INFLUENCE OF AIRFRAME STIFFNESS ON THE AERODYNAMICCOEFFICIENTS OF A HIGH ASPECT RATIO WING

Luís A. V. Pelegrineli∗ , Felipe F. Liorbano∗ , Henrique O. H. Natori∗ , Ricardo A. Angélico∗University of São Paulo, São Carlos School of Engineering, Department of Aeronautical

Engineering∗

Keywords: flexible wings, experimental aerodynamics, aerodynamic coefficients, wind-tunnel tests.

Abstract

The use of new materials into airframe enabledlighter and more flexible aircraft. The structuralflexibility of aerodynamic surfaces can signifi-cantly change aircraft performance. In this con-text, the present article aims to furnish experi-mental data (lift and drag coefficients) of flexi-ble wings by tests performed at EESC-USP opencircuit wind tunnel. The model consists of arectangular wing with 0.60 m semi-span and as-pect ratio of 8.88 with a Clark-Y airfoil with twopossible stiffness. The stiffer configuration wasadopted as a reference condition. Non-invasivetechniques were employed to measure twist an-gle and the displacement of the wing. For thiswing model tested with 20.5 m/s (Re≈ 191,000),it was shown that the effects of flexibility onthe aerodynamic coefficients considering perfor-mance parameters were adverse, especially forhigh angles of attack, where the aerodynamicloads are higher. Also, the identification of wingdisplacement and its torsion contributes to a bet-ter understanding of the aerodynamic behavior.Wing torsion and displacements measurementsare presented for three angles of attack (0◦, 6◦

and 14◦). The experimental results presentedherein can be used for validation of computa-tional codes on flexible wings.

1 Introduction

With the advent of new materials on aeronauti-cal industry, the shape of modern aircraft compo-

nents may change during flight due to high struc-tural deflections. For instance, the wings with-stand aerodynamic loads which can change theirformat significantly, especially on airplanes withhigh aspect ratio wings. Some airplanes, likeBoeing 787 [1] and Airbus A380 [2], when un-der the ultimate-load wing-up bending test, havetheir wing tips deflected more than 7.5 m. Thischange in the wing shape plays an important roleon the aircraft aerodynamic coefficients and, con-sequently, on its performance. In such a way, itis important to estimate the losses on the aircraftperformance due to these effects. To accomplishthis, it is necessary to understand the influenceof the geometric nonlinearities of a wing on itsaerodynamic coefficients. In this context, someauthors investigated experimentally and analyti-cally the aerodynamic behavior of flexible liftingsurfaces.

Tang and Dowell [3] performed wind tunneltests to investigate the effect of bursts on flexiblewings with high aspect ratio. Albertani [4] car-ried out wind tunnel tests in MAVs (Micro AirVehicles), integrating visual image correlation tomeasure structural deflections.

Hooker et al. [5] presented a method to pre-dict aeroelastic deformation (twist and bending)via FEM (Finite Element Method) which wasvalidated by wind tunnel tests.

Nguyen and Chaparro [6] presented a cou-pled aerodynamic-nonlinear finite element modelfor a wind tunnel model designed to have about10 % wingtip deflection. This wind tunnel modelrepresents the state of the art of high aspect ratio

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PELEGRINELI , LIORBANO , NATORI , ANGÉLICO

wings and was used to validate a real-time adap-tive drag optimization control strategy. The de-scription of this wind tunnel model was presentedby Vassberg et al. [7] and was motivated by theneed for contemporary experimental databases tovalidate computational fluid dynamics (CFD) ap-plications.

This highlights the importance of experimen-tal aerodynamic data of flexible lifting surfaces.The present article aims to investigate the influ-ence of structural flexibility on the aerodynamiccoefficients of a high aspect ratio flexible wingvia wind tunnel testing, providing experimen-tal data of flexible wings to validate FSI (Fluid-Structure Interaction) computational codes.

2 Experimental procedure

2.1 Wing model

The model consists of a rectangular wing with0.60 m semi-span and aspect ratio of 8.88 with aClark-Y airfoil. The airfoil was selected due to itshorizontal lower surface, which facilitates its po-sitioning at the wind tunnel. In addition, as pre-sented by Marchman and Werme [8], the Clark-Y airfoil performs well at low Reynolds numbers(50,000≤ Re≤ 200,000) for wind tunnel tests.

The wing model was designed to allow thechange in stiffness by introducing or removingthe spars, as shown in Figure 1. This way, thesame model can be used for tests with differentstiffness levels, diminishing the influence on theresults due to causes other than the variable wingcurvature and twist in span direction when sub-mitted to aerodynamic load. The model consistsof a hexagonal brass spar 5.6 mm width acrossthe flat and a removable 8.0 mm diameter steelbar. For the configuration with higher stiffness,the steel spar is introduced to increase the stiff-ness of the system. These spars are connected tothe aerodynamic balance located at the right wallside of the test section.

