Adaptive Stiffness Structures for Air Vehicle Drag Reduction

The development of several adaptive internal structures concepts is described that are aimed towards enabling adaptive static aeroelastic shape control of an aircraft wing in flight. It is shown how changes in the position, orientation and stiffness of the internal wing structure can be used to change the bending and torsion stiffness properties of a wing, and hence to control the aerodynamic performance, in particular the lift and drag characteristics. Two approaches that implement the adaptive internal structures technology are described, based upon the rotation and chordwise translation of the spars. Following the description of conceptual studies to illustrate the concepts, the design, manufacture and testing of two wind tunnel models is described. The experimental results were found to show good agreement with static and dynamic aeroelastic behaviour predictions from Finite Element models. The feasibility of implementing the adaptive internal structures approach on full-size aircraft is discussed. 1.0 INTRODUCTION Since the beginning of powered flight, aeroelastic phenomena have had a significant influence upon aircraft structural design. In particular, as many prototype aircraft have been destroyed due to the occurrence of either flutter or divergence, it has been accepted that aircraft lifting surfaces have to be built to be stiff, and therefore heavy, enough so that neither phenomenon occurs within the desired flight envelope. Indeed, the most common “fix” to deal with flutter problems that might arise within aircraft development programmes is to add extra mass to the structure. This requirement has been termed the “aeroelastic penalty”. Civil and military aircraft are designed to have optimal aerodynamic characteristics (maximum lift/drag ratio) at one point and fuel condition in the entire flight envelope. However, the fuel loading and distribution changes continuously throughout the flight, and aircraft often have to fly at non-optimal flight conditions due to air traffic control restrictions. The consequent sub-optimal performance has more significance for commercial aircraft as they are more flexible than military aircraft and also have fuel efficiency as a performance parameter of far greater importance. There is also much recent interest in High Altitude Long Endurance (HALE) aircraft which are designed to fly for several days at a time and have a greater proportional of fuel to take-off weight than other aircraft, the resulting changes in the aeroelastic shape throughout the flight can be substantial. Fuel efficiency is becoming increasingly important for civil and HALE aircraft, and any approach that enables better aerodynamic performance throughout a flight needs to be investigated and developed. In recent years there has been a growing interest in the development of aircraft structures that allow aeroelastic deflections to be used in a beneficial manner and to enhance aerodynamic performance. For instance, the Active Flexble Wing, Active Aeroelastic Wing programs investigated the use of using leading and trailing edge control surfaces to control the wing shape. The Morphing Program developed a RTO-MP-AVT-141 15 1 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED Cooper, J.E. (2006) Adaptive Stiffness Structures for Air Vehicle Drag Reduction. In Multifunctional Structures / Integration of Sensors and Antennas (pp. 15-1 – 15-12). Meeting Proceedings RTO-MP-AVT-141, Paper 15. Neuilly-sur-Seine, France: RTO. Available from: http://www.rto.nato.int/abstracts.asp. 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THIS PAGE unclassified Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 number of active aeroelastic concepts based upon smart materials and structures. In Europe, the 3AS (Active Aeroelastic Aircraft Structures) research program also developed and demonstrated various active aeroelastic concepts, primarily in the areas of adaptive attachments, three surface aeroelastic aircraft and novel aeroelastic leading edge and wing tip devices. The Variable Stiffness Spar (VSS) approach demonstrated the use of rotating spars for roll control. The concept of adapting the shape of an aircraft in flight has been given the generic title of morphing, however, the activity can be divided clearly into two different categories: • Planform morphing – where the aircraft planform is changed. Recent work has investigated the use of telescopic, deployable and variable sweepback wings using a range of mechanical and pneumatic devices. One of the main drivers behind the use of this technology is the capability of changing mission mid-flight, e.g the development of UCAVs that are able to loiter with high aspect ratio wings, but can then change their aspect ratio as they change to an attack role. Other examples have included the use of telescopic wings for roll control. Of course, there are many examples of military aircraft that have flown for many years with variable sweep wings. • Performance morphing – where the lifting surface planform remains the same but the aerodynamic shape (and hence performance) can be changed either through adjustments in the twist and bending behaviour along the wingspan, or changes in the camber and/or the leading and trailing edge shapes. Such a capability could be used to maintain the best possible lift-drag ratio throughout the flight however, there is also the possibility of implementing roll-control (analogous to the Wright Brother’s “wing-warping”) which has gained some significance recently with the interest in control-surface-free UAVs in order to improve observability characteristics. Part of the 3AS research programme, as described in this paper, was devoted towards the performance morphing category, and investigated the use of changes in the internal aerospace structure in order to control the static aeroelastic bending and twisting behaviour of a number of simple wings. Other work has examined the use of smart materials (e.g. piezo and shape memory alloys) to achieve this goal however, there is still a lot of work and material development required in order to develop the considerable forces required to twist and bend (and maintain that deflection) a wing during flight. In this paper, the Adaptive Internal Structures concept for control of the wing static aeroelastic shape inflight is introduced and illustrated with two different approaches: changing the chord-wise position of the spars, and rotating the spars. The design, manufacture and testing of several adaptive aeroelastic wind tunnel models based on the two concepts is described. Comparisons are made with the results obtained from Finite Element simulations. Finally, the feasibility of applying the Adaptive Internal Structures concept to full-size aircraft is discussed. 