Different systems and strategies have been invented in order to reduce the noise level inside the fuselage of aircrafts. First of all passive methods like adding materials with high damping or vibration absorbing qualities were used. Due to mass reduction as a major aspect in aircraft design a lot of research is focused on active noise reduction (ANR). The level of attenuation gained by an ANR - system is depending on several attributes of the system like hardware and software in use. Another important parameter, which has a great impact on the performance, is the positioning of the actuators and sensors. Because of the high number of possible arrangements of actuators and sensors in three dimensional spaces, it is almost impossible to determine the optimal positions by experimental work. Therefore numerical optimization is applied.
In this paper a hybrid evolutionary algorithm is introduced for the calculation of appropriate configurations for a fixed number of actuator and sensors out of a high number of possible positions for an ANR - system within a military aircraft. The presented COSA - algorithm (cooperative simulated annealing) connects qualities of two well known optimization algorithms, the simulated annealing (SA) and genetic algorithm (GA). A general description of the algorithm and the acoustical basics will be provided together with an overview of the results.
In some types of aircraft tonal interior noise with high sound pressure level (up to 110 dB(A)) occurs at low frequencies (f < 500 Hz). Typical examples are propeller driven aircraft, for which the excitation frequencies are given by the blade passage frequency (BPF) and its higher harmonics. The high tonal noise levels at these frequencies can occur due to the fact that the blades' profiles are only optimized in terms of aerodynamics. The acoustic properties are usually not taken into account. In order to obtain an acceptable interior noise level, and to guarantee both work-safety and comfort in the aircraft interiors, passive methods are commonly used - e.g. adding material with high damping or vibration absorbing qualities. Especially when low frequency noise has to be reduced, adding material results in a lot of unwanted additional weight. In order to avoid this extra weight, the concept of active noise reduction (ANR) and tunable vibration absorber systems (TVA), which focus on the unwanted tonal noise, are a good compromise of treating noise and the amount of additional weight in aircraft design. This paper briefly discusses two different possible methods to reduce the low frequency noise. The noise reduction of tuned vibration absorbers (TVA) mounted on the airframe are nowadays commonly used in propeller driven aircraft and can be predicted by vibroacoustic finite element calculations, which is described in this paper. In order to abide to industrial safety regulations, the noise level inside the semi closed loadmaster area (LMA) must be reduced down to a noise level, which is even 8 dB(A) below the specified cargo hold noise level. The paper describes also the phases of development of an ANR system that could be used to control the sound pressure level inside the LMA. The concept is verified by experimental investigations within a mock up of the LMA.
The increasing industrialization and markets across the globe do result in noise pollution that affects humans. In order to reduce the sound pressure level (SPL) of disturbing noise active noise control (also known as noise cancellation, active noise reduction (ANR) or anti-noise) is a good option. Herewith unwanted noise from a primary sound source can be reduced significantly by anti-noise generated from a secondary source: At present commercial active noise reduction systems are using moving-coil loudspeakers as actuators. These actuators need a quite large built-in volume and they are not lightweight. Therefore the industrial application of ANR in vehicles is limited. To reduce these difficulties the use of flat loudspeakers made of electromagnetic films seems to be a promising approach. It is a precondition for the use of such new technologies within an ANR- system to have a basic understanding of the dynamic systems behaviour and the sound transmission behaviour of such a lightweight active component: This paper describes the investigation of a flat panel speaker which is based on electrostatic loudspeaker technology. First of all the passive transmission properties have been measured in a test bed. The passive acoustic insulation has been analyzed and weak spots in the frequency response were discovered. Afterwards the flat panel speaker has been used as actuator in an ANR-System to support insulation at those frequencies. An adaptive filter (FxLMS) was adjusted to the panel and the reduction capabilities of a single-output system have been determined.
In propeller driven aircraft the main source for internal noise are tonal disturbances caused by the propeller blades that are passing the fuselage. In a certain four propeller military transport aircraft the maximum sound level in the cabin can reach up to 110 dB(A), not taking into account any noise control treatments. Inside the semi closed loadmaster working station (LMWS) the sound level must be reduced down to 86 dB(A). It is proposed to reach this goal with an active noise control system, because passive solutions are to heavy at low frequencies.
Optimal positions of the loudspeakers are found by finite element calculations. These positions have been realized in a full-scale test bed. A reduction of the sound pressure level of more than 30dB within a specified volume was achieved at a frequency of 100 Hz.
HiFi speakers are used as secondary actuators in this test bed. These speakers are heavy and have unsuitable geometric dimensions for an aircraft. Therefore, other actuators, e.g. flat panel speakers, will be investigated with respect to the application in a mock-up of the LMWS.
