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On the Use of Adaptive Internal StructuresFor Wing Shape Control

Michael Amprikidis*, Jonathan E. Cooper†, Chris Rogerson‡ and Gareth Vio§

University of Manchester, Manchester, M13 9PL, UK.

This paper describes part of a research programme investigating the development of “adaptive internalstructures” concepts to enable active aeroelastic control of aerospace structures. A number of different conceptshave been considered as part of the EU funded Active Aeroelastic Aircraft Structures (3AS) project that allow thebending and torsional stiffness of aircraft wings to be controlled through changes in the internal aircraft structure.The aeroelastic behaviour, in particular static bending and twist deflections, can be controlled through changes in theposition, orientation and stiffness of the spars. In this paper, finite element models are used to explore the use ofrotating spars to vary structural stiffness, thus adjusting the static aeroelastic wing twist and bending shape, and thusaltering the lift and drag properties. The effect on the flutter characteristics is also explored. A number ofexperimental studies of the concepts are also described.

I. IntroductionThere is a growing interest in the development of active aeroelastic structures to allow aeroelastic deflections to

be used in a beneficial manner 1,2. Such an approach will lead to more efficient aircraft designs. For example, thewing twist could be adjusted throughout the entire flight in order to maintain a shape giving optimal lift-drag ratiofor maximum range, and also as a means of roll control. Other concepts are being developed to change the wingleading and trailing edge shape in order to adjust the lift coefficient, and also to change the wing planform shape. Inrecent years, a number of research programmes, for example the Active Aeroelastic Wing3 and the MorphingProgramme4, have started to develop active aeroelastic concepts. In Europe, the 3AS (Active Aeroelastic AircraftStructures) research programme5 has the aim of developing and demonstrating various active aeroelastic concepts.

Part of the 3AS research programme, described in this paper and previously6,7, is devoted towards investigatingthe use of changes in the internal aerospace structure ( the so-called adaptive internal structures approach) in order tocontrol the static aeroelastic behaviour. Possible applications include the continuous adjustment of wing shape tomaintain an optimal lift drag ratio, and also roll control of control-surface free UAVs. Such an approach is desirableand arguably advantageous compared to other possible concepts. For instance, the use of leading and trailingcontrol surfaces to control wing twist can lead to increased drag and poor observability characteristics. Smartmaterials (e.g. piezo and shape memory alloys) have also received considerable attention for such an application, butstill suffer from limits in the amount of force required to twist a wing. It is felt that conventional materials should beused initially until smart materials reach maturity. UAVs are likely to play a key role in exploring active aeroelastictechnologies due to the reduced power needed to adjust smaller structures and the much smaller cost in theirdevelopment.

The key idea exploited in the Adaptive Internal Structures approach is to make use of the aerodynamic forcesacting upon the wing to provide the moment to twist the wing. Consider the schematic of the wing shown in figure1, with the lift acting at the aerodynamic centre on the quarter chord. By changing the position of the shear centre ofthe wing, the bending moment (and hence the amount of twist) will also change. A far smaller amount of energy isrequired to adjust the structure compared to that required to twist the wing and keep it in that shape. Similar effectscan be achieved by using the internal structural changes to alter the value of the torsional stiffness. Such approachesare very attractive for adaptive aeroelastic wing concepts as they can be used in conjunction with a conventionalwing structure and are not such a radical step forward.

* Research Assistant, School of Mechanical, Aerospace and Civil Engineering, Manchester, M13 9PL, UK.† Professor, School of Mechanical, Aerospace and Civil Engineering, Manchester, M13 9PL, UK. SMAIAA.‡ Research Student, School of Mechanical, Aerospace and Civil Engineering, Manchester, M13 9PL, UK§ Research Assistant, School of Mechanical, Aerospace and Civil Engineering, Manchester, M13 9PL, UK.

46th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics & Materials Conference18 - 21 April 2005, Austin, Texas

AIAA 2005-2042

Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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The two most successful of the approaches of changing the shear centre location and torsional stiffnesspreviously considered at the University of Manchester6,7 have been:

• changing the chord-wise position of one or more of the spars, and• rotating the spars.

Both approaches have been demonstrated experimentally to enable successful control of bending and twist of anumber of simple wind tunnel models. The internal structure of both concepts is shown in figure 2. The rotating sparapproach provided much faster (around half a second) change between maximum and minimum stiffness, andconsequently would be much more suitable for roll control application as well as lift/drag applications. The changeof wing shape for on-line adjustment of drag (or lift/drag) does not need to be so quick, and so both approacheswould be feasible.

In this paper, some initial findings are shown that examine the changes in the aerodynamic dynamic performancethat can be achieved using the rotating spars approach. A number of finite element models of simple rectangularwings were generated and used to investigate the changes in the aerodynamic performance and the aeroelasticcharacteristic that can be obtained by varying the orientation of forward and rear spars.

II. Numerical Aeroelastic ModellingA range of finite element models of a similar size to the adaptive internal structure wind tunnel models were

generated using MSC NASTRAN. The initial objective was to reproduce the static and dynamic aeroelasticbehaviour of both wings. All models are straight, untapered wings with semi-span 0.775m and chord 0.25m.However, in this paper the emphasis is on examining the range of aerodynamic behaviour that is achievable. Thestatic and dynamic aeroelastic results were obtained using the MSC NASTRAN and ZAERO software packages.