The wing ribs are made of epoxy resin andhave weight relief holes, which are also usedto accommodate the inertial measurement units(IMUs) used to measure the twist angle along the

wingspan. In both configurations, the wing de-flection is allowed by the introduction of smallgaps between the ribs, as shown in Figure 1.Thus, the ribs do not influence the wing stiffness,even when the deflection is high. The wing modelin its configuration with two spars is shown inFigure 2.

x

z

x

y

z

y

Rigid wingspar socketFlexible wingspar socket

Wing segments

GapsRigid configuration

Flexible configuration

(a)

(b)

(c)

Fig. 1 Wing model: (a) scheme of the wingmodel; (b) wing segment made of epoxy resin;(c) wing sockets for different spars.

Fig. 2 Wing model with brass and steel spars(rigid configuration).

2.2 Wind tunnel and model instrumentation

The instrumentation used in the experiments wasaimed at extracting aerodynamic data, such as thedrag and lift coefficients for a wing with differentstiffness levels, as well as data related to the rota-tion of wing sections around the x and y-axes. Be-sides, the wing was photographed in its y-z planeto obtain data related to its deflection along thez-axis.

Concerning aerodynamic data, an NI-9237 module in conjunction with a NationalInstrumentsTM cDAQ-9191 chassis was usedto communicate the software LabVIEW withthe balance coupled with the wind tunnel. Forthe dynamic pressure measurement, it was used

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INFLUENCE OF AIRFRAME STIFFNESS ON THE AERODYNAMIC COEFFICIENTS

a Pitot tube connected to a TSITM DP-CALC8705-M-GB micromanometer. For the acquisi-tion of the temperature and the local atmosphericpressure an ITWH-1170 meteorological stationof InstrutempTM was used.

Along the wingspan, IMUs (MPU 6050-GY521) integrated into an Arduino MegaTM boardwere arranged in five sections, and thus, the ro-tation around the x and y-axes of these sectionswere calculated from measurements of the com-ponents of the gravitational acceleration in the x,y and z directions.

For the acquisition of the deflections of thewing in the z-direction, a CanonTM EOS-RebelT7i camera was used along with a CanonTM EF-S 18-55 mm f/4-5.6 IS STM lens.

The deflection of the wing was computed bypost-processing the images acquired throughoutthe tests. The images obtained are in RGB for-mat. The color of a pixel in an RGB image is thecombination of three colors: red (R), green (G)and blue (B). Then, the color of a pixel located atx = (x,y) is

I (x) = arIr(x)+agIg(x)+abIb(x) (1)

where Ir(x), Ig(x) and Ib(x) are the red, greenand blue layers of I ; and ar, ag, ab are multipli-ers.

In the first step, the multipliers were chosen tohighlight the region of interest. The green layerhad an intensity distribution that highlighted thewing, but the red layer was also required since thewalls of the closed test section interfered in theidentification of the wing root. Then, a thresh-old filter based on Ir and Ig binarized the image,giving a new image I (x), i.e.

I (x) = (I (x)> aT1 max(Ig)) ∩ (Ig− Ir)> aT2

(2)where max(Ig) returns the maximum intensity ofIg and aT1 and aT2 are threshold parameters.

Next, the wing contour was identified andmidpoints between the upper and lower boundswere computed. These points were transformedto the aerodynamic coordinate system and thepixels coordinates were converted to millimeters.

This conversion factor was obtained by correlat-ing the dimensions of a checkerboard and its re-spective dimensions in pixels on the plane con-taining the midpoints. Finally, a cubic curve wasused to fit the data.

2.3 Wind tunnel tests

The wind tunnel tests were performed at theAeronautical Engineering Department of the SãoCarlos School of Engineering of the Universityof São Paulo in a blowing type wind tunnel witha closed test section. The experimental procedurewas conducted as follows:

1. The wing model was positioned at thewind tunnel and connected to an aerodynamicbalance at an angle of attack of -8 degrees.

2. The balance was tared in a wind-off con-figuration.

3. The wind tunnel was turned on and theaerodynamic forces measurements were takenfrom the angle of attack range of -8 to 16 de-grees for the configuration with lower stiffness.For each angle of attack, the temperature, the dy-namic pressure of the flow and the atmosphericpressure were measured.

