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Aircraft Ground Vibration Testing (GVT) is a key part of the aircraft development process, which aims to obtain experimental vibration data of the overall structure of the aircraft in order to validate and optimize its structural dynamics model.
The modal parameters determined through this test are used to validate the analytical model, and the modal characteristics are also used to predict the aircraft’s flutter in order to create a safe flight envelope prior to flight operations. Aircraft Ground Vibration Testing is therefore a key process in the aircraft design and development cycle.
The ground vibration test validated our understanding of the fundamental dynamic behavior of the aircraft. The test measures the vibration modes of the aircraft, including the resonance frequency, damping characteristics and mode shape of each mode.
You can compare this process to playing the violin. The vibration characteristics of a violin string depend on its mass and stiffness. Similarly, ground vibration testing works by stimulating the aircraft structure to vibrate at different vibration modes or resonant frequencies, which are then measured using sensors. This is similar to how a violinist uses a bow to excite the vibrations of the strings to produce sound frequencies.
The dynamic response of an aircraft can be described by its vibration modes. No matter what the aircraft encounters during operation, its dynamic behavior can be characterized by these vibration modes. For example, when a gust of wind blows through an aircraft, the time domain response of the aircraft can be represented by these vibration modes.
In some special cases, the vibration of an aircraft structure can interact with the unstable aerodynamics around it, a typical example being the famous collapse of the Tacoma Narrows Bridge.
The bridge’s torsional vibration mode resonated with the instability of the surrounding air flow, causing the bridge to collapse.
It can be seen that accurate measurement and understanding of aircraft vibration modes is crucial to predicting and avoiding such dynamics problems. Ground vibration test is the key link to obtain these key dynamic parameters.
The entire process of ground aircraft vibration testing requires careful and timely management in several key stages.The first is the preparation phase, where the results of the simulation model can be used to optimize the test. Having an efficient hardware system helps optimize the excitation and data measurement processes. During the measurement process, field data analysis can help determine if the experimental configuration needs to be adjusted.
Once all the measurements have been made, a large amount of test data needs to be processed and transmitted efficiently. Post-processing of these test data can produce the necessary results to meet compliance standards and ensure safety.
All in all, from preparation to data processing, engineers need to carefully control the ground vibration test to ensure the smooth implementation and validity of the results. Only in this way can the test truly provide reliable experimental data support for the verification and optimization of aircraft dynamics model.
Through finite element analysis (FEA) model, we can simulate the vibration characteristics of complex test structures. Subsequent actual ground vibration testing (GVT) can help optimize these dynamic simulation models. The experimental test data can be used to continuously refine and improve the numerical simulation model. However, when conducting extensive modal testing of large aircraft components, there are often a number of uncertainties.
How many sensors are needed for aircraft ground vibration testing?
What are good measuring positions for these sensors on aircraft components?
In order to fully excite all modes, what are the good drive point locations for placing the modal vibrator?
Are the boundary conditions established correctly so that the first flexible mode is well separated from the highest rigid body mode?
Finite element (FE) models play a crucial role in optimizing vibration test setup. For large and complex structures, it is often necessary to connect all sensors at once for modal testing. However, if the number of sensors or the number of measurement channels in the data acquisition system is limited, then the method of mobile response modal testing can be adopted. In this case, the FE model can help simulate the strength of the mass load effect and provide a basis for the test layout.
In addition, the FE model can effectively determine the optimal driving point position to ensure that the global vibration mode of the aircraft structure can be fully excited. Depending on whether all modes of interest are included in the frequency response function (FRF) plot, you can also plan the number of drive points you need.
The Modal Assurance Standard (MAC) analysis not only helps to optimize the number of measurement points and improve spatial resolution, but also to select the best sensor placement to uniquely identify each vibration mode.
Finally,FE models can also generate geometric grids for experimental modal testing, contributing to the deep integration of experimental testing and numerical simulation.
In short,FE models play a key role in optimizing all aspects of vibration testing, laying the foundation for improving the quality of experimental tests and the reliability of finite element models.
The setup and instrumentation for GVT modal testing is time-consuming and complex. A large number of sensors are needed to capture the response of the entire aircraft assembly. It is important to ensure that each sensor is carefully mounted on the measuring point in a precise orientation. The error space of these high channel count tests can negatively affect the results. Using simple features like “Read all TEDS” and highlighting the measurement points mapped on the grid can greatly help this process.
