Overview of Adaptronics in Aeronautical Applications

Kurzfassung

In the first half of the eighties the development of new kinds of structural systems, called „smart“, „intelligent“, or „adaptive“, was initiated first in the USA and soon after in Japan. Apart from suitable sensors and controllers, emphasis was placed on actuator systems of thermally, electrically or magnetically activaitable materials (piezo-electrics, electrostrictives, magnetostrictives, electro- and magnetorheological fluids, etc.) capable of being inte-grated into appropriate structural components, thus enabling the structure to adapt automatically to changing environmental and operational conditions. Approximately five years later similar efforts were also started in Europe, especially in Germany, leading to the development of so-called „adaptronic“ structures. These structures are characterized by their multifunctionality, i.e. adaptronic actuator systems are structurally integrated such that they serve not only as actuators but also as loadcarrying members of the structure itself. With respect to the fail-safe principle prevalent in aircraft design, this implies that the actuator elements ought to be operated parallel to passive load-carrying structural elements, allowing not only a sensible distribution of operationally induced loads on active and passive structural elements, but sufficient residual strength after a failure of the actuation function as well. The Institute of Structural Mechanics (ISM) at the German Aerospace Center is working on several projects, concerning adaptive structures. This paper focuses on some aeronautical applications, that are subject of current investigations. The following applications have the objective to increase the efficiency of cruise and high lift flight of transport airplanes by deforming the wing trailing edge and building up a bump at the upper surface of the wing. Since the actuation is relatively slow, a so called quasi static actuator concept can be applied. Wings of modern transport airplanes have a fixed geometry, optimized for a certain design point in cruise flight, described by the parameters altitude, mach number and aircraft weight. Since these parameters vary constantly, the airplane normally flies outside the design point, leading to a decreased performance. Changing the geometry of the airfoil during cruise flight by deforming the wings trailing edge leads to a much higher efficiency. The ISM has developed two variable chamber concepts to realize an adaptation of the airfoil, the finger concept and the belt rib concept. Both concepts are even capable of a spanwise differential deformation, enabling them to change the spanwise lift distribution in order to decrease the bending moment at the wing root, allowing a lighter construction of the wing itself. A bump on the upper wing surface decreases the wave drag significantly. The bump is extended in that area of the profile, where an expansion shock due to local supersonic flow speed oc-curs. The position of the bump can be adapted to the position of the expansion shock, thus providing a configura-tion with the lowest possible drag. During high lift flight with flaps extended, these are deformed in spanwise direction due to the applied airload. With a structurally integrated actuator system it is possible to re-deform the flap in order to provide an optimal gap size between wing and flap, leading to a much better high lift behavior. There are also projects, where the dynamic behavior of aerospace structures is increased by means of adaptron-ics. One example is concerned with vibrations of the vertical stabilizer on fighter aircraft due to vortexes, gener-ated by the wings leading edge. They are reduced by means of an active interface, made of piecoceramic stack actuators, thus resulting in reduced structural loading. Adaptronic solutions can also be applied to the suppression of vibrations in propeller-driven airplanes. If some elements of the passive strut/truss structure between power plant and airframe are substituted by active parts, it is possible to reduce propeller induced vibrations, leading to a higher passenger comfort and reduced dynamic loading of the airframe. Present helicopter research mainly focuses on the improvement of the aerodynamic efficiency and on the reduc-tion of vibrations and acoustic emission. Adaptronics have a high potential to efficiently achieve this goal. The Adaptive Blade Twist concept, developed by ISM, allows to directly control the twist of the helicopter blades by smart adaptive elements and through this to positively influence the main rotor area which is the primary source for helicopter noise and vibrations. The goal of this paper is to demonstrate the current state of the art of adaptronic applications in the field of aero-nautics with some examples, to point out existing deficits, and to identify new application areas, in order to pre-pare the ground for a rapid transfer of adaptronic concepts into industrial practice