4. An 8.0 mm steel spar was introduced to thewing model to increase its stiffness.

5. The steps 1 to 3 were repeated for the newwing configuration.

After the aerodynamic forces measurements,the same wing was instrumented with the pre-viously described IMUs along the semi-wingspan and the photographic camera was posi-tioned downstream of the wing model to take thepictures used for the deflection measurements.Then, the steps 1 to 5 were repeated for thisinstrumented wing for both stiffness configura-tions. This time, for each angle of attack, theIMUs data and a photo was taken for each angleof attack. These data were used to compute wingdeflection and twist along the semi-span. All thetests were performed at the same Reynolds num-ber, approximately 191,000.

The results obtained for the aerodynamicforces (lift and drag) and measured dynamic pres-sure were corrected for the solid blockage, the

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PELEGRINELI , LIORBANO , NATORI , ANGÉLICO

wake blockage and the downwash effects. Thecorrection methods used were presented by Popeet al. [9].

3 Results

The airframe stiffness influences the aircraft per-formance, and it must be quantified to get closerto the real aircraft characteristics. Herein, asmentioned before, its influence is experimentallyinvestigated by the testing of two configurations:rigid (R) and flexible (F). The wing aerodynamiccoefficients – lift coefficient vs. angle of attackand lift coefficient vs. drag coefficient – can beseen in Figure 3. For low values of lift coefficient,CL between −0.25 and 0.25, the stiffer and flex-ible configurations present almost the same be-havior. With the increase of the lift coefficientmagnitude, CL > 0.25, the difference between thecurves also increases, with the stiffer configura-tion presenting the higher values. On the otherhand, the significant differences for the drag oc-cur for CL < 0.6. The flexible configuration – incomparison with the stiffer one – decreases theaircraft performance in takeoff and landing due tothe CLmax reduction. The reduction of CLmax was5.85 % for a Reynolds number about 191,000.Also, for typical climb and cruise lift coefficients,0.3 < CL < 0.6, the flexible configuration shows ahigher drag coefficient, leading to a performancereduction in these mission profile phases. For in-stance, for a CL equals to 0.4, the drag coefficientdifference is 16 %.

For the flexible configuration, the aerody-namic coefficients depend on the dynamic pres-sure, once the wing shape varies with the air-speed. The wing motion due to the dynamic pres-sure decomposes into flexion and torsion. Themodel was built to be flexible only along the axisy, so that, the lift is the driving force of the wingmovement. The wing flexion was monitored bythe camera and its torsion by the inertial sen-sors embedded into the wing. The wing displace-ment for the stiff and flexible configurations canbe seen in Figure 4(a) and 4(b), respectively, forthe angles of attack 0◦, 6◦, and 14◦. The flex-ible configuration presented a wingtip displace-

ment equals 90, 128, and 168 mm, for α = 0◦, 6◦

and 14◦, respectively.The identification of the wing displacement

is a non-invasive technique and does not influ-ence the wing response. However, improvementsmust be implemented to take into account thewing profile geometry during the post-processingof the images. For instance, for low angles ofattack, the algorithm identifies as the upper andthe lower bounds, the higher profile thickness,and the trailing edge, respectively. Notwithstand-ing, for higher angles, it identifies the tangentpoint formed by the camera line-of-sight with theprofile and the trailing edge as upper and lowerbounds. At last, for small angles of attack, theopposite occurs.

The inertial sensors embedded into the wingmeasured the wing torsion. The angle measure-ments for α = 0, 6 and 14 degrees can be seenin Table 1. The average values obtained arein agreement with the expected wing behavior.The location of the shear center leads to a wingwashout when the wing is loaded. For the previ-ous angles of attack, a washout of 1.00, 1.28, and1.71 was observed, respectively. The significantdeviations observed are considered to be mainlydue to the model oscillations during the tests andthe IMU signal conditioning. Authors considerthat improvements in the signal conditioning al-gorithms can reduce the angle deviation. It isimportant to highlight that the sensors and ca-bles weight, as well as their location, influencethe wing response. Although these sensors canchange the wing response, they do not interferedirectly in the surrounding airflow. The valuesobserved for IMU 5 (located closer to the wingroot) show that the angle of attack set differs fromthe wing root one. Unconstrained 150 mm be-tween the wing root and balance fixture can ex-plain this difference.

4 Conclusions

The experimental investigation showed that dif-ferent levels of airframe stiffness for a given wingplanform and flow condition have significant ef-fects on its aerodynamic lift and drag coefficients

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INFLUENCE OF AIRFRAME STIFFNESS ON THE AERODYNAMIC COEFFICIENTS

CL

α [°]-10 -5 0 5 10 15 20

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

CL

CD0 0.05 0.1 0.15 0.2

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

(a) (b)

Rigid wingFlexible wing

Rigid wingFlexible wing

I

II

III

(c)

Fig. 3 Aerodynamic coefficients for rigid and flexible configurations: (a) lift coefficient vs. angle ofattack and (b) drag vs. lift coefficient. (c) snapshots.

