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AbstractGround Vibration Tests have been performed ona replica of the Group for AeronauticalResearch and Technology in Europe(GARTEUR) testbed structure SM-AG19. Theaim of the Ground Vibration Tests was to studythe impact on modal data stemming from thetest data, from the boundary conditions used tomimic a free-free condition. For the tests an air-spring support system was designed with theaim to give a behaviour that is in resemblancewith the behaviour of real aircraft testconditions.

To eliminate a potential, unwanted, impact onthe test results generated by the wing tofuselage interface as it was designed for theoriginal structure (SM-AG19), a deviation fromthe original interface has been made on thestructure for this particular study.

1 IntroductionIn the aerospace industry, finite element (FE)models are traditionally used to represent theglobal structural dynamics of an aircraft. Themodels are for example used for aeroelasticanalyses of the aircraft and to predict theresponses (loads and accelerations) due todynamic excitations. Hence, it is vital that themodels represent the essential dynamics of thereal structure. Due to certification requirementsand flight test permits, aeroelastic stability mustbe verified based on validated test data such asthose stemming from a Ground Vibration Test(GVT). For this purpose, either modal datastemming from the GVT directly and/or from an

FE-model being successfully correlated to GVTdata can be used. In either of these cases above,the damping values are almost always takenfrom the GVT alone and can therefore not bevalidated.

The modal data estimated from an ExperimentalModal Analysis (EMA) of GVT data are alwaysaffected by the test procedure and itsomnipresent imperfections. One possible sourceof biased modal data is the suspension used [3].Especially, experimentally obtained dampingvalues are well known to have relatively highuncertainties. It is also well known that dynamicmodels’ response predictions are highlydependent on the damping values used. Inaddition, damping is a notoriously difficultparameter to estimate correctly. It is also wellknown that in EMA, the estimated damping of astructure is prone to effects on damping fromthe suspension. An example of the difficulties inobtaining free-free boundary conditions and itseffects on natural frequencies and dampingestimates has been reported [2].

At SAAB Aeronautics, as in the aerospaceindustry in general, air-spring support systemsare generally used to establish as close aspossible a free-free condition during a GVT ofan aircraft. The aircraft can usually only besupported by the air-springs at predefinedpositions which cannot easily be changed. Onthe GARTEUR SM-AG19 testbed structure [1]however, the support positions can easily bechanged and the effect can be studied.

A STUDY OF THE ACCURACY OF GROUNDVIBRATION TEST DATA USING A REPLICA OF THE

GARTEUR SM-AG19 TESTBED STRUCTUREPär Gustafsson*, Andreas Linderholt**

*SAAB Aeronautics, ** Linnaeus University

Keywords: GVT, GARTEUR, suspension, modal damping, air-springs

P.Gustafsson, A.Linderholt

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2 Ground Vibration TestThe GVTs of the SM-AG19 replica wereconducted at the Linnaeus University (LNU) inVäxjö, Sweden, as a collaboration between thedepartment of mechanical engineering at LNUand the structural dynamics department atSAAB Aeronautics.

2.1 Testbed StructureThe structure used in this study is a replica ofthe testbed used in a round robin workperformed in the 90’s by the Structure andMaterials Action Group of GARTEUR, SM-AG19. The replica structure is shown in Fig. 1.Examples of the design criteria for the originalstructure were; to be suited for instrumentationsimilar to an aircraft and to have closely spacedmodes in order to make the problem even morechallenging [1]. The original model also had aviscoelastic material located on the uppersurface of the wing in order to accomplishsufficient damping [1]. The structure used inthis study, was not equipped with such aviscoelastic material.

Fig 1. The replica of the GARTEUR SM-AG19testbed structure at Linnaeus University. Here, thestructure is in its reference support condition.

