The term "smart structures" is commonly used for structures which have the ability to adapt to environmental conditions according to the design requirements. As a rule, the adjustments are designed and performed in order to increase the efficiency or safety of the structure. Combining "smart structures" with the "sophistication" achieved in materials science, information technology, measurement science, sensors, actuators, signal processing, nanotechnology, cybernetics, artificial intelligence, and biomimetics, one can talk about Smart Intelligent Structures. In other words, structures which are able to sense their environment, self-diagnose their condition and adapt in such a way so as to make the design more useful and efficient.
The concept of Smart Intelligent Aircraft Structures offers significant improvements in aircraft total weight, manufacturing cost and, above all, operational cost by an integration of system tasks into the load carrying structure. It also helps to improve the aircraft's life cycle and reduce its maintenance. Individual morphing concepts also have the ability to decrease airframe generated noise and hence reduce the effect of air traffic noise near airports. Furthermore, cruise drag reductions have a positive effect on fuel consumption and required take-off fuel load.
Morphing structures
Fixed geometry wings are optimized for a single design point, identified through altitude, Mach number, weight, etc. Their development is always a compromise between design and off-design points, referred to a typical mission. This is emphasised for civil aircraft where flight profiles are almost standard. Nevertheless, it may occur to fly at high speeds and low altitude with light weight over a short stretch or to fly at low speeds and high altitude with maximum load for a longer range. The lift coefficient would then range between 0.08 and 0.4, with the aircraft experiencing up to 30% weight reduction as the fuel is consumed. These changes could be compensated by wing camber variations, to pursue optimal geometry for any flight condition, thus improving aerodynamic and structural performance.
Existing aircraft cannot change shape without aerodynamic gaps, something that can be solved with Smart Intelligent Structures. By ensuring the detailed consideration of structural needs throughout the entire lifetime of an aircraft and focusing on the structural integration of needed past capabilities, Smart Intelligent Aircraft Structures will allow aircraft designers to seriously consider conformal morphing technologies. The reduced drag during take-off, cruise and landing for future and ecologically improved civil aircraft wings can be achieved through naturally laminar wing technology, by incorporating a gapless and deformable leading edge device with lift providing capability. Such a morphing structure typically consists of a flexible outer skin and an internal driving mechanism (Figure 1). Current aircraft designs already employ winglets aimed at increasing the cruise flight efficiency by induced drag reduction. Smart intelligent Structures propose a state of the art technology that incorporates a wingtip active trailing edge, which could be a means of reducing winglet and wing loads at key flight conditions.
Structural health monitoring
Another component of an "intelligent" aircraft structure is the ability to sense and diagnose potential threats to its structural integrity. This differs from conventional non-destructive testing (NDT) by the fact that Structural Health Monitoring (SHM) uses sensors that are permanently bonded or embedded in the structure. Composite materials, which are highly susceptible to hidden internal flaws which may occur during manufacturing and processing of the material or while the structure is subjected to service loads, require a substantial amount of inspection and defect monitoring at regular intervals. Thus, the increasing use of composite materials for aircraft primary structure aircraft components increases substantially their life cycle cost. According to some estimates, over 25% of the life cycle cost of an aircraft or aerospace structure, which includes pre-production, production, and post-production costs, can be attributed to operation and support, involving inspection and maintenance. With sensing technology reducing in cost, size and weight, and sensor signal processing power continuously increasing, a variety of approaches have been developed allowing integration of such sensing options onto or into structural components.