Assessment of Nonlinear Structural Response in A400M GVT

Javier Rodríguez Ahlquist, José Martinez Carreño, Héctor Climent, Raúl de Diego and Jesús de Alba

Airbus Military

John Lennon s/n, 28906 Getafe (Madrid) – Spain e-mail: Javier.I.Rodriguez@casa.eads.net, Hector.Climent@casa.eads.net

Nomenclature: Aeroelastics, Ground Vibration Test, GVT. ABSTRACT

As part of the flutter clearance and aircraft certification process, a Ground Vibration Test (GVT) was performed on the first Airbus A400M out of production line. The A400M is a military transport aircraft with a maximum take-off weight of 141 Tm and capable of take off and landing on unprepared runways. Its powerplant is formed by four TP400 turboprop engines with a combined power of 44,000 SHP. With the A400M being a relatively large four-engine aircraft, its structural dynamic response is characterized by a considerably rich modal density in the frequency range of interest of the test, up to 30 Hz. The pylon-mounted turboprop engines are more flexible that alternative motorizations, contributing to an even denser modal base. Following usual practice, most relevant modes, including those of powerplant and control surfaces, were appropriated (tuned) at different excitation levels. This allowed assessing the magnitude and character of structural nonlinearities inherent to real structures. Selected results are presented in this document together with some of the lessons learned.

Figure 1: A400M MSN001 during GVT at the Flight Test Centre in Seville (Spain)

Proceedings of the IMAC-XXVIIIFebruary 1–4, 2010, Jacksonville, Florida USA

©2010 Society for Experimental Mechanics Inc.

1 INTRODUCTION

The A400M Ground Vibration Test was carried out in autumn 2008 on premises of the A400M Final Assembly Line in Seville (Spain). Five different aircraft configurations (light A/C, open ramp, refueling pods ON, extended flaps and heavy A/C) were tested in the frame of five weeks. EADS Military Transport Aircraft Division (MTAD), now Airbus Military, with headquarters in Getafe (Spain) was responsible of the test, with technical assistance of LMS International and Alava Ingenieros. Airbus Military has its roots in the former Construcciones Aeronáuticas S.A. (CASA), founded in 1923 and since then the largest Spanish aircraft manufacturer. In 1999 CASA merged together with DASA and Aerospatiale-Matra to form the EADS (European Aeronautic Defence and Space) company. In April 2009 military transport aircraft activities were reorganized and integrated with the name of Airbus Military into Airbus, the commercial aircraft division of EADS. With an experience in dynamic testing of more than 35 years, activities of the former CASA in this field have seen a dramatic increase in the last years with four full-scale GVT’s: A310 Boom Demo (2006), A330 MRTT (2007), C295 underwing pods (2008) and A400M (2008). These tests justified the considerable investment made in state of the art test instrumentation, including an acquisition system of 768 channels, and the development of the required competence in GVT preparation, execution and analysis. Compared to other GVT’s, the high-wing and T-tail configuration of the A400M represented a challenge in terms of robustness and stability for the platforms required for shaker installation. Excitation points are high relative to floor level, while important loads were to be applied because of the aircraft size. This motivated selecting a significantly more robust design for powerplant and tail group platforms, far from the conventional temporary scaffolding commonly used in other tests.

Figure 2: A400M GVT scaffolding outline (top) and details (bottom)

A second distinctive feature in the GVT setup is the suspension system. Bungee suspensions are usually superior compared to pneumatic suspensions, as they allow lowering the frequency of aircraft rigid body modes (RBM). This is desirable, as the aircraft dynamic response is less affected by support conditions. The limitation is normally aircraft weight. The specially developed bungee suspension for this GVT, makes the A400M one of the largest aircraft ever tested on bungees. The system allowed a RBM range comprised between 0.3 Hz and 1.3 Hz, 40% lower than the first flexible mode.

Figure 3: A400M GVT suspension system: MLG (left) and NLG (right) In parallel with test hardware design and manufacture, a dynamic finite element model (FEM) of the aircraft was adapted to reproduce GVT conditions including suspension system and mass states of the various GVT aircraft configurations. The engines were introduced in the model using dynamic condensation of the detailed engine FE model.

Figure 4: A400M GVT pre-test GVT FE model

500 accelerometers were distributed over the entire aircraft structure and additionally 200 on specific aircraft systems. Accelerometers were positioned on selected structural hard points at pre-defined locations ensuring perfect correspondence between test and FE model.

Figure 5: Test wireframe model with accelerometer distribution in the A400M GVT

57 different excitation points were used during the test. Different shaker types covered various requirements in terms of maximum load and stroke. Maximum levels up to 1000 N and 25 mm stroke were reached during the test. Of the 256 acquisitions performed for the five aircraft configurations, 60% of them corresponded to sweep sine excitation (Phase Separation Method, PSM), 33% to modal appropriation of 25 different modes (Phase Resonance Method, PRM), and the rest was dedicated to rigid body mode determination (hammer and manual excitation) and random with multiple shakers.

