An analytical design technique for an active flutter-suppression and gust-alleviation control system is presented. It is based on a rational approximation of the unsteady aerodynamic loads in the entire Laplace domain, which yields matrix equations of motion with constant coefficients. Some existing rational approximation schemes are reviewed, and a new technique which yields a minimal number of augmented states for a desired accuracy is presented. The state-space aeroelastic model is used to design a constant gain, partial-feedback control system, which simultaneously assures stability and optimizes any desired combination of gust response parameters throughout the entire flight envelope.

Design for Active Flutter Suppression and Gust Alleviation Using State-Space Aeroelastic Modeling

A study is presented on the unsteady aerodynamic loads due to arbitrary motions of a thin wing and their adaptation for the calculation of response and true stability of aeroelastic modes. In an Appendix, the use of Laplace transform techniques and the generalized Theodorsen function for two-dimensional incompressible flow is reviewed. New applications of the same approach are shown also to yield airloads valid for quite general small motions. Numerical results are given for the two-dimensional supersonic case. Previously proposed approximate methods, starting from simple harmonic unsteady theory, are evaluated by comparison with exact results obtained by the present approach. The Laplace inversion integral is employed to separate the loads into 'rational' and 'nonrational' parts, of which only the former are involved in aeroelastic stability of the wing. Among other suggestions for further work, it is explained how existing aerodynamic computer programs may be adapted in a fairly straightforward fashion to deal with arbitrary transients.

Unsteady aerodynamic modeling for arbitrary motions

The primary objective of this paper is to demonstrate that the field of aeroelasticity continues to play a critical role in the design of modern aerospace vehicles, and several important problems are still far from being well understood. Furthermore, the emergence of new technologies, such as the use of adaptive materials (sometimes denoted as smart structures technology), providing new actuator and sensor capabilities, has invigorated aeroelasticity, and generated a host of new and challenging research topics that can have a major impact on the design of a new generation of aerospace vehicles.

Renaissance of Aeroelasticity and Its Future

A modeling method for constructing a state-space aeroservoelastic mathematical model for time-domain analysis with a low number of aerodynamic lag states is presented. The modeling method employs the minimumstate method for rational approximation of tabulated unsteady aerodynamic force coefficients at various reduced-frequency values. The approximation method is modified to deal with weighted aerodynamic data and with alternative constraint combinations. Two weighting types are analyzed and discussed. The first weighting is normalizing the aerodynamic data to maximum unit value of each aerodynamic coefficient. The second weighting is one in which each tabulated coefficient, at each reduced-frequency value, is weighted according to the effect of an incremental error of this coefficient on aeroelastic characteristics of the system. This weighting yields a better fit of the more important terms at the expense of less important ones. The analytical developments are presented and numerical examples, which demonstrate various features of this method, are shown to yield significant reduction in model size per given accuracy relative to other rational approximation methods.

Time-domain aeroservoelastic modeling using weighted unsteady aerodynamic forces

Active flutter suppression, which is a part of the group of flight vehicle technologies known as active controls, is an important contributor to the effective solution of aeroelastic instability pr...

https://arc.aiaa.org/doi/pdf/10.2514/1.C034442

Aircraft Active Flutter Suppression: State of the Art and Technology Maturation Needs

The first flight test demonstration of active flutter suppression has been successfully completed. The Control Configured Vehicles (CCV) B-52 test airplane was twice flown 10 knots faster than its flutter speed relying solely on an automatic control system for adequate damping. The design, safety considerations, mechanization, ground testing, and flight testing of the flutter mode control system are reported. Comparisons between flight test and theoretical results are presented. The system was tested at heavy and light airplane weights and tested for compatibility with simultaneous ride control, maneuver load control, fatigue reduction, and augmented stability.

Active Flutter Suppression—A Flight Test Demonstration

Active flutter suppression - A flight test demonstration

A theoretical evaluation of the structural performance of benefits to be expected after installing a state-of-the-art stability augmentation system (SAS) on the B-52 is presented. The new system is expected to provide additional stability in the airplane rigid body motions and the low-frequency structural vibration modes. The mathematical model used in this study included rigid body motions, a large number of normal vibration modes, control surface rotations, and the SAS transfer functions. The dynamic response of the airplane to random atmospheric turbulence was determined using harmonic analysis techniques with provisions made to include the effects of nonlinearities due to SAS saturation. Quantitative estimates of the changes in airplane structural response indicate that fatigue-damage accumulation rates and peak-load amplitudes, due to atmospheric turbulence, can be significantly reduced without degrading the flutter margin.

Kenneth L. Roger

Papers

Active Flutter Suppression—A Flight Test Demonstration

Active flutter suppression - A flight test demonstration

Evaluation of B-52 structural response to random turbulence with stability augmentation systems.

Gust design envelope of interacting loads.

Nonplanar doublet lattices