Derivation of S-domain aircraft models

Today, the design of modern control systems is greatly facilitated by the use of computer based design and analysis tools. More specifically, the design of aircraft light controls has benefited immensely from these tools. The ability to simulate control laws in a virtual environment allows for the design of complex algorithms whose functionality and performance may be evaluated during the design phase of a project rather than during the flight testphase. Besides improving the control engineer's ability to create a superior design, these tools reduce cost by uncovering design flaws and performance deficiencies before the flight test program commences. Additionally, the transition from analog to digital control law implementation in flight control systems have made simulation on a digital computer more important. Before one can begin the analysis of a set of control laws, an accurate representation of the plant being controlled must be derived. In the case where one wishes to evaluate an autopilot the plant is the aircraft being controlled. Therefore, the dynamics of the airframe must be modeled. This paper presents a framework for the derivation of a Laplace (s-domain) representation of an airframe based on raw aerodynamic data from both wind tunnel testing and actual flight tests. This representation can then be implemented using a dynamic systems modeling tool such as MATRIXx. This process has been carried out successfully on aircraft such as the C-141B and A-4D at AlliedSignal Aerospace Guidance and Control Systems by the author. Once this model has been generated, it is used to design and analyze the control laws targeted for that airframe.