally occur. When that is structurally impossible, other solutions are required, such as the installation of dampers between blade root and disk to dissipate vibration energy via friction in the case of resonance.
However, their effectiveness cannot be verified using the traditional testing methods at component level, such as on a spin test rig, which is capable only of calculating the centrifugal stress acting on the rotor and therefore on the blades; it cannot simulate vibrations. In the shaker test, on the other hand, individual blades are clamped to a shaker unit and vibrated, but this method lacks the simultaneous action of centrifugal forces. For this reason, the only way of verifying the effectiveness of the dampers in the past was by test running the complete engine. This led to extremely time- and cost-intensive design variations and lengthy wear tests.
The engineers at MTU Aero Engines in Munich found a solution to this dilemma around five years ago: an excitation rig. In this test rig, explains Dr. Ulrich Retze from the component testing department, an individual rotor is set in rotary motion; simultaneously, by pumping in air, forces are applied to the blades whose strength and frequency can be set such that the vibrations at the blades correspond to those that occur in a real engine. Now various dampers can be tested at an early stage of development and without extensive alterations, and comparisons can be made with a non-damped system.
EXCITATION RIG IN ACTION
The excitation rig essentially consists of the existing spin test rig, to which the air injection system and other additional functionalities were added. Existing methods can be used to record and evaluate the vibrations. A key role is played here by the non-contact blade vibration measurement system (BSSM) developed in house by MTU, which is also used for testing with complete engines. Put simply, the system measures the times at which a blade tip passes a certain spot. Arithmetical conclusions about vibration behavior can then be drawn from the differences in time intervals.
In view of the clear advantages of the new method, it is unsurprising that prospective users were already lining up before development of the excitation rig was even fully completed. First up was an order from MTU Maintenance, who had developed a new repair method for the high-pressure compressor used in the V2500 engine and urgently required information about the wear and damping behavior of various damping wire configurations.
Subsequently, the test rig was also able to prove its worth in the development of new engines—namely, while testing the low-pressure turbine of Pratt & Whitney’s PW1000G Geared Turbofan™ family. The versions PW1500G (for Bombardier’s CSeries) and PW1100G-JM (for Airbus’s A320neo family) are already in flight testing, while the first flight of the Mitsubishi Regional Jet MRJ90 (with PW1200G engines) is imminent.
The excitation rig has also already been used for research projects. As part of the European Clean Sky program (see below), research is being carried out to further develop Geared Turbofan technology, and here MTU was able to use the rig to test a new low-pressure turbine technology it had developed even before the first run of the demonstrator engine.
Clean Sky and Clean Sky 2
Clean Sky is the most comprehensive research program ever undertaken in Europe for the development of new technologies to improve the environmental sustainability of aviation. Six Integrated Technology Demonstrators (ITDs) form part of the project, which was launched in 2008 and will run until 2017. In the SAGE (Sustainable and Green Engines) ITD, five engine demonstrators are being constructed and tested for different power classes and market segments. MTU Aero Engines is responsible for the demonstrator designed to validate improved Geared TurbofanTM technology.
Clean Sky 2 is already underway. The successor program was launched in 2014 and is due to be completed in 2024. MTU Aero Engines is involved as one of 16 lead companies in the aviation sector.
- MTU’s activities are focused primarily on further optimizing the low-pressure turbine and the high-pressure compressor.
- Core topics include developing lighter and more temperature-resistant materials and improving the aerodynamics.
- Components such as compressors and turbines are no longer to be considered separately, but as integral parts of the engine.