Piergiovanni Marzocca

Nonlinear Adaptive Control of an Aeroelastic Two-Dimensional Lifting Surface

Adaptive control of a nonlinear two-dimensional wing-flap system operating in an incompressible flowfield is studied. An output feedback control law is implemented, and its performance toward suppressing flutter and limit cycle oscillations, as well as reducing the vibrational level in the subcritical flight speed range, is demonstrated. The control law proposed here is applicable to minimum phase systems, and conditions for stability of the zero dynamics are provided. The control objective is to design a control strategy to drive the pitch angle to a setpoint while adaptively compensating for uncertainties in all of the aeroelastic model parameters. It is shown that all of the states of the closed-loop system are asymptotically stable. Furthermore, an extension is presented to include flap actuator dynamics. Simulations have been presented to validate the efficacy of the proposed strategy. Pertinent conclusions have been outlined.

Supersonic/Hypersonic Flutter and Postflutter of Geometrically Imperfect Circular Cylindrical Panels

A theoretical investigation of the flutter and postflutter of infinitely long thin-walled circular cylindrical panels in a supersonic/hypersonic flowfield is presented. In this context, third-order piston theory and shockwave aerodynamics are used in conjunction with the geometrically nonlinear shell theory to obtain the pertinent aeroelastic governing equations. The effects of in-plane edge restraints and small initial geometric imperfections are also considered in the model. The objective is twofold: 1) to analyze the implications of nonlinear unsteady aerodynamics and structural nonlinearities on the character of the flutter instability boundary and 2) to outline the effects played, in the same respect, by a number of important geometrical, physical, and aerodynamic parameters characterizing the aeroelastic system. As a by-product of this analysis, the implications of these parameters on the linearized flutter instability behavior of the system are captured and emphasized. The behavior of the aeroelastic system in the vicinity of the flutter boundary is studied via the use of an encompassing methodology based on the Lyapunov first quantity. Numerical illustrations, supplying pertinent information on the implications of geometric and aerodynamic nonlinearities, as well as of other effects, such as curvature and thickness ratios, on the flutter instability and on the character of the flutter boundary are examined, and pertinent conclusions are outlined.

Flutter, Postflutter, and Control of a Supersonic Wing Section

A number of issues related to the flutter and postflutter of two-dimensional supersonic lifting surfaces are addressed. Among them there are the 1) investigation of the implications of the nonlinear unsteady aerodynamics and structural nonlinearities on the stable/unstable character of the limit cycle and 2) study of the implications of the incorporation of a control capability on both the flutter boundary and the postflutter behavior. To this end, a powerful methodology based on the Lyapunov first quantity is implemented. Such a treatment of the problem enables one to get a better understanding of the various factors involved in the nonlinear aeroelastic problem, including the stable and unstable limit cycle. In addition, it constitutes a first step toward a more general investigation of nonlinear aeroelastic phenomena of three-dimensional lifting surfaces.

Aeroelastic instability and response of advanced aircraft wings at subsonic flight speeds

A unified aeroelastic model developed towards investigating the flutter instability and subcritical aeroelastic response to a sharp-edged gust load in the compressible subsonic flight speed range is presented. The aircraft wing is modeled as an anisotropic composite thin-walled beam featuring circumferentially asymmetric stiffness lay-up which generates preferred elastic couplings. A number of non-classical effects such as transverse shear, warping restraint, and the 3-D strain effects are incorporated in the structural model. The unsteady aerodynamic loads in subsonic flow are based on 2-D indicial functions in conjunction with aerodynamic strip theory extended to a 3-D wing model. The numerical results reveal that elastic tailoring and warping restraint play a significant role on the flutter instability and dynamic response of composite aircraft wings.

Model-Free Control Design for Multi-Input Multi-Output Aeroelastic System Subject to External Disturbance

In this paper, a class of aeroelastic systems with an unmodeled nonlinearity and external disturbance is considered. By using leading- and trailing-edge control surface actuations, a full-state feedforward/feedback controller is designed to suppress the aeroelastic vibrations of a nonlinear wing section subject to external disturbance. The fullstate feedback control yields a uniformly ultimately bounded result for two-axis vibration suppression. With the restriction that only pitching and plunging displacements are measurable while their rates are not, a high-gain observer is used to modify the full-state feedback control design to an output feedback design. Simulation results demonstrate the efficacy of the multi-input multi-output control toward suppressing aeroelastic vibration and limit cycle oscillations occurring in pre and postflutter velocity regimes when the system is subjected to a variety of external disturbance signals. Comparisons are drawn with a previously designed adaptive multi-input multi-output controller.

