TABLE OF CONTENTS As we discussed in Chapter 7, an aircraft must have dynamic stability characteristics that generally meet specified criteria if the pilot is to consider the handling qualities of the aircraft acceptable. In many aircraft designs, a tradeoff exists between acceptable performance and acceptable dynamic stability characteristics. For example, to meet a performance requirement, aircraft drag may be minimized by decreasing the size of the horizontal tail. This, of course, will decrease the short period damping ratio and may result in handling qualities that do not meet Level 1 requirements. Most modern aircraft incorporate feedback control systems that augment the dynamic stability characteristics of an aircraft so that excellent performance and handling qualities are achievable. An aircraft feedback control system senses key aircraft motion parameters and, through control laws (computer calculations) and control surface actuators, deflects the appropriate control surface to oppose undesired motion (for example, add damping) and/or amplify the pilot's control command. Because of these feedback control systems, most modern aircraft have excellent handling qualities and are rated by test pilots very high on the Cooper-Harper scale. This chapter introduces the fundamentals of feedback control system design needed to tailor the dynamic stability characteristics of an aircraft.
We are familiar with aircraft transfer functions and how they are used to represent the output/input for the aircraft. Transfer functions are used for a variety of applications and include dynamic behavior because they are based on the differential equations that define the system.
