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«Aeroservoelastic and structural dynamics research on smart structures conducted at NASA Langley Research Center Anna-Maria %vas McGowan, W. Keats ...»

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Aeroservoelastic and structural dynamics research on smart structures

conducted at NASA Langley Research Center

Anna-Maria %vas McGowan, W. Keats Wilkie, Robert W. Moses, Renee C. Lake,

Jennifer Pinkerton Florance, Carol D. Wieseman, Mercedes C. Reaves, Barmac K. Taleghani,

Paul H. Mirick, and Matthew L. Wilbur

Aeroelasticity and Structural Dynamics Branches

NASA Langley Research Center

Hampton, Virginia


An overview of smart structures research currently underway at the NASA Langley Research Center in the areas of aeroservoelasticity and structural dynamics is presented. Analytical and experimental results, plans, potential technology pay-offs, and challenges are discussed. The goal of this research is to develop the enabling technologies to actively and passively control aircraft and rotorcraft vibration and loads using smart devices. These enabling technologies and related research efforts include developing experimentally-validated finite element and aeroservoelastic modeling techniques;

conducting bench experimental tests to assess feasibility and understand system trade-offs; and conducting large-scale windtunnel tests to demonstrate system performance. The key aeroservoelastic applications of this research include: active twist control of rotor blades using interdigitated electrode piezoelectric composites and active control of flutter, and gust and buffeting responses using discrete piezoelectric patches. In addition, NASA Langley is an active participant in the DARPA/ Air Force Research Laboratory/ NASA/ Northrop Grumman Smart Wing program which is assessing aerodynamic performance benefits using smart materials.

Keywords: aeroelasticity, smart structures, piezoelectric actuators, active fiber composites, rotorcraft, buffet load alleviation, individual blade control, aeroservoelasticity, shape memory alloys, damping augmentation, piezoelectric power consumption

1. INTRODUCTION The Aircraft Morphing program’ at the NASA Langley Research Center is a new 6-year program to develop smart devices for airframe applications to enable self-adaptive flight for a revolutionary improvement in efficiency and safety. In this context, a smart device is one that senses and reacts to its local environment to achieve some overall system benefit such as an increase in performance or maintaining flightworthiness in the event of other failures. The goals of the Aircraft Morphing program are to develop and mature smart devices and the enabling technologies needed to embed these devices in aircraft structures (and thus create an “active component”) and efficiently use them to provide cost-effective system benefits. Research in the Aircraft Morphing program covers a wide range of disciplines and supports the integrated program focus areas of active aerodynamics, active noise control and active aeroelastic control. This paper presents an overview of the current research conducted in the areas of active aeroelastic control and the related structural dynamics research activities conducted in the Aircraft Morphing program as well as other Langley programs. A review of previous smart structures-based aeroservoelastic research activities at NASA Langley is also presented.

The goals of applying smart devices to aeroelastic problems are to control the aerodynamic andor structural characteristics of air vehicles to improve flutter characteristics and reduce gust, buffeting and maneuver loads of fixed-wing vehicles and to reduce dynamic responses and loads on rotorcraft. These benefits also result in reduced emissions and increased performance and safety. In many cases, applications of smart devices will take advantage of the inherent flexibility in air vehicles to create more efficient structural designs. At NASA Langley, much of the effort in the area of controlling dynamic aeroelastic phenomena is focused towards applying piezoelectric-based actuators for active strain actuation because of their high bandwidth, the wealth of knowledge available on piezoelectrics and previous experience with piezoelectric-based devices.

Research using other smart materials is being conducted in the Smart Wing program in collaboration with the Defense Advanced Research Projects Agency (DARPA), the Air Force Research Laboratories (AFRL), and the Northrop Grumman Corporation. In this program, shape memory alloys, Terfenol-D, and piezoelectric actuators are being used for wing shape control for improved aerodynamic and aeroelastic performance.

The use of piezoelectric materials and other smart materials for structural vibration control using active strain actuation has been intensely studied since the early 1980’s. Active strain actuation typically refers to dynamically or statically straining Further author information: Email: a.r.mcgowan@larc.nasa.gov, Telephone: 757-864-2846, Fax: 757-864-8678, Mailing Address: Mail Stop 340, NASA Langley Research Center, Hampton, VA 2368 1-2 I99 (bending or twisting) a structure to achieve control. With a bandwidth of approximately 20 KHz, piezoelectric materials have been the materials of choice for applications requiring high bandwidth, such as aeroelasticity and acoustics. In addition, since a very thin layer of piezoelectric materials can be surface-bonded to or embedded within a structure, these materials can be nonintrusive and very effective structural controllers. For active strain actuation, in-plane actuation of patch-like piezoelectric actuators is often used. Another promising emerging technology for active strain actuation is the use of interdigitated electrode piezoelectric fibers woven into a composite laminate, The key benefits of using piezoelectric actuators for aeroelastic control versus conventional control surfaces are: increased control bandwidth, mechanical simplicity, lack of control lag, control of localized areas subject to vibratory or fatigue problems, and the nonintrusive nature of flat piezoelectric actuators.

The key drawbacks are potentially low control authority in some cases and the additional hardware required to power piezoelectrics. Also, the control authority of piezoelectric actuators is independent of aerodynamic forces, which can be a benefit or a drawback depending on the application.

