For highly sophisticated, subsonically unstable fighter airplanes flying supersonically, a joint strategy to lay out the flight control system restricting design loads must be adopted. Such a strategy does not sacrifice performance since the majority of performance-critical design cases are not load critical. A carefree maneuvering control system limits design parameters (accelerations, acceleration rates, velocities, attitudes) in such a way that design loads are not exceeded. The design loads are defined in an interdisciplinary approach between flight control systems designers, loads engineers, and aeroelasticians. To minimize wave drag supersonically requires low thickness-to-chord ratios on surfaces and, hence, gives considerable static aeroelastic effects at high dynamic pressures. Control surface geometry cannot be selected using rigid aerodynamic derivatives as in the past, since this will lead to high structural mass penalties as well as too large hydraulic actuators requiring increased power supply. Therefore, this geometry must be found by applying modern structural optimization algorithms that deliver the exchange rate between structural weight penalties and the required hydraulic power, which is a function of control surface efficiency. The higher the actuator area is to produce high forces at load critical cases, the higher is the volume at low dynamic pressures where large deflections with high rates of all control surface are required. This gives the design case for the hydraulic pump and engine power takeoff. The interdisciplinary design method as applied to a modern fighter aircraft is described in this paper.
Extensive research programs have been conducted at Messerschmitt-Bolkow-Blohm (MBB) to investigate the application of active flutter and mode control to achieve increased flutter margins. Such techniques are of special interest for airplanes that already have a full command and stability augmentation system together with fast responding control surface actuators and that carry heavy wing mounted stores. A flutter suppression system (FSS) was installed on the F-4F, and this system was flight tested. The control law was found by applying optimal control theory, thus minimizing the control surface motion due to disturbances and providing the required stability margins. During the test it was found that the dynamic properties of the wing-pylon-store system change considerably with vibration amplitude because of play and preload.
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