In order to ensure a repeatable behaviour in thewing to fuselage attachment for all supportconfigurations, the wing was welded to thefuselage instead of only bolted as for theoriginal structure. By doing this one abandonsthe original structure somewhat since thebehavior of the wing/fuselage interface will beslightly different. However it was considered

necessary in order to avoid unwanted variationsin the GVT results due to the interface itselfwhen changing the support condition.

The total mass of the structure used in this studyis 41.7kg, with a wing span of 2.0m and afuselage length of 1.5m.

2.2 GVT equipment and setupThe test object was excited using a modal shop2025E shaker via a stinger rod and PCBTLD288D01 impedance head. The acceleranceswere measured using PCB T356A16 triaxial and352A56 single axis sensors. LMS ScadasMobile units were used for the data acquisition.

A small scale air-spring support system wasmanufactured for the testbed structure. The aimof that system was to give a similar dynamicbehaviour for the testbed structure as its largescale counterpart does for real aircraft, e.g. rigidbody mode frequencies lower than about onethird of the first elastic eigenmode. For thetestbed structure this implies rigid bodyfrequencies lower than about 2Hz. The smallscale air-spring support system consists of threeair-bellows which are all connected to one largepressure vessel, into which air is suppliedthrough a pressure regulator. The pressurevessel consists of two barrels, see fig. 2. Theair-springs supporting the structure are shown infig. 3.

Fig 2. The two-barrel pressure vessel for the air-springsupport system.

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A STUDY OF THE ACCURACY OF GROUND VIBRATION TEST DATAUSING A REPLICA OF THE GARTEUR SM-AG19 TESTBED STRUCTURE.

Fig 3. The air-spring system supporting the structure.

The air-spring support system was designed tofacilitate accurate and fast positioning of eachindividual air-spring, by having predefinedpositions on a support frame structure.

Sensors were placed according to Fig. 4 andFig. 5. All three Degrees-Of-Freedom (DOFs)were measured at position 1, 3, 9, 16, 21, 23 and27 with triaxial accelerometers. At position 10-14, 16-20 and 22 on the wings and position 24-26 and 28 on the horizontal tail, only the –ZDOFs (the normal direction perpendicular to thewing area) were measured. On the fuselage, atpositions 2, 4-8, 29 and 30, measurements weremade in the –Y DOFs (the sideway direction)only.

Fig 4. The sensor placements and position numbers onthe wing and horizontal tail.

Fig 5. The sensor placements and position numbers onthe fuselage.

The structure was excited by a singleelectromagnetic shaker, which excited thestructure in the +Z DOF at position 11, see Fig.4. The shaker was attached to the structurethrough a stinger rod and an impedance head,measuring both the force applied and thecorresponding acceleration for the same DOF,which enabled the achievement of a high qualitydirect point Frequency Response Function(FRF).

As a reference for the measurements involvingthe air-springs, the structure was tested whensupported by cords, connected at thewing/fuselage interface and to the rear part ofthe fuselage of the structure, see Fig. 1.

When involving the air-springs, both thepositions and the internal pressure level of thesystem were varied. The air-springs supportingthe structure under the wing, were positioned inthree ways: a) as close to the fuselage aspossible i.e. at a distance equal to 6.25% wingspan, b) at 27.5% wing span and c) at 42.5%wing span, all at 50% chord. The position of theair-spring supporting the rear fuselage was keptconstant throughout the GVTs. With the air-springs positioned at 42.5% wing span, theirinternal pressure level was reduced in two steps.Unfortunately the pressure indicator of theregulator was not sensitive enough for the exactpressure level to be monitored. The sensor andactuator setup was done once and thereafterpreserved during all measurements.

All measurements presented in this paper arebased on stepped sine measurements with anexcitation force level equal to 0.5N, and anacceptable force amplitude deviation of 1dB. Asa baseline, a frequency range of 5-65Hz wasselected with a frequency step size of 0.05Hz.

3.3 GVT resultsAll the experimental modal analysis has beenperformed using LMS Test.Lab© utilizingPolyMAX©, which is a least-squares complexfrequency-domain estimator method.Eigenmodes were selected based on located

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stable poles in the stabilization diagram and theComplex Mode Indicator Function (CMIF).

