Bradley S. Liebst

Design of an active flutter suppression system

An active control law is synthesized for the suppression of wing flutter for a mathematical model of a flight test vehicle. Eigenvalue placement is used to synthesize a full state controller that satisfies performance specifications on control surf ace activity and that exhibits excellent gain and phase margins. A simple frequency response matching technique is used to design a realizable compensator which approximates the feedback properties of the full state controller. The performance of the control system using this compensator is evaluated at various flight conditions and found to be satisfactory. In addition, eigenvector shaping is used to enhance the gust load alleviation capabilities of the flutter control system.

Pitch Control System for Large-Scale Wind Turbines

The purpose of this analysis is to study the design of a pitching blade segment control system for the NASA-DOE MOD 0 wind turbine to alleviate some of the problems associated with shear, tower shadow, and gravity phenomena, such as shortened lifetime and noise generation. The classical linear quadratic Gaussian optimal regulator approach is used in the control formulation. A quasisteady aerodynamic analysis incorporating wind shear and tower shadow is utilized. An equivalent hinge model describes the turbine structural dynamics. The study shows that the proposed control system can provide significant vibration and noise reduction as well as a cleaner power signal, better gust response, and increased annual energy output.

ACTIVE FLUTTER SUPPRESSION USING EIGENSPACE AND LINEAR QUADRATIC DESIGN TECHNIQUES

Eigenspace (ES) and Linear Quadratic (LQ) techniques are used to design an active flutter suppression system for the DAST ARW-2 flight test vehicle. The performance of the ES and LQ controllers are very similar in meeting control surface activity specifications. The ES controller provides reduced wing root bending moment and shear but torsional stress is slightly higher than with the LQ controller. The ES controller also results in improved flutter boundaries compared with the LQ controller. The LQ controller exhibits significantly better phase margins at the flutter condition than does the ES controller but the LQ design requires large feedback gains on actuator states while the ES does not. This results in reduced overall actuator gain for the LQ design.

Design of a multivariable flutter suppression/gust load alleviation system

This paper discusses the use of eigenspace techniques for the design of an active flutter suppression/gust load alleviation system for a hypothetical research drone. One leading-edge and two trailing-edge aerodynamic control surfaces and four sensors (accelerometers) are available for each wing. Full-state control laws are designed by selecting feedback gains which place closed-loop eigenvalues and shape closed-loop eigenvectors so as to stabilize wing flutter and reduce gust loads at the wing root while yielding acceptable robustness and satisfying constraints on rms control surface activity. These controllers are realized by state estimators designed using an eigenvalue placement/eigenvector shaping technique which results in recovery of the full-state loop transfer characteristics. The resulting feedback compensators are shown to perform almost as well as the full state designs. They also exhibit acceptable performance in situations in which the failure of an actuator is simulated.

Wind Turbine Gust Load Alleviation Utilizing Curved Blades

The aeroelastic response of curved (i.e., tip sweep in planform) wind turbine blades is analyzed. Numerical results for a 3-bladed, 20-ft radius wind turbine with fiberglass blades were obtained for various tip sweeps. Tip sweeps up to 5 ft appear to provide little tip deflection and bending moment reductions in response to gusts. However, blades with much lower torsional rigidities than the fiberglass blades were shown to provide up to 279b in tip deflections and bending moment relief during gusts.

Asymptotic approximations for systems incorporating fractional derivative damping

Viscoelastic constitutive relationships incorporating fractional derivatives have been previously shown to be extremely useful in describing the frequency dependent behavior of common damping materials. However, the implementation of such models in the analysis of damped mechanical systems is somewhat complicated by the fact that polynomial equations with noninteger order exponents must be solved. This paper develops accurate approximations from which the damping factor and damped natural frequency of such systems may be obtained by evaluating relatively simple algebraic expressions.

Application of Eigenspace Techniques to Design of Aircraft Control Systems

Eigenspace techniques allow the control system designer to use feedback to place eigenvalues and shape eigenvectors so as to modify closed loop dynamic response characteristics to achieve performance objectives. In this paper the theory of eigenspace design is reviewed, extended and applied to several aircraft control problems.

Eigenspace design of an active flutter suppression system

An active control system is designed for the suppression of wing flutter in a flight test vehicle. Eigenvalue placement is used to synthesize a full state controller which satisfies performance specifications on control surface activity and which exhibits excellent gain and phase margins. The use of limited state feedback is examined; however, it is found that a simple frequency response matching technique can be used to design a realizable compensator which reproduces the feedback properties of the full state controller. The performance of the control system using this compensator is evaluated at various flight conditions and found to be satisfactory. In addition eigenvector shaping is used to enhance the gust load alleviation capabilities of the flutter control system.

Computational simulation of wing rock in three degrees-of-freedom for a generic fighter with chine-shaped forebody

Modern fighter aircraft have been associated with lateral self-excited limit cycle oscillation known as ‘wing rock’. Simulations of wing rock have been encouraged to develop a complete understanding of the fluid mechanism that triggers and drives the oscillation, as well as for prediction purposes. Previous simulations of wing rock in wind/water tunnels were almost exclusively limited to a single degree-of-freedom in roll, due to the difficulty encountered in mounting the model to freely oscillate in more than one degree-of-freedom. Numerical simulations, utilising computational fluid dynamics, were also limited to roll-only degree-of-freedom. The loss of simulation accuracy due to the reduction of the actual wing rock degrees-of-freedom to roll-only has not as yet been fully investigated. In this study wing rock is numerically simulated in three degrees-of-freedom: roll, sideslip, and vertical motion for a generic fighter model. The unsteady Euler equations are coupled with the rigid-body dynamic equations through an innovative sub-iteration algorithm to simultaneously solve the coupled equations. The effect of including the sideslip and vertical degrees-of-freedom was found to delay the onset angle-of-attack of wing rock by 5° and reduce the limit cycle amplitude by about 50% with the frequency remained almost unchanged.