Introduction to Aircraft Aeroelasticity and Loads

7

Introduction to Control

Control systems are used in a wide range of engineering applications and industries, to enable a system (e.g. an aircraft) to respond in some desired way when subjected to some form of external input. For example, a gust load alleviation system might use acceleration measurements on the aircraft to detect the motion due to turbulence and then employ the control surfaces in such a way as to reduce the loads acting on the aircraft structure. Other inputs could be provided by the pilot and the control system could limit the loads generated. The systems used on aircraft for control in flight may be electrical, mechanical, hydraulic or pneumatic and perform a widely differing range of tasks, e.g. provision of required stability and handling properties, carefree handling, manoeuvre load alleviation, etc. Modern aircraft have extremely sophisticated flight control systems (Pratt, 2000) that play an important role in the aeroelastic and loads behaviour of the aircraft, so it is important to understand key issues of control.

This chapter will examine some of the basic control tools and definitions that need to be understood before the application of control systems to aeroelastic systems, the science of aeroservoelasticity (or structural coupling), which will be tackled in Chapter 12. The aircraft flight control system will be considered further in Chapters 14 and 22. Many textbooks cover basic control theory, e.g. Raven (1994) and Dorf and Bishop (2004).

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