Throughout the study, the Modal AssuranceCriterion (MAC) [4] was calculated in order tocompare modal matrices stemming from the testdata. The MAC number between mode i andmode j is defined as:

{ } { }{ } { }( ) { } { }( )j

Tji

Ti

jT

iijMAC

ffff

ff2

= (1)

Rigid body natural frequencies were estimatedfor support condition with the air-springspositioned at 42.5% wing span and with anominal pressure level. The highest rigid bodynatural frequency for movements within theXZ-plane was estimated to 0.9Hz.

3.3.1 Reference configurationWhen examining the stabilization diagrams forthe reference configuration, the number ofeigenmodes present in the frequency range 30-40Hz is not obvious. However when examiningthe FRFs in more detail using a Nyquistdiagram, see Fig. 6, one can more clearly notethe presence of three modes within thefrequency range; the first two eigenmodes areclosely spaced.

Fig 6. The Nyquist diagram of synthesized FRFs (g/N)P21:-Z/P11:+Z (red), P22:-Z/P11:+Z (blue), frequencyrange 32-39Hz.

The modal analysis performed for the referenceconfigurations resulted in eight eigenmodeswithin the frequency range 5-65Hz, see Table 1.and Fig. 7.

Table 1. Results for reference configuration.Mode number Frequency

[Hz]Damping

%1 5.36 0.632 16.46 0.503 33.29 0.614 34.14 1.015 37.22 0.616 50.12 0.337 54.04 0.138 56.91 0.11

Fig 7. Eigenmode 1-8 for the reference configuration.

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A STUDY OF THE ACCURACY OF GROUND VIBRATION TEST DATAUSING A REPLICA OF THE GARTEUR SM-AG19 TESTBED STRUCTURE.

Fig 8. The AUTOMAC-matrix for the referenceconfiguration.

Fig 9. The AUTOMAC for the referenceconfiguration.

3.3.2 Air-spring support system positionAs for the reference configuration, eighteigenmodes were estimated for each position ofthe air-springs. EMA estimates of theeigenfrequencies and their associated dampingvalues are presented in Table 2. Synthesizeddirect point FRFs for a frequency range of 32-39Hz are presented in Fig. 10. MAC matricesfor the reference configuration versus each ofthe air-spring position are presented in Figs. 11-13 together with an eigenmode comparison plotof eigenmode number three in Figs. 14-16.Eigenmode number three is chosen to beillustrated due to the low MAC numbersassociated with it.

Table 2. Results for different air-spring supportpositions, a = 6.25%, b = 27.5% and c = 42.5% wingspan.Modeno. #

Freqa[Hz]

Dampa%

Freqb[Hz]

Dampb%

Freqc[Hz]

Dampc%

1 6.32 0.60 6.18 0.47 6.16 0.482 16.50 0.77 16.62 0.79 16.64 0.813 33.19 0.45 33.18 0.32 33.18 0.424 34.08 0.62 34.36 0.83 34.13 0.645 37.24 0.13 37.71 0.71 38.09 1.046 50.21 0.25 50.31 0.28 50.37 0.347 54.05 0.12 54.16 0.14 54.13 0.148 56.90 0.12 57.07 0.13 57.07 0.10

Fig 10. Synthesized direct point FRFs with air-springssupporting the wing at different wing span position.

Fig 11. The MAC-matrix for the referenceconfiguration versus air-spring support at 6.25% wingspan.

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Fig 12. The MAC-matrix for the referenceconfiguration versus the air-spring support at 27.5%wing span.

Fig 13. The MAC-matrix for the referenceconfiguration versus the air-spring support at 42.5%wing span.

Fig 14. Eigenmode number 3, the referenceconfigurations (left) and the air-springs at 6.25% wingspan (right). MAC=0.90.

