Moti Karpel

Dynamic Response of Aeroservoelastic Systems to Gust Excitation

Frequency-domain and time-domain approaches to dynamic response analysis of aeroservoelastic systems to atmospheric gust excitations are presented. The discrete and continuous gust inputs are defined in either time-domain or stochastic terms. The various options are formulated in a way that accommodates linear control systems of the most general form. The frequency-domain approach is based on the interpolation of generalized aerodynamic force coefficient matrices and the application of Fourier transforms for time-domain solutions. The time-domain approach uses state-space formulation that requires the frequency-dependent aerodynamic coefficients to be approximated by rational functions of the Laplace variable. Once constructed, the state-space equations of motion are more suitable for time simulations and for the interaction with control design algorithms. However, there is some accuracy loss because of the rational approximation. The spiral nature in the complex plane of the gust-related aerodynamic terms is discussed, and means are provided for dealing with the associated numerical difficulties. A hybrid formulation that does not require the rational approximation of the gust coefficients is also presented for optional use in discrete gust response analysis. The various methods were utilized in the ZAERO software and applied to a generic transport aircraft model.

Reduced-Order Models for Integrated Aeroservoelastic Optimization

Recent developments of modal-based aeroservoelastic modeling techniques extended the applicability of the modal approach to almost all of the aeroservoelasticity related aspects of aircraft structural and control design. The various techniques are reviewed and combined for an integrated design optimization scheme, where stress, static - aeroelastic, closed-loop flutter, control margins, time response, and continuous gust constraints are treated with a common basic model. The structure is represented in the basic model by a set of low-frequency normal modes of a baseline design. Design changes are adequately addressed without changing the generalized coordinates. Typical difficulties of the modal approach are alleviated by various optional fictitious-mass and modal-perturbation techniques. Static modes can be added during the optimization process for better convergence to the optimal solution. Minimum-state rational approximation of the unsteady aerodynamics leads to an efficient state-space model that can be augmented by any combination of linear-contro l components. A physical weighting algorithm is used to improve the aerodynamic approximations and to select modes for truncation or residualizati on. The reduced-size models and the associated analytic sensitivities to design changes facilitate extremely efficient and adequately accurate on-line optimization sessions. LA] [A0], [A,], [A2] b

Gust loads alleviation using special control surfaces

Control laws are designed for the alleviation of dynamic gust loads on a wind-tunnel model of a transport aircraft using wing-mounted control surfaces. Three different control surfaces are used: the symmetrically actuated main ailerons, special underwing forward-positioned control surfaces at about 0.8 of the wingspan, and special wing-tip forward-positioned control surfaces. The 5.3-m-span cable-mounted wind-tunnel model was constructed at the TsAGI laboratories and is being tested as there part of the 3AS 5th Framework European Commission research project. The length of one-minus-cosine vertical gust velocity profile is tuned to yield maximal wing-root bending moment All the control laws are based on simple low-pass filters for easy and robust application in the wind tunnel. Each is based on single input of a wing-tip accelerometer, which is shown to react sufficiently before the peak of the wing-root bending moment

Structural optimization using computational aerodynamics

A recently developed methodology for aircraft structural design, based on nonlinear airloads, is extended to include a modal-based optimization option and is employed with a new computational aerodynamics code for loads analysis. Nonlinear maneuver loads are evaluated by a computational scheme that efficiently combines fluid dynamics iterations with iterations for elastic shape deformations and trim corrections. An efficient design process is obtained by performing several structural optimization runs during one maneuver load analysis, where each optimization is based on the interim nonconverged airloads. To allow for the efficient application of the method with large finite element structural models and many constraints, the discrete-coordinate optimization scheme is replaced by a modal-based optimization where a set of low-frequency vibration modes of the baseline structure is used to represent the structure throughout the optimization, both for response analysis and for sensitivity analysis. Comparative modal-based and discrete-coordinate design cases are shown to converge to the same optimal design variable values, even though they do not follow the same path. Two flow solvers are used, one of which is a newly developed Euler/Navier-Stokes computational aerodynamics code that is capable of handling complex geometries by using the Chimera overset grid method. The method avoids the problem of mesh discontinuities due to elastic shape deformations and control surface deflections because the displacements of each component affect only the components mesh. The method is demonstrated with a wing-fuselage-elevator transport aircraft model performing symmetric and antisymmetric maneuvers at Mach 0.85.

Efficient aeroelastic analysis using computational unsteady aerodynamics

A methodology for efe cient evaluation of generalized aerodynamic forces (GAFs) in transonic e ows for use in e utter analysis is presented. GAF matrices are evaluated from a reduced-order model (ROM), which comprises the generalized aerodynamic forces recorded from a time-accurate computational e uid dynamics (CFD) analysis in response to a modal step excitation in each structural mode. With the step response database, that is, the ROM, the comprehensive CFD analysis is replaced by a simple convolution scheme to compute the GAFs. The forces due to excitation of one mode at a given Mach number for all reduced frequencies can be computed from a single step response. Comparison of the GAFs computed from the ROM to those computed by direct sinusoidal excitation of the boundary conditions in a CFD run demonstrate, that for small amplitudes of excitation, the ROM is capable of predicting the unsteady aerodynamic forces very accurately. The use of ROM offers a signie cant reduction in computational time and makes the calculation of CFD-based unsteady aerodynamic forces for e utter analysis feasible. The CFD-based GAFs are used to conduct a e utter analysis of the AGARD 445.6 wing at several Mach numbers, and the results are compared to wind tunnel test results.