Figure 6. Excitation points used during A400M GVT

2 RESULTS

In total 70 aircraft flexible modes were identified in Configuration 1 up to 45 Hz. This figure does not include local modes corresponding to propeller, landing gear and others. The aircraft architecture, with T-tail and fuselage rear door and delivery ramp, resulted in empennage modes at lower frequency than for other aircraft of comparable size. Powerplant was source of an important number of modes in the range of interest. These included vertical lateral and roll of each powerplant and at least two types of engine bending modes. As expectable in four-engine aircraft, each mode type derived in series of four modes as the different engines coupled together. Propeller blade bending modes turned out to be located towards the higher end of the range of interest, producing an important number of low-damped modes in a relatively narrow frequency range. Structural nonlinearity observed throughout the test can be freely categorized in:

1) Conventional nonlinearity 2) Nonlinearity affecting modal shapes 3) Combined nonlinearity with multiple mechanisms

1) Conventional nonlinearity Real structures usually present nonlinear behavior up to some extent. Significant sources of nonlinearity in aerospace structures include riveted metallic construction, nacelle latches, hydraulic actuators of control surfaces and elastomeric engine mounts. The degree of nonlinearity for a given structural resonance will depend on the modal displacements at the locations where nonlinearity is originated. The pylon-based engine mounting system is known to be more flexible than alternative truss designs. On top of it, turboprop engines are significantly more flexible than turbofan engines with comparable size. This originated a considerable number of engine and Engine Mounting System (EMS) modes below 30 Hz. Localized nonlinear damping is mainly attributable to high performance elastomeric engine mounts meant to decouple the engines from the rest of the aircraft.

The main footprint of nonlinear effects affecting a vibrating structure is the variation in terms of frequency and amplitude of its structural response when excited with different levels of energy. This can be immediately observed in the resulting Frequency Response Functions (an example is shown in Figure 7).

Figure 7. Driving point FRF’s corresponding to engine hub excitation along Y-axis with various force levels: 250 N (red …); 500 N (green --..); 750 N (blue ___). Apart from frequency shift, nonlinear response can be associated to significant FRF peak skewness at affected resonances. This skewness can in turn produce equivocal pole stability plots using conventional modal identification algorithms in frequency domain. During the A400M GVT, certain highly damped, highly force-dependent structural resonances showing positive skewness were systematically fitted by default by means of two poles: a low frequency pole with low damping fitting the left flange, and a higher frequency and higher damping fitting the right flange. This two-pole decomposition, which from lower level acquisitions was known to be unphysical, was avoided by narrowing the analysis range around the affected resonance and reducing considerably the size of the modal model. A graph comparing a measured FRF with a fitted double pole and a single pole modal model is shown in Figure 10. Synthesized modal models derived from FRF’s show difficulties in reproducing nonlinear peak skewness.

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Figure 8. Measured driving point FRF vs. fitted modal model for highly damped force-level dependent resonance (EMS roll mode). Top: two-pole model (default); bottom: single-pole model.

Linearity plots show modal properties (frequency, damping) as functions of excitation force and vibration amplitude, which can all be accurately measured when using harmonic excitation. The divergence of measured data with respect to a linear model can be used to assess the character and magnitude of the dominating sources of structural nonlinearity. A few examples as obtained in the A400M GVT are shown in Figure 9 through Figure 11. All reported examples correspond to homogenous excitation, this is, maintaining exciter arrangement. For these cases, variation of modal frequency ranged within 5% for a force level ratio (max/min) of ca. 4. Damping shows larger variations, especially for the case of elevator antisymmetric rotation (Example 3), where damping more than doubled when testing with different force levels. When comparing results for non-identical exciter arrangements, the variation of modal frequencies is larger with maximum variation in excess of 10%. Modal properties derived from different excitation arrangements are generally more difficult to assess. These results can be compared with earlier studies [3]. It becomes apparent that the number of excitation levels needed to characterize structural nonlinearities varies depending on each mode, and that force-level dependence can be difficult to predict. Given that modal appropriation (PRM testing) is considerably time demanding, proper planning is required ahead of the GVT to ensure that nonlinear response is properly characterized at representative force and structural deformation levels.

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Figure 9. Engine roll mode: modal frequency and damping vs. displacement at resonance (left) and displacement vs. excitation level (right).

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Figure 10. Rudder rotation: modal frequency and damping vs. displacement at resonance (left) and displacement vs. excitation level (right).

