Adaptive Autopilot Design for the F-8 DFBW
The main difficulty in the application of control systems to the longitudinal dynamics (pitch angle) of aircraft arises from the large variation in plant parameters which occur during the course of normal flight operations. This variable nature of aircraft dynamics represented a challenge for the application of any control system, and particularly for the application of adaptive control systems. The design and experimental evaluation of an adaptive predictive automatic pilot for the pitch angle dynamic control of NASA's F-8 supersonic aircraft was carried out in 1975 using the facilities of the Charles Stark Draper Laboratory in Cambridge, Massachusetts, USA. The results obtained were excellent, since the autopilot was able to achieve a desired aircraft’s dynamic response under a wide range of flight conditions. This application is also of historical interest as it was the first real time application of adaptive predictive control systems. Chapter 7 of the book “Adaptive Predictive Control: From the Concepts to Plant Optimization” describes the autopilot implementation and its performance.
Due to stability requirements, aircraft design traditionally requires that the centre of mass be located between the aerodynamic centre and the elevator as represented in the attached figure. In this way, the torque originated by the lift force, applied in the aerodynamic centre, and the weight, applied at the centre of mass, is compensated by the torque produced by the elevator position.
The automatic pilot was designed to operate between the human pilot and the aircraft, as illustrated in the attached figure. Moving the rudder, the human pilot enters the desired set point to the automatic pilot through a simple gain, using the measurements of the aircraft's response to generate the control signal, that is, the elevator position. The objective of the automatic pilot was to allow the human pilot to execute changes in the pitch angle easily and with the desired dynamics, irrespective of flight conditions while avoiding any kind of oscillations.
