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Adaptive Stiffness Structures for Air Vehicle Drag Reduction
J. E. Cooper
School of Mechanical, Aerospace and Civil Engineering University of Manchester Oxford Road, Manchester, M13 9PL, UK. jecooper@manchester.ac.uk

ABSTRACT
The development of several adaptive internal structures concepts is described that are aimed towards enabling adaptive static aeroelasticshape 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 aredescribed, 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 implementingthe 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 haveto 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”1. Civil and military aircraft are designed to have optimalaerodynamic 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 aremore 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 theflight 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 toenhance aerodynamic performance1,2. For instance, the Active Flexble Wing3, Active Aeroelastic Wing4-6 programs investigated the use of using leading and trailing edge control surfaces to control the wing shape. The Morphing Program7 developed a
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.

RTO-MP-AVT-141

15 - 1

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