Introduction to Aircraft Aeroelasticity and Loads

12

Aeroservoelasticity

The science of aeroservoelasticity (ASE) extends the aeroelastic interactions between aerodynamic forces and a flexible structure, discussed in Chapters 8, 9 and 11, to include a control system, introduced in Chapter 7. The classic Collar aeroelastic triangle can be extended to form the aeroservoelastic pyramid shown in Figure 12.1, where there are now forces resulting from the control system as well as the aerodynamic, elastic and inertial forces. ASE effects (Zimmermann, 1991; Pratt, 2000; Librescu, 2005) are becoming of increasing importance in modern aircraft design as it is usual nowadays to employ some form of flight control system (FCS) (Pratt, 2000; see also Chapter 14) to improve the handling and stability, flight performance and ride quality throughout the flight envelope, and also to reduce loads and improve service life. For commercial aircraft, the FCS might include a gust and/or manoeuvre load alleviation system in addition to a control system that meets the basic handling requirements. Modern military aircraft are often designed for carefree handling and the ability to fly with reduced, or unstable, open loop static stability so as to improve their manoeuvrability; however, they can only stay airborne through the use of the FCS. All control implementations involve the use of sensors, usually accelerometers and rate gyros placed at the aircraft centre of mass and air data sensors (e.g. angle of incidence, air speed). Some form of control input ...