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Figure 11. Elevator antisymmetric rotation: modal frequency and damping vs. displacement at resonance (left) and displacement vs. excitation level (right). 2) Nonlinearity affecting modal shapes In the previous section it was shown how modal frequency and damping can depend on excitation level, now it will be seen how modal shapes may also change. This was especially evident in the case in the A400M in a frequency range comprising the inner engine pitch and yaw. These modes presented considerable difficulty when attempting appropriation at high force levels. The reason can be inferred from Figure 12. The representation of modal frequencies as function of excitation force reveals multiple intersections as force levels are increased. This is produced as different modes show different sensitivity to changing excitation level and exciter arrangement.

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Figure 12: Force level dependence of EMS pitch and yaw modes.

While modal identification of these modes turned out straightforward using very low excitation levels (random excitation), higher forces reached using sweep sine excitation produced occurrence of lateral and vertical modes in a very narrow frequency range. This does not just affect modal shapes, as it is illustrated in Figure 13, but also test quality. Combined vertical and lateral displacement, whose ratio varies as force levels change, make difficult to ensure perfect alignment between shaker and structure. Out-of-excitation-axis displacement is not only difficult to avoid, its amplitude is also considerable, as a result of high force levels being exerted at low frequency. A large number of acquisitions were devoted to powerplant characterization at low frequency, using different exciter arrangements. In spite of all attempts, phase purity of the modes in this range, as quantified by means of Modal Phase Collinearity, resulted lower than average for the rest of the test.

Figure 13. Engine pitch mode DUUD as derived from acquisition using random excitation (a), and with sweep sine excitation at engine hubs along Z-axis (complex mode shapes). Observe difference in lateral powerplant component between both cases.

3) Combined nonlinearity with multiple mechanisms

For certain modes combining control surface and powerplant response, it turned out a priori difficult determining the dominant source of nonlinearity from the list of usual suspects (hydraulic actuators, elastomeric engine mounts). This issue is a key consideration when selecting the optimum excitation point(s). Testing provided the necessary insight (see Figure 14). Representing the modal frequency of one of these modes as function of the excitation force turned out that for much lower excitation levels, modal frequency is lower when exciting directly at the control surface. On the other hand, engines could be excited at much higher levels than the rest of the structure during Sustained Engine Imbalance testing, a side activity of the GVT during which maximum constant levels of 600 to 1000 N were reached. Aileron excitation is probably more representative, but questions arise whether true convergence can always be reached within admissible excitation levels. Appropriation of control surface modes was performed exciting them directly.

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Figure 14: Left: Modal shape of structural resonance involving aileron rotation and outer engine aft bending. Right: Dimensionless modal frequency as function of excitation force: engine hub excitation (red diamond) and aileron excitation (blue triangle)

3 SUMMARY

For the A400M GVT a state-of-the-art test setup was used to characterize the aircraft structure in terms of modal frequency, damping, stiffness and mass. Extremely robust support structures allowed reaching considerably high excitation levels without perturbing aircraft response. This allowed characterizing structural nonlinearities for relevant aircraft modes. Challenges derived from high modal density and modal coupling increased the complexity of modal identification, with the relatively flexible engine and engine mounting system playing a key role. Different aspects have been reviewed surrounding the topic of nonlinear behavior: modal shape modification and combined nonlinear mechanisms. Every GVT is different, and this is likely to remain so as new materials and designs are incorporated in future aircraft. In any case, the A400M GVT has turned out to be one of most complex GVT’s performed in recent times. The aircraft size and number of test configurations, the flexibility of the turboprop engines, high modal density and structural nonlinear response represented altogether a significant technical challenge that had to be addressed within a tight test schedule. The quality of the A400M GVT test results owes to more than two years of preparation activities and the fruitful collaboration between Airbus Military, its technical partners LMS and Alava Ingenieros and the close collaboration with Airbus, with special mention to its Aeroelasticity Department in Bremen.

4 REFERENCES

[1] Ewins D.J., “Modal Testing: Theory and Practice”, Research Studies Press Ltd., 1984.

[2] Worden K. and Tomlinson G.R., “Nonlinearity in Structural Dynamics”, Institute of Physics Publishing, 2001.

[3] Göge D., Sinapius M. and Füllekrug U. (DLR), “Non-Linear Phenomena in GVT of Large Aircraft”, Proc. of IFASD (International Forum on Aeroelasticity and Structural Dynamics), Amsterdam, Netherlands, 2003.

[4] Peeters, B., Climent, H., de Diego, R., de Alba, J., Rodriguez-Ahlquist, J., Martinez-Carreño, J., Hendricx, W., Rega, A., García, G., Deweer, J., and Debille, J., “Modern Solutions for Ground Vibration Testing of Large Aircraft”. Proceedings of IMAC 26, the International Modal Analysis Conference, Orlando (FL), USA, 2008.

[5] Oliver M., Rodríguez Ahlquist J., Martinez J., Climent H., de Diego R. and de Alba J., “A400M GVT: The Challenge of Nonlinear Modes in Very Large GVT’s”. Proc. of IFASD (International Forum on Aeroelasticity and Structural Dynamics), Seattle, USA, 2009.