NASA Langley has been actively investigating the control of aeroelastic response using advanced concepts such as smart devices for many years. Current research activities at NASA Langley are primarily motivated by the successes of the previous efforts and the many issues raised by these studies and additional research conducted by several others in the field. Previous smart fixed-wing aeroelastic research efforts conducted at NASA Langley are summarized in reference 2. Some of these efforts are briefly reviewed in the current document to highlight key issues and discuss several related follow-on research activities.

In general, the research approach is primarily focused in three key areas: 1) developing experimentally-validatedstructural and aeroservoelastic modeling techniques; 2) conducting analytical and bench experimental tests to assess feasibility and understand system trade-offs; and 3) conducting large-scale wind-tunnel tests to demonstrate system performance.

Investigations on applying smart devices to both fixed-wing vehicles and rotorcraft are presented herein. Presented first is research on developing and validating analytical methods and conducting bench tests on models where piezoelectric patch actuators are used for control. Then studies on applying patch piezoelectric actuators to suppress tail buffeting are discussed.

A summary of the DARPAI AFRL/ NASA/ Northrop Grumman Smart Wing program is given next followed by a discussion of research on a relatively new actuator called THUNDER. An overview of future research on fixed-wing aeroelasticity is also presented. Lastly, a discussion of active piezoelectric fiber composites and how they are being used in NASA Langley’s Active Twist Rotor program is presented.


2.1 Piezoelectric Aeroelastic Response Tailoring Investigation (PARTI) Much of the research at NASA Langley on the development of analytical and experimental methods for patch piezoelectric actuators was motivated by the results and “lessons learned” from a program called the Piezoelectric Aeroelastic Response Tailoring Investigation (PARTI). The PARTI program, which took place from 1991-1996, was a cooperative effort between NASA Langley and the Massachusetts Institute of Technology3. This program was an analytical and experimental study using a relatively large, multi-degree-of-freedom aeroelastic testbed. The objectives were to demonstrate the ability of strainactuated adaptive wings to control aeroelastic phenomena, including wing flutter suppression and gust load alleviation, and to develop experimental and analytical techniques. To accomplish these objectives, a wind-tunnel model was designed and fabricated, aeroservoelastic analyses were performed, and the model was ground and wind-tunnel tested in NASA Langley’s Transonic Dynamics Tunnel (TDT)4. During the program, active flutter suppression and reduced gust loads using piezoelectric actuation on a 4 foot long semi-span wing were successfully demonstrated in wind-tunnel testing. Flutter dynamic pressure was increased 12% and peak wing root bending moment due to gust was reduced by 75%.

Although the model used during the PARTI program was a plate-like model (versus a monocoque structure), the experimental results show clear evidence that piezoelectric technology may provide a viable alternative to conventional aeroelastic control techniques. As a result of this program, an extensive database of experimental information has been gathered that is instrumental in understanding the many issues associated with applying strain actuation technology to dynamic problems. Three issues were identified during the PARTI program as key areas where additional research is needed.

These issues (research areas) and the current effort to address these issues are:

Research area: The development of detailed structural and aeroelastic models is needed to promote a better I) understanding of the local and global effects of piezoelectric actuation.

Current activity: Finite element and aeroservoelastic modeling and validation research have both been initiated in the Aircraft Morphing program.

2 ) Research area: The investigation of piezoelectric power consumption characteristics during active control may enable a more realistic evaluation of the economic viability of using piezoelectric actuators on full-scale vehicles.

Current activity: Ground tests are planned to investigate the efficiency of active and passive control schemes, using a method for predicting the power consumption of piezoelectric actuators that was developed in a follow-on study to the PARTI program.

Research area: The development of improved control and optimization techniques is needed to determine how to 3) efficiently use the piezoelectric capabilities to their hllest extent.

Current activity: Thus far, two studies have used data from the PARTI wind-tunnel tests to examine how to optimally use the piezoelectric actuators to control aeroelastic response; and, further research is planned in this area via open and closed-loop ground tests.

2.2 Finite Element Modeling and Validation for Patch Piezoelectric Actuators During the PARTI program, a finite element model of the PARTI wind-tunnel model was created and the natural frequency and mode shape results were validated with ground vibration tests. Although global structural dynamic data (mode shape and natural frequency data) could generally be modeled with sufficient accuracy, local deformation and force data were very difficult to model accurately with reasonable computational efficiency. It was found that considerably small finite element mesh sizes and a judicious use of finite elements were needed to capture the abruptly changing strain field in the area immediately next to the piezoelectric actuators. Thus, the analytical model used during the PARTI program was primarily used for structural design (including actuator and sensor placement) and preliminary control assessments. For final windtunnel testing, all control laws were designed using experimental data. Although existing modeling techniques generally yield good theoretical results, they can be complicated and difficult to implement even for simple structures.

The objectives of current research at NASA Langley on finite element modeling and validation on patch piezoelectric actuators are to develop and validate simple and accurate techniques for modeling complex structures containing piezoelectric actuators using commercially-available FEM codes such as MSCMASTRAN. Finite element models of structures of increasing complexity, such as composite plates, box beams, and monocoque sections will be developed and validated. The first modeling technique to be investigated is described in reference 5. This analytical technique involves reducing a finite element model, which is typically composed of a large number of equations, to a low order modal model that includes the “Ritz” states to improve convergence of low frequency modes. These Ritz states are derived from the static deflection of the structure when piezo-generated forces are applied. Temperature-induced expansion is used to simulate voltage actuation. The first model and testbed will consist of a composite cantilever plate with piezoceramic patches surface-mounted near the root.

The design, modeling, fabrication and testing of a composite box beam will follow.

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