Robust Aeroservoelastic Design with Structural Variations and Modeling Uncertainties

An aeroservoelastic analysis procedure has been integrated with structural optimization and robust-control design tools. The multidisciplinary analysis and design process can address structural constraints, performance, and robust aeroservoelastic stability requirements simultaneously. The design process is expanded to account for structural variations and modeling uncertainties utilizing a reduced-order state-space model of the aeroelastic plant that is advantageous for efficient low-order controller design. Reduced-order modeling introduces unstructured modeling errors. Structural mass variations, such as those associated with a wide range of changeable external stores of a fighter aircraft, are also interpreted as model uncertainties. The models are expressed in a form compatible with standard robust control design procedures such as the μ synthesis and analysis technique incorporated in this study. The robust control system can then be included in an optimization process where more elaborate aeroelastic models are used for tuning selected structural variables and control gains for minimizing specific design objectives under stability, performance, and structural integrity requirements. To provide enhanced physical insight, the μ analysis procedure is supplemented by a technique for computing stability boundaries in multidimensional parametric uncertainty spaces that is based on the classical single-input/single-output gain-margin approach. The integrated design procedure is demonstrated with a realistic model of a fighter aircraft with four wing control surfaces and a wing-tip missile with variable inertial properties.

Models for Aeroservoelastic Analysis with Smart Structures

Modal-based mathematical models for the analysis of control-augmented aeroelastic systems are expanded to facilitate the use of distributed strain actuators. Smart actuators are constructed of structural elements, such as piezoelectric patches, that change their shape due to electric inputs. When embedded in the structure, the deforming smart elements introduce structural strains that change the shape of the structure and, consequently, the aerodynamic load distribution. The voltage‐strain relations, the overdetermined nature of the elastic actuator‐ structure equilibrium, the relatively large number of involved interface coordinates, and the high importance of local strains that typically limit the actuator performance require substantial modifications in the modeling process compared to that with common control-surface actuators. Fictitious masses are used in a way that causes the inclusion of important actuator strain information in the modal data with a minimal increase in the number of structural states. A control mode is defined by the static deformations due to a unit static voltage command. Huge dummy masses may be used to generate the control modes as part of a standard normal-modes analysis. State-space aeroservoelastic equations are constructed by the use of the minimum-state rational aerodynamic approximation approach. Two options are given for the introduction of control forces: a direct application of the forces, and an indirect application through the control mode. A numerical application for an unmanned aerial vehicle with a piezoelectric-driven control surface demonstrates the two options and shows that the control-mode option has some numerical advantages.

Analysis and Wind Tunnel Testing of a Piezoelectric Tab for Aeroelastic Control Applications

A concept for exploitation of a piezoelectric actuator by using aeroelastic amplification is presented. The approach is to use the actuator for excitation of a tab that occupies the rear 25 % o ...

Aeroservoelastic Gust Response Analysis for the Design of Transport Aircrafts

Frequency-domain and time-domain approaches to the calculation of dynamic loads due to response of aeroservoelastic systems to atmospheric gust excitations are presented. The discrete and continuous gust inputs are defined in either time-domain or stochastic terms. The various options are formulated in a way that accommodates linear control systems of the most general form. The frequency-domain approach is based on the interpolation of generalized aerodynamic force coefficient matrices and the application of Fourier transforms for timedomain solutions. The time-domain approach uses state-space formulation that requires the frequency-dependent aerodynamic coefficients to be approximated by rational functions of the Laplace variable. Specific difficulties associated with the spiral nature in the complex plane of the gust-related aerodynamic terms is discussed and resolved. The two approaches are adapted for efficient application in realistic design environment with numerous combinations of flight conditions and weight configurations. An actual process of calculating gust-response design loads for the A400M transport aircraft is demonstrated, showing significant controlsystem effects.

Aeroelastic Control Using Distributed Floating Flaps Activated by Piezoelectric Tabs

In this paper, a novel aeroservoelastic effector configuration that is actuated by piezoelectric tabs is presented. The effector exploits trailing-edge tabs installed on free-floating flaps (FFFs). These flaps are used to prevent flutter from occurring and to alleviate loads originating from external excitations such as gusts. A vertical tailplane wind-tunnel model with two free-floating rudders and a flutter control mechanism were designed, and the aeroelastic stability and response characteristics have been modeled numerically. The controller uses the tailplane tip acceleration as a sensor and sends control signals to the piezoelectrically actuated tabs. Wind-tunnel experiments were performed to demonstrate the feasibility of the technology. It was demonstrated experimentally that the flutter speed associated with the free rudders could be increased by 80%. The same controller, applied to the external rudder, was used to alleviate the aeroelastic response of the tailplane to the excitation of the other ...