For ground vibration testing, soft ropes are suspended from the aircraft to simulate free-free boundary conditions. A general rule of thumb to ensure good boundary conditions is to set the frequency of the highest rigid body mode to less than 1/10 of the first flexible mode.
There are multiple accelerometers on the plane. It is excited with multiple mode shakers and multiple reference frequency response functions (FRF) are obtained. The high channel number data acquisition system can effectively process these large channel test data sets.
GVT’s modal shakers are typically mounted on the wings and tail of the aircraft. The modal shaker can also be mounted on the fuselage of the aircraft. These multiple-input multiple-output (MIMO) modal tests can use multiple output excitation types (e.g., random, sinusoidal, etc.).
The EDM Modal software is a complete test and analysis suite that intuitively handles the geometry, measurement and processing of these GVT modal tests. A variety of efficient curve fitting algorithms can be used to analyze MIMO FRF to extract the modal parameters of aircraft components
After setting up an experimental setup with appropriate boundary conditions and connecting all sensors to the hardware system and laying out measurement and excitation points, GVT modal tests are performed to obtain all FRFS. Several excitation signals can be used to perform these GVT measurements. Modal hammers can be used for impact measurement. Modal shakers can use a variety of output excitation types, such as shape random, burst random, sine, swept-frequency sine, pseudo-random, etc. Careful selection of the output excitation signal helps to optimize the accuracy of the measurement results.
Random excitation signals are beneficial for exciting a wide frequency range and help capture all modes of interest of the aircraft in a short period of time.
The phase and amplitude of periodic randomness are random from one block to another. Pseudorandom is also a random waveform of the period type. The difference with periodic random is that only the phase is random when generating a pseudo-random block.
With periodic random or pseudo-random excitation signals, the user can adjust two parameters: the number of delays and the number of cycles.
Due to the nature of the periodic response, these data blocks can be averaged in the time domain. After that, a spectrum is computed to generate all the corresponding spectrum, including the FRF signal. This process repeats the frequency spectrum average number of times, which is 10 times in this setting.
The use of periodic output waveforms significantly extends the test time, but the end result is an accurate measurement.
Shape Random and Burst shape random excitation allow users to specify the driver PSD profile. This allows more energy to be concentrated in a specific frequency range. Since the excitation level can be adjusted over the frequency range, by improving the excitation to the structure under test, this will increase the signal-to-noise ratio in areas with lower response levels.
Dedicated sinusoidal excitations, such as sweep and step sinusoids, provide a very good signal-to-noise ratio by focusing all the energy on one frequency point at a time. When testing these large structures, the wideband excitation is not possible to deal with the nonlinearity of the structure, and the test level may be too far from the operating vibration level. The quality of FRF obtained by sinusoidal excitation is higher than that obtained by random noise excitation.
After the test is ready and the above Settings are configured, perform the GVT modal measurement to obtain the MIMO FRF measurement. These different excitation techniques are used to test different configurations of aircraft components to observe the effects of flight flutter. Test various configurations based on different payload and fuel configurations to observe changes in the extracted modal parameters.
Ground vibration tests are important to observe and evaluate the flutter characteristics of aircraft and help establish a safe flight envelope before takeoff. The results of these GVT modal tests can also be used to validate and calibrate simulation models to improve the accuracy of numerical analysis. Therefore,GVT can be said to be an indispensable key link in the aircraft development process.
To meet this demand,JOEO offers a hardware chassis system with a high channel count that can effectively handle the large number of data acquisition and synchronization tasks involved in large-scale experimental testing, ensuring the dynamic range and accuracy of the measurements. At the same time,EDM Modal software can intuitively and efficiently process large amounts of geometric, measurement and post-processing data from complex aircraft assemblies.
The parameters such as natural frequency, damping ratio and mode of vibration obtained by the test can provide a basis for the subsequent optimization design, which is helpful to reduce the risk of aircraft flutter and ensure flight safety and operation efficiency. In short,Ground Vibration testing and its data analysis are very important for the evaluation and verification of aircraft dynamics characteristics, and it is one of the core technologies in the field of aeronautical engineering.