Table 1 Angles measurements from inertial sen-sors for flexible wing configuration.

α = 0◦

IMU θx [◦] θy [

◦]1 9.32 ± 3.58 0.06 ± 6.24782 8.57 ± 3.00 -0.01 ± 4.06493 7.87 ± 3.31 0.41 ± 3.36934 6.45 ± 2.92 0.31 ± 6.29235 3.71 ± 2.97 1.06 ± 8.0587

α = 6◦

IMU θx [◦] θy [

◦]1 13.2 ± 4.58 4.59 ± 7.752 12.3 ± 3.91 4.68 ± 1.303 11.3 ± 4.56 5.20 ± 2.354 9.52 ± 3.28 5.04 ± 4.985 5.36 ± 3.04 5.88 ± 6.50

α = 14◦

IMU θx [◦] θy [

◦]1 16.5 ± 6.94 10.1 ± 5.822 15.6 ± 4.85 10.2 ± 2.763 14.3 ± 6.65 10.8 ± 2.244 12.3 ± 5.28 10.6 ± 5.165 7.01 ± 3.27 11.8 ± 6.82

(a)

(b)

Fig. 4 Wing displacement in z-direction identi-fied by image processing: (a) stiff and (b) flexibleconfigurations.

and, for consequence, in some performance pa-rameters. In general, the effect of flexibility forthe presented wing was negative, showing thatthese effects must be taken into account in theaircraft design since its initial steps. However,investigations of the phenomenon for differentwing planforms, stiffness and Reynolds numbersmust and will be conducted in the continuity ofthe present work. Also, the usage of non-invasivemeasurement techniques for wing displacements

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PELEGRINELI , LIORBANO , NATORI , ANGÉLICO

and twist showed to be very useful and reliable,although special care must be taken to avoid wingspar torsion between the wing model root and thebalance. The results of the present article can beused in the validation of computational codes re-garding the flexible wings modeling.

References

[1] Paur J. Boeing 787 passes incred-ible wing flex test. Available in:https://www.wired.com/2010/03/boeing-787-passes-incredible-wing-flex-test/, 2010.Acessed: 2017-07-28.

[2] Kingsley-Jones M. Airbus A380 test wingbreaks just below ultimate load target. Availablein: https://www.flightglobal.com/news/articles/airbus-a380-test-wing-breaks-just-below-ultimate-loa-204716/, 2006. Acessed: 2017-07-28.

[3] Tang D and Dowell E H. Experimental and the-oretical study of gust response for high-aspect-ratio wing. AIAA Journal, 2002.

[4] Albertani R, Stanford B, Hubner J P and IfjuP G. Aerodynamic coefficients and deforma-tion measurements on flexible micro air vehi-cle wings. Experimental Mechanics, 47(5):625-635, 2007.

[5] Hooker J R, Burner A W, Valla R. Static Aeroe-lastic Analysis of transonic wind tunnel mod-els using finite element methods. AIAA Journal.1997;(2243):254-261, 1997.

[6] Nguyen N T, Ting E, Chaparro D. Nonlinearlarge deflection theory with modified aeroelas-tic lifting line aerodynamics for a high aspectratio flexible wing. 35th AIAA Applied Aero-dynamics Conference. 2017;(June):1-26.

[7] Vassberg, J C, DeHaan, M C, Rivers, S M andWahls, R A. Development of a common re-search model for applied CFD validation stud-ies. AIAA Applied Aerodynamics Conference,AIAA 2008-6919, August 2008.

[8] Marchman III J F and Werme T D. Clark-YAirfoil Performance at low Reynolds numbers.AIAA 22nd Aerospace Sciences Meeting, 1984.

[9] Pope A, Barlow J B and Rae Jr. W H. Low-speedwind tunnel testing. Third edition. John Willeyand Sons. 1999; p. 713.

5 Contact Author Email Address

mailto: luis.pelegrineli@usp.br

Copyright Statement

The authors confirm that they, and/or their companyor organization, hold copyright on all of the origi-nal material included in this paper. The authors alsoconfirm that they have obtained permission, from thecopyright holder of any third party material includedin this paper, to publish it as part of their paper. Theauthors confirm that they give permission, or have ob-tained permission from the copyright holder of thispaper, for the publication and distribution of this pa-per as part of the ICAS proceedings or as individualoff-prints from the proceedings.

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