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

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Flutter suppression using active controls based on the concept of aerodynamic energy

The application of active control technology to suppress flutter was demonstrated successfully in the transonic dynamics tunnel with a delta-wing model. The model was a simplified version of a proposed supersonic transport wing design. An active flutter suppression method based on an aerodynamic energy criterion was verified by using three different control laws. The first two control laws utilized both leading-edge and trailing-edge active control surfaces, whereas the third control law required only a single trailing-edge active control surface. At a Mach number of 0.9 the experimental results demonstrated increases in the flutter dynamic pressure from 12.5 percent to 30 percent with active controls. Analytical methods were developed to predict both open-loop and closed-loop stability, and the results agreed reasonably well with the experimental results.

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Development and demonstration of a flutter-suppression system using active controls

The paper presents the design of a symmetric, active, gust load alleviation system for a regional transport aircraft, based on a static output feedback with a constrained structure. The design is carried out on a comprehensive finite state aeroservoelastic model, including sensor units and actuator transfer functions, and verified by taking into account saturated control positions, rates, and hinge moments. The controller is designed within a quadratic optimal framework, through a second-order Hessian-based optimization algorithm, exploiting block diagonal Schur transformations of the closed-loop state equations and performance weightings. An accurately chosen worst discrete gust and a reference flight condition provide a baseline design, which is significantly effective in alleviating continuous turbulence loads. Such a reference design proves itself robust enough to alleviate atmospheric loads over the complete flight envelope and is eventually further improved and robustified through a simple bilinear ...

Gust Load Alleviation for a Regional Aircraft Through a Static Output Feedback

This study develops an active control technology to reduce the incremental dynamic loads of a large four-engine transport airplane flying through a gust field The mathematical model of the proposed gust-alleviation system features composite structural motions (for example, rigid-body motions, elastic vibrations, and deflections of control surfaces) and unsteady aerodynamic forces induced by the structural motions and the gusts A clear outline of the procedure is first provided to determine the aeroservoelastic equation of the system Then, an adaptive feedforward controller that uses the preview information of the gust sensed by an onboard alpha probe is designed to operate the ailerons symmetrically to alleviate the wing-root bending moment induced by the gust The rigid-body motions due to travelling gusts are also compensated for using symmetrical deflections of the elevators To solve the problems of weight drift and weight bias that are commonly encountered in adaptive control, the circular leaky l

Gust Load Alleviation on a Large Transport Airplane

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.

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

Results and comparisons wi th theoiet ical predictions are given for a f l ight test performance evaluation of an advanced stab i l i ty augmentation system (SAS). The new SAS, developed for instal lat ion in the B-52G-H fleet, provides control of low frequency structural modes a s wel l as the conventional cont ro l of airplane r ig id body motions. Fl'ight test resul ts are presented showing the SAS performance i n terms of mode damping, fatigue damage rates, maximum expected stress, and r ide quali t y for f l ight through turbulence. Comparisons are made between theoret ical ly predicted and experimental resul ts. The f l ight test resii l ts show signi f icant reductions in dynamic iesponse to turbulence wi th the advanced stab i l i ty augmentation system. Reductions i n response of the low frequency antisymmetric structural modes and the Dutch ro l l mode were obtained with SAS. Lateral loads on the f in and aft fuselage during f l ight through turbulence were reduced by moie than 20 percent. Fatigue damage rates due to turbulence were reduced more than 50 percent for these same structural locations. The f l ight cont ro l system configuration and test procedures used to evaluate the SAS performance are presented. lntioductian An A i r Force sponsored study was conducted by The Boeing Company dui ing 1964 and 1965 to determine the changes to the E-52 f l ight control andstabil ityaugmentation systems that would provide meaningful improvements in the airplane structuial l i fe and i n aerodynamicand structural s tab i l i ty in severe turbulence. This study was conducted as a part of a continuing program to provide 5-51 f leet longevity andeffectiveness to meet Air Force requirements during the next decade. The results of the study program, available in August 1965, indicated that s igni f icant reductions in structural fatigue and peak loads could beexpected i f an advanced stab i l i ty augmentation system were instal led on the 8-52, Development of the prototype s tab i l i ty augmentation system was accomplished during 1966 and 1967. Reference 1 summar izes SAS analyses and synthesis. Structural analyses conducted and a summary of theanalyt ical resul ts obtainedare presented in Reference 2. The system selected for development included both pi tch and yaw stab i l i ty augmentation. A prototype model of the advanced SAS was designed, fabr icated, and installed on a B-52H f l ight test airplane. F l ight testing of the prototype SAS was completed in 1967 to optimize and demonstrate the SAS perfoimance in terms of reducing peak structural loads and fatigue damage rates. Theopt imizat ion was accomplished within the boundaries of adequate handling qual i t ies and dynamic s tab i l i ty of the airplane. The fol lowing sections present a general description of the f l ight control system configuration, the f l ight test approach,and results obtained. The f l ight test included f lut ter , SAS optimization, and performance test ing. Performance test ing included evaluation of handling qual i t ies (Reference 3) and dynamic response to atmospheric turbulence. General resul ts obtained during gust response test ing are described i n th is paper, inc luding comparisons to analyt ical predictions. Piototvpe SAS Configuration A general description of the ex is t ing 8-52 f leet f l ight configuration, which includes a yaw-damper, i s given i n Reference 2 along wi th study ground rules,SAS variations considered, and the SAS configuration selected for prototype f l ight test ing. The two ax is SAS consists of structural and r ig id body motion sensors, and hydraulicactuators to posi t ion the elevator andrudder. These same hydraulic actuators a lso posi t ion the control surfaces on command from the primary f l ight control system and autopi lot . The yaw SAS funct ional configuration, i l lustrated in Figure 1, ut i l izes a yaw rate gyro located at Body Station695 (wing rear spar to body inteisect ion) and a lateral accelerometer 10cated at Body Station 1719 (stabil izer rear spar to body intersection). Separate gain functions, scheduled a s a function of impact pressure, and separate f i l te rs are used for the rate gyro and accelerometer s ignals. The two channels are summed after f i l ter ing and gain scheduling operations,and thecombinedsignal drives the SAS servo which posi t ions the rudder through the hydraulic actuator. The pi tch SAS, i l lustrated in Figure 2, u t i l i zes a pi tch rate signal derived from a rate gy io located at Body Station i321 (located between the wing and empennage). T h i s pi tch rate s ignal i s shaped through a f i l ter and gain scheduled as a funct ion of impact pressure. Th is channel then drives a S A S servo which posi t ions the elevator throiigh the hvdraulic actuator. F l i gh t Test ing F l ight test ing of the prototype SAS was in i t iated i n January 1967 to evaluate performance in terms of airplane dynamic response t o turbulence. The evaluation was designed to determine dynamic load reduction for the new system. F l i gh t f lut ter test ing was necessary to establ ish s tab i l i ty boundaries prior to demonstrating the system. Test ing in smooth air was accomplished to optimize the SAS configuration. The discussion of f lutter and optimization test ing in the fo l lowing paragraphs i s generally restricted to the yaw ax is ; however, s imi lar techniques were employed for the p i tch ax is .

John B. Dempster

Papers

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

Flight test evaluation of an advanced stability augmentation system for B-52 aircraft.

Summary of flight load environmental data taken on b-52 fleet aircraft