Fig 15. Eigenmode number 3, the referenceconfigurations (left) and the air-springs at 27.5% wingspan (right). MAC=0.55.

Fig 16. Eigenmode number 3, the referenceconfigurations (left)and the air-springs at 42.5% wingspan (right). MAC=0.70.

3.3.3 Air-spring support system pressureWith the air-springs supporting the structure at42.5% wing span, the air-spring support internalpressure was reduced in two steps. The 42.5%wing span position was chosen since the largestimpact was expected for this position. Since thepressure regulator monitor was not accurateenough, the precise pressure level could not bemonitored. The pressure level was thereforreduced to levels based on geometrical aspectsof the air-springs as depicted in Fig. 17.

Fig 17. Air-springs at the nominal (left), reduced(center) and low (right) pressure level.

In an early stage of the study, the mostsignificant impact of reducing the air-springpressure level from the nominal one was shownto be for frequencies above 30Hz. Therefor thefrequency range for these measurements waszoomed into 30-55Hz instead of 5-65Hz.

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A STUDY OF THE ACCURACY OF GROUND VIBRATION TEST DATAUSING A REPLICA OF THE GARTEUR SM-AG19 TESTBED STRUCTURE.

EMA estimates of the eigenfrequencies andtheir associated damping values are presented inTable 3. Synthesized direct point FRFs for afrequency range of 36-54Hz are presented inFig. 18. The MAC matrices and illustratedeigenmodes including all comparable estimatesfor the reference configuration versus each air-spring pressure level condition are presented inFigs. 19-22.

Table 3. Results for different air-spring pressurelevels.Modeno. #

Freqnom[Hz]

Dampnom%

Freqred[Hz]

Dampred%

Freqlow[Hz]

Damplow%

1 6.16 0.482 16.64 0.813 33.18 0.42 33.35 0.36 33.33 0.424 34.13 0.64 33.85 0.87 33.57 0.985 38.09 1.04 39.88 1.67 43.32 2.586 50.37 0.34 50.78 0.52 51.48 0.737 54.13 0.14 54.15 0.13 54.28 0.208 57.07 0.10

Fig 18. The synthesized direct point FRFs with air-springs supporting the wing at 42.5% wing spanposition with different pressure levels.

Fig 19. Eigenmodes 3-7 for the reference configuration(left) and with the air-springs positioned at 42.5%wing span at reduced pressure level (right).

Fig 20. The MAC-matrix for the referenceconfiguration versus the air-spring system at 42.5%wing span and at the reduced pressure level.

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Fig 21. Eigenmodes 3-7 for the reference configuration(left) and with the air-springs positioned at 42.5%wing span at low pressure level (right).

Fig 22. The MAC-matrix for the referenceconfiguration versus the air-spring system at 42.5%wing span with low pressure level.

4 ConclusionEMA estimates for the reference configurationdo not compare as accurately as desired with theoriginal structure as presented in [1]. Someoverall characteristics and the closely spacedeigenmodes within the frequency range of 30-40Hz are somewhat comparable in terms ofestimated eigenfrequencies but someeigenmodes can be judged, simply from a visualinspection not to compare well. Two sources ofthe deviation from the original structure areassociated with the welded wing/fuselageinterface and the fact that the additional viscousdamping is not included. These are believed notto be the only sources causing the deviations.However, for the study presented in this paper,the slightly different behaviour is of lessimportance.

In principle, the designed air-spring supportsystem worked well. The highest rigid bodymotion in the XZ-plane has a natural frequencyof about 0.9Hz, which is well below one third ofthe lowest estimated elastic eigenfrequency.Hence, the setup is believed to be fairlycomparable with an ordinary full scale GVTsetup.

