The Airbus Loads and Aero-elastics division in Toulouse, France, uses LMS hybrid technologies and software to update structural models of aircraft, combining CAE and Test data. Airbus uses CAE models as part of the aero-elastic clearance process applied to their aircraft development programs. During the modeling, they match the Finite Element (FE) models with vibration test results and embed their entire structural dynamics refinement process (pre- and post-processing) in LMS Gateway. This increases model quality, which in turn leads to more accurate predictions concerning dynamic phenomena, such as comfort in turbulence. LMS Gateway is used by Airbus in several of its recent aircraft development programs, such as A340-600 in 2001, and A340-500 and A318 in 2002.
Hybrid simulation
A Finite Element (FE) model must be of the highest quality to be effective when used to assess a design in preparation for physical test. It must accurately reflect real functional performance to be truly useful. To better anticipate how an aircraft design will respond to loads in operational conditions, Airbus has developed a strategy for adjusting CAE models with test results. The use of models that are matched to test data raises the level of confidence engineers have in their predictions.
Once the models are updated, simulations can be performed with improved accuracy. Both the ground movement of the plane and its behavior during flight are simulated. With the improved model, Airbus engineers determine detailed ground loads and flight loads to investigate the aircraft’s safety, handling and passenger comfort as well as the internal loads that apply to the integrated aircraft model. In addition, engine imbalance studies are performed. With test data from Ground Vibration Tests (GVT), they can simulate the expected dynamic response levels during operation. A high-quality matching between the dynamic model and the vibration test results is a must to guarantee the safety during the flight envelope clearance.
GVT - The experimental data
Ground Vibration Testing (GVT) is done on the first full assembly of a given model to get its basic modal data – frequencies and damping characteristics. The GVT measures the low-frequency modes by means of normal modes acquisition and frequency response function acquisition, the latter using random excitation. The aircraft is put on a pneumatic suspension to generate almost free/free test conditions. To simulate real GVT conditions, the experimental data is used to tune the parameters of the FE model. Firstly, the pneumatic suspension used in an actual GVT is reproduced. This is achieved by changing the stiffness of spring elements to update the rigid-body modes in accordance with the test data. Secondly, the flexible aircraft modes are reproduced.
Correlating Test and CAE data
Hybrid simulation is a powerful tool for making engineering judgements, provided that the values of the modes in the model correspond to the test data. To check the model modes for accuracy, Airbus uses LMS Gateway. In this software, each measured mode is correlated to the numeric mode of the model. First the geometrical nodes are paired to bring the FE model in line with the measured structure. Then modal pairs (measured and numeric), containing mode shape and frequency information, are compared. Divergences between initial values for measured and simulated data are indicated on visual representations made in LMS Gateway. For instance, the Modal Assurance Criterion (MAC) matrix compares the measured and synthesized transfer functions, and the measured and numerical mode shapes, etc.
Changing parameters to update the model
To adjust the global modes of the hybrid model to be more accurate, the Airbus engineers added a stick model to the FE model. Commonly used for dynamics calculations, a stick model is a simple beam model containing force and load information. Airbus analysts positioned the stick model of the aircraft within the numeric model along the main force transmission paths, wing, fuselage, Horizontal Tail Plane (HTP) and fin.
Using information from the mode-pair table, they changed parameters in the stick model to update the FE model. Each section of the model could be manipulated for stiffness, torsion and bending, bringing the modes closer to realistic values. To account for engine modes, engineers worked on the engine pylon area, which attaches the engine to the wing. For this area, Airbus worked with the parameters of the FE model rather than working with an additional stick model.
With an updated model, one that correlates well with reality, structural responses can be simulated with a high degree of confidence. This technique requires a very high quality model, but isolates with a high degree of accuracy those changes that will have an effect on the modes of the structure. The results of the Airbus updating were displayed in a final mode-pair table, showing a close degree of correlation.
Hybrid modeling at Airbus
Hybrid simulation provides Airbus with an effective tool to improve their development process. The use of GVT data helps them to raise the level of accuracy when matching between test and simulation. The dynamic behavior of the model compares to a satisfactory degree with in-flight test results. Airbus engineers use LMS solutions to perform faster and more precise correlations and to further enhance the quality of FE models for accuracy. With each updating iteration, the frequency and mode-shape matching further improved for accuracy. Airbus sees hybrid modeling as an industrial application that forms an integral part of their aircraft development processes.