Nonlinear Open-/Closed-Loop Aeroelastic Analysis of Airfoils via Volterra Series

Determination of the subcritical aeroelastic response to arbitrary time-dependent external excitation and determination of the flutter instability of open/closed-loop two-dimensional nonlinear airfoils constitute the main topics. To address these problems, Volterra series and indicial aerodynamic functions are used, and, in the same context, the pertinent aeroelastic nonlinear kernels are determined. Flutter instability predictions obtained within this approach compared with their counterparts generated via the frequency eigenvalue analysis and via experiments reveal excellent agreements. Implications of a number of important parameters characterizing the lifting surface and control law on the aeroelastic response/flutter are discussed, and pertinent conclusions are outlined.

Aeroelastic Response of Nonlinear Wing Sections Using a Functional Series Technique

This paper addresses the problem of the determination of the subcritical aeroelastic response and flutter instability of nonlinear two-dimensional lifting surfaces in an incompressible flow-field via indicial functions and Volterra series approach. The related aeroelastic governing equations are based upon the inclusion of structural and damping nonlinearities in plunging and pitching, of the linear unsteady aerodynamics and consideration of an arbitrary time-dependent external pressure pulse. Unsteady aeroelastic nonlinear kernels are determined, and based on these, frequency and time histories of the subcritical aeroelastic response are obtained, and in this context the influence of the considered nonlinearities is emphasized. Conclusions and results displaying the implications of the considered effects are supplied.

Aeroelastic response of a 2-D airfoil in a compressible flow field and exposed to blast loading

This paper deals with the generation and use of proper aerodynamic indicial functions toward the aeroelastic formulation of two-dimensional lifting surfaces in the subsonic compressible, linearized transonic, supersonic and hypersonic flight speed regimes. The indicial function approach enables one to treat in an unified way (i.e. in the time and frequency domains) the subcritical aeroelastic response and the flutter instability of lifting surfaces. Validations of the aerodynamic model are documented and excellent agreements are reported. In addition, closed form solutions and aerodynamic derivatives for different flight speed regimes are obtained; comparisons, and results displaying the aeroelastic response to blast loads are presented, and pertinent conclusions are outlined.

Linear/Nonlinear Supersonic Panel Flutter in a High-Temperature Field

An analysis of the flutter and postflutter behavior of infinitely long flat panels in a supersonic/hypersonic flowfield exposed to a high-temperature field is presented. In the approach to the problem, the thermal degradation of thermoelastic characteristics of the material is considered. A third-order piston theory aerodynamic model in conjunction with the von Karman nonlinear plate theory is used to obtain the pertinent aerothermoelastic governing equations. The implications of temperature, thermal degradation, and of structural and aerodynamic nonlinearities on the character of the flutter instability boundary are analyzed. As a byproduct, the implications of the temperature on the linearized flutter instability of the system are discussed. The behavior of the structural system in the vicinity of the flutter boundary is studied via the use of an encompassing methodology based on the Lyapunov First Quantity. Numerical illustrations, supplying pertinent information on the implications of the temperature field and of the thermal degradation are presented, and pertinent conclusions are outlined.

Aeroelasticity of Time-Delayed Feedback Control of Two-Dimensional Supersonic Lifting Surfaces

Determination of the nature of the critical flutter boundary (benign/catastrophic) and its control constitute important issues that can be addressed within the nonlinear formulation of lifting surface theory. The main attention of this paper consists in the development of a computational approach enabling one to get a better understanding on time-delayed dynamics as applied to this important aeroelastic problem, and more specifically, to two-dimensional supersonic lifting surfaces. The analysis is based on the reduction of the infinite-dimensional problem to one described on a two-dimensional center manifold. Results presenting the implication of the linear/nonlinear timedelayed feedback control on two-dimensional supersonic lifting surfaces are addressed, and pertinent conclusions are drawn.