Changing the positions of the air-springssupporting the wing had a large impact oneigenmodes number three and five. Foreigenmode number three, the eigenvectorchanges significantly with a MAC number aslow as 0.55 for the air-springs positioned at27.5% wing span compared to the referenceconfiguration. Its eigenfrequency and associateddamping value are however not stronglyaffected. For eigenmode number five theopposite apply, the eigenvector is not affectedand it has a MAC number of 1.00 compared tothe reference configuration, for all investigatedair-spring positions. However, theeigenfrequencies and their associated dampingvalues change drastically. Especially that is truefor the damping value of eigenmode No. fivethat shows an increase from 0.13% to 1.04%. Itshould be noticed that the change in positions ofthe air-springs is also excessively largecompared to the possibilities to move the

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A STUDY OF THE ACCURACY OF GROUND VIBRATION TEST DATAUSING A REPLICA OF THE GARTEUR SM-AG19 TESTBED STRUCTURE.

suspension positions in a real full scale GVTwhich in general are none.

A change in the pressure of the air-springsaffected eigenmodes number three and fiveprincipally in the same way as a change in itspositons. Here, the changes are however moresevere, especially for eigenmode number fivefor which an increase in eigenfrequency and theassociated damping value of about 5Hz and1.5% (units) respectively are found. The changein frequencies and MAC numbers foreigenmodes number three and four show thatthe two eigenmodes are becoming even moreclosely spaced when reducing the pressure ofthe air-springs. For the “low” air-spring pressurelevel the difference in frequency between thetwo eigenmodes is only about 0.15Hz. If thepressure level would have been reduced evenmore, the two eigenmodes would most likelyhave changed order. This is well known as themode switching phenomenon.

The study shows that the support used for theSM-AG19 replica structure which is supposedto be somewhat similar to a real aircraftstructure support, strongly affects the estimatedmodal parameters. The effect on closely spacedeigenmodes is such that it may introduceunwanted difficulties when performing an FE-model correlation based on GVT data,especially if the used support has resulted in thechanging of the order of eigenmodes.

5 Future workBy welding the wing to the fuselage instead ofbolting it, the original structure was abandoned.Depending on the objectives of future studies, astructure more similar to the original in thissense should be developed, possibly alsoincluding the extra viscous damping which isapplied to the wing of the original structure. Tobe able to compare data with previous studiesbased on comparable structures, this isconsidered as a requirement. To continue thework related to the air-spring support system, amore sensitive regulator should be included, bywhich pressure levels can be monitored moreaccurately. This will facilitate a better control of

the structure support conditions. The air-springsshould also be included in an FE-model forcomparison and further investigations of thedrastic changes shown for some eigenmodesdue to its characteristics.

6 AcknowledgementThe authors would like to thank Mr. MatsAlmström, the laboratory technician at theLinnaeus University for his work with thetestbed structure and the air-spring supportsystem used in the GVTs.

References[1] E. Balmès , J. Wright, GARTEUR group on ground

vibration testing, Results from the test of a singlestructure by 12 laboratories in Europe, 15th IMAC,1997, pp. 1346-1352.

[2] V. Crupi, An Unifying Approach to assess thestructural strength, International Journal of Fatigue,vol. 30, pp. 1150-1159, 2008.

[3] T. G. Carne, D. T. Griffith and M. E. Casias, Supportconditions for experimental modal analysis, Soundand Vibration, pp. 10 - 16, 2007.

[4] Allemang, R. J., Brown, D. L., A CorrelationCoefficient for Modal Vector Analysis, Proceedings,International Modal Analysis Conference, pp. 110-116, 1982.

8 Contact Author Email AddressThe authors may be contacted by email throughthe following addresses:

mailto: par.gustafsson@saabgroup.commailto: andreas.linderholt@lnu.se

Copyright StatementThe authors confirm that they, and/or their company ororganization, hold copyright on all of the original materialincluded in this paper. The authors also confirm that theyhave obtained permission, from the copyright holder ofany third party material included in this paper, to publishit as part of their paper. The authors confirm that theygive permission, or have obtained permission from thecopyright holder of this paper, for the publication anddistribution of this paper as part of the ICAS 2016proceedings or as individual off-prints from theproceedings.