2.0 ADAPTIVE INTERNAL STRUCTURES The key idea exploited in the Adaptive Internal Structures approach is to use the aerodynamic forces acting upon the wing to provide the forces and moments to bend and twist the wing, rather than trying to apply the forces via some form of actuator. Consider the schematic of the wing shown in Figure 1, with the lift acting at the aerodynamic centre on the quarter chord. By changing the position of the shear centre of the wing, the bending moment, and hence the amount of twist, will also change. It is envisaged that a far smaller amount of energy is required to adjust the structure compared to that required to twist the wing and to maintain the shape. Adaptive Stiffness Structures for Air Vehicle Drag Reduction 15 2 RTO-MP-AVT-141 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED Figure 1. Effect of Position and Orientation of the Spars on the Flexural Axis Position. Such an approach is very attractive for adaptive aeroelastic concepts, where the lifting surface deflections could be adjusted gradually throughout the flight to maintain an optimal aerodynamic performance. The concept could also be applied for roll control and gust or manoeuvre load alleviation, however, there would be a far greater requirement in these cases for the structural stiffness to change rapidly. The actual application, and how the adaptive internal structure concept was applied, would have a great influence upon whether the required roll or load alleviation performance could be achieved, and this has been investigated elsewhere. It is envisaged that the adaptive internal structures concept is not suitable for high frequency applications (e.g. flutter or buffet loads suppression.). It should also be noted that the approach is best applied to the tip end of the lifting surface, where the structural stiffness is less and also the influence upon the aerodynamic forces is greatest. Also, only some elements of the wing internal structure would need to be adaptive. Reference needs to be made to the Variable Stiffness Spar (VSS) work that investigated and demonstrated the use of rotating spars to provide roll control of a wing and to influence the control effectiveness. Part of the work described in this paper builds upon the VSS research, but is aimed towards changing the wing shape using multiple spars with the goal of improving the lift / drag characteristics. 3.0 CONCEPTUAL MODELLING Consider the aeroelastic analysis of a simple high aspect ratio rectangular wing in order to demonstrate how the static and dynamic aeroelastic behaviour can be controlled through movement (either translation or rotation) of the spars. As the purpose is to demonstrate the concept, only a rudimentary aeroelastic analysis has been employed. Modified strip theory aerodynamics was implemented along with thin walled structural theory and a Rayleigh-Ritz assumed shapes approach in order to study the static and aeroelastic behaviour of the wing, considering only the wing box itself. Of interest is the variation of the torsion constant, the position of the shear centre, the static twist, and also the effect upon the flutter and divergence speeds in relation to the position or orientation of the spars. All of these parameters can be calculated explicitly using this analytical approach, which is not the case if a FE model were used. Schematics of the proposed concepts are shown in Figures 2 and 3. In the first, the simple rectangular wing box is made up of three identical uniform spars. The outer two spars remain in the same position, whereas the middle spar can be moved anywhere within the wing box. By moving the chord-wise position of the central spar, it is possible to change the torsional stiffness and also the position of the local shear centre, but the bending stiffness remains constant. In the second approach, the spars are able to rotate, thereby changing their bending stiffness in both vertical and horizontal directions as well as the torsional stiffness and shear centre position. In both cases, changing the wing section bending and torsional stiffness characteristics leads to changes in aeroelastic twist and bending. Lift Flexural axis Spars Adaptive Stiffness Structures for Air Vehicle Drag Reduction RTO-MP-AVT-141 15 3 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED Figure 2. Rectangular Wing Box with Movable Central Spar Figure 3. Rectangular Wing Box with Rotating Leading and Trailing Edge Spars Figure 4 shows the changes in the torsion constant, shear centre position, wing tip twist, flutter and divergence speeds with respect to the position of the middle spar for the moving spar concept. All the results have been normalised with respect to the values found with the spar in the central position. These results demonstrate how the aeroelastic twist can be controlled, simply by moving the spar. However, the twist is not simply a linear relation of the spar position as although the torsion constant and shear centre position have the expected symmetric behaviour with the maximum value occurring when the spar is placed at the central position, the aerodynamic moment is related to the distance of the shear centre from the aerodynamic centre at the quarter chord. The aeroelastic characteristics are dependent upon not just the torsional and bending (constant in this case) stiffness, but also the distance between the flexural axis and the aerodynamic centre. Although the bending stiffness does not change, there is still a coupling between the bending and torsion behaviour in the aeroelastic model. Inspection of the bottom three plots in Figure 4 demonstrates the relative Fixed