Editorial

Compiled by Dr. David Kennedy, Cardiff School of Engineering, Cardiff UK
Email kennedyd@cf.ac.uk

The design of aircraft and spacecraft is a complex multi-disciplinary activity. Aerodynamic, propulsion, structural and systems design requirements lead to multiple, and often competing, objectives and constraints.  Following an initial specification of the design requirements, an iterative conceptual design procedure explores numerous alternative design concepts.  The preliminary design stage next involves more rigorous analysis and testing of a number of the most promising concepts.  The chosen configuration is finally submitted to a detailed design process which provides the engineering data required for tooling, manufacture and certification.

The design of aircraft and spacecraft is a complex multi-disciplinary activity. Aerodynamic, propulsion, structural and systems design requirements lead to multiple, and often competing, objectives and constraints.  Following an initial specification of the design requirements, an iterative conceptual design procedure explores numerous alternative design concepts.  The preliminary design stage next involves more rigorous analysis and testing of a number of the most promising concepts.  The chosen configuration is finally submitted to a detailed design process which provides the engineering data required for tooling, manufacture and certification.

Aerodynamic and structural design is concentrated in the preliminary design stage.  Many different load cases and flight scenarios have to be considered, and even using today’s most powerful computers the computational requirements can be excessive.  While the detailed design stage will undoubtedly engage the most sophisticated software and modelling techniques, during preliminary design there is a need for simpler approaches which are fast, yet accurate and reliable.  Considerable academic effort is being expended in this area, resulting in a wealth of literature on analytical methods and optimization.  It is essential to broaden the dialogue between universities and industry, in order for researchers to improve their understanding of current industrial requirements, and also to facilitate the exploitation of research outcomes into practical methods and tools for design of the next generation of aerospace structures.

This virtual issue of the Journal of Sound and Vibration presents a selection of recent advances in the study of the vibration of aerospace structures and their components, concentrating on the themes of modelling and design, damage detection, noise reduction and active control.

Modelling and design
Advanced lightweight composite materials are commonly used in place of metals in the wings and fuselage of military, and increasingly in civil, aircraft.  Although material and manufacturing costs are higher than those for metals such as aluminium, their superior ratios of strength and stiffness to weight result in lower mass designs and the all-important consequent reductions in fuel consumption and atmospheric emissions.  Because composite laminates can be optimized to specific loading requirements, the design process involves detailed modelling and analysis, typically involving many unknowns and the need to satisfy practical manufacturing constraints which are difficult to incorporate in numerical models.

Woodcock (2008) presents a hybrid modelling approach for the free vibration of such laminates.  He combines the advantages of a single-layer model using first-order shear deformation theory and a pure multi-layer model, so that the number of unknowns is independent of the number of layers and a rigorous transfer matrix analysis can be developed from the interface conditions.  This approach provides efficient, accurate predictions of component-level behaviour for use in optimization at the preliminary design stage.

Damage detection
Laminated composite materials are susceptible to delamination, cracking and other forms of impact and fatigue damage.  Often the damage occurs internally and is difficult to detect.  Changes in vibration characteristics provide an important means of assessing such damage.

Wang et al. (2005) describe an analytical model for a fibre-reinforced composite beam with an edge crack, vibrating in coupled bending and torsion.  Their results indicate how the natural frequencies and mode shapes vary with the material properties as well as with the crack location and depth.  This approach may be used as a tool to detect cracking, to investigate its propagation, and to predict its effect on future behaviour of the structure (e.g. flutter speed).

Trendafilova et al. (2008) develop a generic vibration-based damage detection methodology based on principal component analysis and pattern recognition, using frequency response functions for damaged and undamaged structures.  Their approach is validated by numerical experiments on a finite element model of an aircraft wing, including both cracks and distributed damage.

LeClerc et al. (2007) report experimental work on a large-scale composite aircraft component.  Strain wave propagation is measured using piezoceramic sensors, in order to locate impact damage on the structure using a neural network approach employing a combination of regression and classification strategies.

Accurate response prediction is important in designing aerospace structures for sonic fatigue. The high computational costs associated with direct time integration of a full non-linear finite element model can be reduced by reduction to a low-order system of non-linear modal equations.  Hollkamp et al. (2005) compare five such reduction methods, using a clamped-clamped beam as an example problem.

Noise reduction
There is an increasing awareness of aircraft noise and its impact on communities, and noise reduction at source has become an integral part of the design process.  Gerhold et al. (2006) compare analytical and experimental results on a scale model of an unconventional airframe comprising a wedge-shaped blended wing body with engines mounted on top of the wing.  Using an equivalent source method, it is shown that the scattering and diffraction of engine-generated sound are sensitive to the engine location and the shape of the airframe, with maximum noise reduction occurring in the region beneath and in front of the airfoil.

Guo (2008) describes a component-based empirical model for the prediction of external noise from aircraft landing gear.  Normalized spectra describe the frequency characteristics in three frequency domains, corresponding to the wheels, main struts and small details of the gear assembly.  Predictions from the model are compared with data from wind-tunnel tests on various commercial airliners, showing good agreement both in parametric trends and in absolute noise levels.

Liu (2008) identifies turbulent boundary layer excitation as an important component of aircraft interior noise, developing models for flat and curved rectangular stiffened panels.  Numerical studies consider the effects of the material, panel geometry, curvature, damping and in-plane tension on the structural response and sound radiation of the panel, with the radiation treated as a free-field problem.

Active control
Research is intensifying into active vibration control using smart materials.  Choi et al. (2007) consider helicopter blades, modelled as rotating thin-walled beams made from laminated composites, with integrated piezoelectric sensors and actuators. Finite element simulations demonstrate the damping effects of a negative velocity feedback control algorithm, and also investigate the effects of design parameters such as rotation speed, pre-twist angle and fibre orientation.

Finally, Sheta et al. (2006) address the problem of vertical tail buffeting in twin-tail fighter aircraft, by developing an active smart material control system using distributed piezoelectric actuators.  High fidelity fluid dynamics, structural dynamics, electrodynamics and control models are integrated into a multi-disciplinary object-oriented computing environment.  Numerical results indicate that this approach effectively alleviates buffeting over a wide range of angles of attack. 

>>See virtual special issue articles