Application of Modern Synthesis to Aircraft Control: Three Case Studies

The role of feedback control in the solution of aircraft stability and control problems is discussed. It is argued that this role is becoming more and more important and is a key to meeting performance objectives for new aircraft. .4s a consequence, the control engineer must develop control laws for applications with many, sometimes conflicting, control objectives and stringent safety requirements. In the past, predominantly classical sy-nthesis techniques have been used in industry to develop control laws for aircraft. However, the so-called modern synthesis techniques that are claimed to improve quality and reduce development cost are having increased practical use in industry. Modern synthesis techniques that offer significant promise of practical applications are discussed briefly, and three case studies of their application to aircraft control problems are presented. The first example involves the redesign of an autopilot control law to improve stability and reduce sensitivity to plant parameter variations. A much improved control law was developed, flight tested, and implemented in the autopilot of the Boeing 767 commercial transport airplane. The second and third examples address the development of control laws for aircraft that rely extensively on feedback control to furnish satisfactory stability and control characteristics. These two applications are typical of the next generation of transport aircraft that will rely extensively on feedback control to improve fuel efficiency. The control laws gave the airplane flight characteristics that are superior to those of current airplanes. The solutions presented could have been obtained using classical synthesis techniques. However, the modern approach reduced the number of design iterations required and appeared to produce better control laws for a given level of practical experience of the control engineer. In our opinion, this approach to control law synthesis will play an increasingly important role in control design for present and future aircraft. Implemented in a user-friendly engineering workstation environment, these techniques offer improvement in quality and reduction in development cost, and for some applications, particularly to future highperformance aircraft. only the modern multiloop synthesis techniques will offer practical and cost-effective solutions.