Efficiency in high speed mechanism can be further increased by use of lightweight construction. But quite often these structures have the drawback of being susceptible to vibrations. This can be overcome by applying the technology of smart structures. Here distributed actuators and sensors made from piezoceramic (PZT) material are capable to actively reduce the unwelcome vibrations if implemented within a control loop. For the optimal design of such kind of mechanism up-to-date simulation tools have to be developed further. To simulate the dynamic behavior of lightweight structures undergoing large motions the multibody approach is a suitable tool. The necessary parameters in the equations of motion for the flexible body can be calculated from the output of a finite element code. The large number of variables from the finite element model have to be reduced to only a few generalized coordinates. Therefore a modal reduction is applied in combination with the introduction of a moving frame of reference. Beyond this technique so called active modes are introduced to represent the impact of the active strain by the PZT patches. These active modes combined with natural modes represent the body deformation within the multibody model.
Within the framework of an idea competition for future-oriented key technologies and their industrial utilization, in 1997 BMBF called for project proposals from industries and research for so-called 'Leitprojekte'. An independent group of experts selected few project proposals from the many submitted, and proposed them to BMBF for promotion. One of these projects is the BMBF-Leitprojekt ADAPTRONIK which is introduced in this paper. Adaptronics describes the field of technology focusing on the development of a new class of so-called smart structures. The Leitprojekt ADAPTRONIK consists of 24 partners from industry and research institutes and is conducted under the responsibility of the German Aerospace Center (DLR). The project focuses on the development and structure-conforming integration of piezoelectric fibers and patches in structures for lightweight construction. It is aimed at active vibration and noise reduction, contour deformation and micro-positioning in the very sense of adaptronics in various industrial applications. The project targets are prototype assemblies from the fields of automotive industry, rail vehicles, mechanical engineering, medical engineering, and aerospace. In the paper the content, the status and an outlook will be presented.
The belt-rib concept for lifting surfaces with variable camber evolved at DLR recently as one of the most promising solutions for the adaptive wing. With the belt-rib idea the adaptive wing issue is approached in a new way: instead of a 'mechatronic' solution with hinges or linear bearings a 'structronic' solution is chosen, where defined, distributed flexibility allow the desired shape changes. The new concept evolves from the classical wing structure. The classical rib, which is in charge of the wing section's stiffness, is replaced by a 'belt rib,' which allows camber changes within given limits while leaving the remaining in-plane stiffness properties of the section widely unchanged. The evolution of the belt-rib concept was accompanied by experimental tests on different prototypes. The last developments concern the construction of a model with solid-state hinges, realized as hybrid glass fiber -- carbon-fiber reinforced composite structure. The model is actuated mechanically by Bowden cables, which can be replaced by shape memory wires in the next development stage. In this paper, the fundamentals of the concept and the most relevant results of the first developments are reported. A description of the new belt-rib design follows, which was implemented in a new prototype. The description of the experimental strength proof and an outline of further development work conclude the paper.
The DLR Institute of Structural Mechanics is engaged in the construction and optimization of adaptive structures for aerospace and terrestrial applications. Due to the FFS- Project, one of the recent works of the Institute is the reduction of buffet induced vibration loads at a fin. The construction of modern aircrafts is influenced b the increasing use of fiber composites. They have more specific stiffness and strength properties than metals. On the other hand the layered structure leads to new kinds of damages like delaminations. In the fin interface there are actuators and sensors integrated. Therefore the fin is connected with a controller. For the extension of this adaptive system towards an on-line tool for health monitoring this controller can be used as an identifier of the structure's modal parameters. The most promising procedure is based on MX filters. These filters constitute the filter coefficients from which a fast transformation procedure extracts the modal parameters. The changes of these parameters are related to the location and extent of the damage. So when using the already integrate controller for system identification, one can have a low-cost on-line damage detection for dynamic adaptive structures. First off-line test at CFRP plates have shown the ability to detect delaminations.
Modern military aircraft are characterized by employment of optimized structural components. New demands on exploitation of lightweight construction technology will arise because even greater flexibility with increased maneuverability is desired. The structural integration of multifunctional, often called 'smart' elements, properly activated to e.g. reduce structural loading, offers great potential to necessary advances in military aircraft design. One major problem of modern military aircraft is the buffet loading on the fin structures. Flying the aircraft at high angles of attack allows vortices, evolving from the leading edge of the wing, to hit the fin and excite structural vibrations. This leads to structural attrition as well as a reduced aircraft maneuverability. With the aim to reduce these fin vibrations, an adaptive structure has been developed which is presented in this paper. A concept is discussed with which the vibrational loads are reduced by introduction of counteracting forces using an 'active interface'. This interface concept is characterized by the integration of active, piezoelectric elements directly into the bending support of the fin structure. To validate the stability of the interface FE calculations and extensive measurements on piezoceramic stack actuators have been performed. The manufactured interface was integrate in an existing test structure and realistically loaded. The result will be given in this presentation.