Sample Finite Element models of one of the wings for both maximum and minimum bending stiffness spar casesare shown in figure 3. It can be clearly seen that a change of rotation through 90 degrees is required to achieve this.The torsional behaviour can be adjusted through rotation of the spars as well which not only alters the torsionalstiffness but also the position of the shear centre. It should be noted that the wing internal structure investigated hereis very stiff in torsion. A finite element model for the moving spar wing structure is shown in figure 4.

III. ResultsTypical wind off mode shapes are plotted in Figures 5.1- 5.4 for the (0,0) [leading edge rotation angle and

trailing edge rotation angle] rotation case . Only the first four bending and torsion modes are shown here as it hasbeen found that these are the ones that have the greatest influence upon the aerodynamic and aeroelastic behaviour.There are no surprises in these results, and the modes can be classified in terms of classical bending and torsionmodes.

Figures 6.1 to 6.4 show vg plots for the (0,0), (0,90), (90,0) and (90,90) beam rotation cases. The wing isreasonably stiff and does not flutter in the speed range considered. However, some differences in the behaviour ofthe natural frequencies do occur and these are due to the relative changes in the stiffness between the various cases.Due to the structural layout of the wing, there is not a great deal of change in torsional stiffness for the range ofbeam rotation angles, whereas the bending stiffness varies a great deal.

Table 1 shows the deflections of the wing at 15 m/s. Comparison of the bending and torsion deflections backsup the interpretation made from the vg plots in that the wing is much stiffer in torsion than it is in bending.

Finally, figures 7.1 – 7.3 show CD, CL and CL/CD plots for the range of leading and trailing edge spar orientations. Itcan be seen that there is a variation which can be controlled via changes in the spar rotation angles.

IV. Conclusions

A number of Finite Element models have been developed to investigate the possible use of the adaptive internalstructures concept. Both rotating spar and moving spar strategies have been examined. The effect of the sparorientation on normal modes, static aeroelastic deflections, dynamic aeroelastic behaviour (vg plots) and theaerodynamic lift and drag has been examined. The results back up what has been found previously using very

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simple analysis and experimental results, in that the concepts can be used to control the static wing displacements toachieve aerodynamic objectives.

What has become very apparent in this study is that wing structures need to be optimized in order to make bestuse of these concepts whilst meeting all other structural and aeroelastic constraints. Work is currently underway atthe University of Manchester to optimize the internal wing spar and rib layouts that incorporate adaptive internalstructures and to establish the benefits of such an approach on realistic full scale wing structures.

AcknowledgmentsThis work was funded by the European Union as part of the Active Aeroelastic Aircraft Structures Research

Programme. Active Aeroelastic Aircraft Structures (3AS) is a research project partially funded by the EuropeanUnion under the New Perspectives in Aeronautics Key Action of the "Competitive and Sustainable Growth"Framework Programme 5. Partners in the project are: EADS-Deutschland, Alenia, EADS-CASA, GAMESA, Saab,CIRA, DLR, INTA, VZLU, KTH, IST, University of Manchester, Politecnico di Milano, Technion and TsAGI.

References

1Pendleton, E., ‘Back to the Future - How Active Aeroelastic Wings are a Return to Aviation’s Beginnings and a Small Step toFuture Bird-like Wings’, RTO-SMP Panel Meeting. 2001.2Flick, P. & Love, M.‘The Impact of Active Aeroelastic Wing Technology on Conceptual Aircraft Design’. AVT Panel. Ottawa.1999.3Pendleton, E., Bessette, D., Field, P., Miller, G. & Griffen, K., ‘ The Active Aeroelastic Wing Flight Research Program’ 39th

SDM Conf 19984Wlezien-R-W; Horner-G-C; McGowan-A-R; Padula-S-L; Scott-M-A; Silcox-R-J; Simpson-J-O. “The Aircraft Morphing Program”SPIE Smart structures and Materials Meet 1998. pp 176-1875Kusmina, S et al. „Review and Outlook for Active and Passive Aeroelastic Design Concepts for Future Aircraft“ ICAS 2002.6Amprikidis, M. & Cooper J.E, “Adaptive Internal Structures for Active Aeroelastic Control” Int Forum on Aeroelasticity andStructural Dynamics. Amsterdam. 2003.7Amprikidis, M. & Cooper J.E, “Experimental Validation of Wing Twist Control using Adaptive Internal Structures”, AIAAStructures, Structural Dynamics and Materials Conference. Palm Springs, 2004

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Figure 1. Changing the Position and/or Orientation of the Spars Changes the Flexural Axis Position

Figure 2. Adaptive Internal Structures Prototype Models – Moving and Rotating Spar Concepts

Figure 3. Typical Finite Element Models for Moving Spar Approach

Lift

Flexural axis

Spars

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Figure 4. Typical Finite Element Model for Moving Spar Approach

Figure 5. Typical Wind Off Mode Shapes for Wind Tunnel Model

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0

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Figure 6.1 vg Plots for (0,0) Case

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Figure 6.2 vg Plots for (0,90) Case

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Figure 6.3 vg Plots for (90,0) Case

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Figure 6.4 vg Plots for (90,90) Case

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Figure 7.1 CL Values for Different Spar Orientations

Figure 7.2 CD Values for Different Spar Orientations

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Figure 7.3 CD / CL Values for Different Spar Orientations

caseLE

(mm)TE

(mm)Tip Twist

(deg)0 0 8.24 8 0.060 45 3.03 2.7 0.080 90 1.92 1.58 0.0845 0 2.84 2.8 0.0145 45 1.78 1.66 0.0345 90 1.32 1.14 0.0490 0 1.71 1.71 090 45 1.25 1.19 0.0190 90 0.99 0.89 0.02

Table 1. Deflections (mm) of the Rotating Spar Model