Elmar Breitbach

Adaptive Blade Twist - Calculations and Experimental Results.

Abstract Applying adaptronics to helicopters has a high potential to significantly suppress noise, reduce vibration, and increase the overall aerodynamic efficiency. This paper presents recent investigations on a very promising specific concept described as Adaptive Blade Twist (ABT). This concept allows us to directly control the twist of the helicopter blades by smart adaptive elements. This influences positively the main rotor area which is the primary source for helicopter noise and vibration. Since the interaction of non-stationary helicopter aerodynamics and elastomechanical structural characteristics of the helicopter blades causes flight envelope limitations, vibration and noise, a good comprehension of the aerodynamics is essential for the development of structural solutions to effectively influence the local airflow conditions and finally develop the structural concept. With respect to these considerations, the ABT concept will be presented. This concept is based on the actively controlled tension-torsion-coupling of the structure. For this, an actuator is integrated within a helicopter blade that is made of an anisotropic material based on fiber composites. Driving the actuator results in a local twist of the blade tip in such a way that the blade can be considered as a torsional actuator. Influencing the blade twist distribution finally results in a higher aerodynamic efficiency. The paper starts at giving a review on conventional concepts and potential adaptive solutions for shape control. After, some calculations of the adaptive twist control concept are presented. These are based on a representative model in which the active part of the rotor blade is simplified with a thin-walled rectangular beam, that is structurally equivalent to a model rotor blade of the Bo105 with a scaling factor 2.54. The calculations are performed using an expanded Wlassow theory. The results are valid for static and dynamic conditions. For the dynamic condition excessive deformations near the blade resonance frequency shall be utilised. Therefore, the actuated blade section has to be properly designed for this precondition. This has been demonstrated and verified in experiments, which will not be discussed in this paper. For experimental investigations on the ABT concept the skin of the outer part of the model rotor blade was manufactured of fibre composite material using the above mentioned tension-torsion-coupling effect with an additional uncoupling layer between skin and spar. The experimental results have shown that near to the resonance frequency dynamic forces of 550±550 N are required for a deformation of ± 3 degrees at the blade tip.

Development and design of flexible Fowler flaps for an adaptive wing

Civil transport airplanes fly with fixed geometry wings optimized only for one design point described by altitude, Mach number and airplane weight. These parameters vary continuously during flight, to which means the wing geometry seldom is optimal. According to aerodynamic investigations a chordwide variation of the wing camber leads to improvements in operational flexibility, buffet boundaries and performance resulting in reduction of fuel consumption. A spanwise differential camber variation allows to gain control over spanwise lift distributions reducing wing root bending moments. This paper describes the design of flexible Fowler flaps for an adaptive wing to be used in civil transport aircraft that allows both a chordwise as well as spanwise differential camber variation during flight. Since both lower and upper skins are flexed by active ribs, the camber variation is achieved with a smooth contour and without any additional gaps.

Overview of adaptronics in aeronautical applications

Abstract 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.

Interdisciplinary Wing Design - Structural Aspects

The following paper describes a multidisciplinary approach to design a wing with almost optimal aerodynamic efficiency during the entire cruise flight. Therefore a tight collaboration between structural mechanics and aerodynamics is necessary. Aerodynamic aspects are described here to ill ustrate the interdisciplinary nature of the design process, but they are not explained very deeply. The paper focuses on structural aspects, e.g. description of the tasks of the wings structural members, their placement within the wing and the modeling of the actual wing structure.

Structural Dynamic Health Monitoring of Adaptive CFRP-structures

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 structures 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.

Automated Synthesis of Robust Controllers for Smart-Structure Applications in Parallel Robots

Industrial robots occupy a central position in automation technology. Especially in the field of handling and assembly, the main domain of industrial robots, an increase of productivity is desired. Serial robots, where drives and links are located in a single chain, are widely used in those applications. A further enhancement of their productivity by shortening of cycle times is limited. In the past years, parallel robots demonstrated their potential in applications with needs for high-dynamic trajectories. Due to their fixed drives and large structural stiffness, they meet the demands for higher accelerations at constant precision, in contrast to serial robots. The direct consequence of large accelerations and decelerations during pick-and-place operations are high inertial forces that lead to vibrations and accordingly large decay times of the effector. To extend the possibilities of parallel robots under this operational conditions and in relation to cycle times and precision, the vibrations have to be suppressed effectively. A key technology for the control of such unwanted effects is smart-structures technology. Smart-structures technology uses structure integrated sensors and actuators to observe and control the vibrations of structures or parts of it. A general problem in smart-structures technologies is the sensitivity of the structures vibration behavior to structural changes. To guarantee the stability of the control loop beyond these changes, a redesign of the controller is necessary. To address this in the majority of cases unattended problem, a new and rapid procedure for a controller development chain including system identification is presented. This paper shows that an automated synthesis of a Robust Controller for a smart-structure system in a complex industrial robot with 4 DOF can be realized and verified. The algorithms are implemented on a real system and proven with measurements.

Adaptronics in airliner design: a new structural approach

The following paper presents the development of an adaptive aeronautical structure at the example of a high lift device, a so-called fowler flap. It shows the passive optimization of the flap with a complex finite element model and illustrates the necessity to develop an active structure, due to the limited possibilities to increase the efficiency of the passive flap. Different kinds of active measures are discussed and it is shown, why an activation of the structure with shape memory alloy (SMA) actuators is preferred. Additionally, the results and findings of experiments with an active spar that was developed, built, and tested at the Institute of Structural Mechanics (ISM) at the German Aerospace Center in Braunschweig are presented.

New results and future plans of the German major project ADAPTRONICS

In 1997 the BMBF announced a highly paid competition for future oriented key technologies and their industrial utilization. 230 proposals from industrial enterprises and research establishments were submitted. An independent group of experts selected altogether only 5 projects which were proposed to the BMBF. One of these projects was the major project ADAPTRONICS which is funded from 1998 to 2002 with a total volume of 25,000,000 EURO. This project is under the direction of the DLR and focuses on the integration of piezoelectric fibers and patches into lightweight structures aiming at active vibration and noise reduction, shape control and micro positioning. The main project target is the implementation of this technology in different industrial branches like the automotive industry, rail technology, mechanical engineering, medical engineering, and aerospace technology. This paper will give an overview of the recent progress and the next steps in the various tasks.