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Meeting	ACGS Committee Meeting 110 - Auburn, Maine - October 2012
Agenda Location	8 SUBCOMMITTEE A – AERONAUTIC AND SURFACE VEHICLES 8.3 F-111 Adaptive Control System
Title	F-111 Adaptive Control System
Presenter	Carl Droste
Affiliation	Lockheed Martin (retired)
Available Downloads*	presentation
	Downloads are available to members who are logged in and either Active* or attended this meeting.
Abstract	This presentation provides an overview of the F111 adaptive flight control system along with a brief review of the methods and tools available for control system design in the 1960 time period. The presentation first discusses the mechanical and electrical implementation of the pitch, roll, and yaw flight control systems on the F-111. Building on this baseline, the presentation discusses the use of model following concepts in the adaptive system design. The F-111 design incorporated an “adaptive mode” that provided a target root (model) for the pitch and roll closed loop responses. To approach the target dynamic response, it was necessary to raise the loop gain to the highest level possible. To accomplish this goal, an adaptive root was developed in the closed response. The system gain was then increased until the adaptive root had a .3 damping ratio. The analog hardware was designed to detect the adaptive frequency using a very complex analog band-pass filter. When the oscillating root was detected, the damping was measured by counting the number of zero crossings. The system then adjusted the loop gain as necessary to achieve the .3 damping. Real world problems forced a small, (“subliminal”) pulse into the system at regular intervals to guarantee excitement of the adaptive mode in calm air. To help the crossing detector function in air turbulence, a variable threshold was applied to the zero crossing detectors. This presentation goes on to discuss operational problems, implementation of the adaptive gain changer in the “digital” flight control computer, and other lessons learned. The F-111 flight control system is probably the only adaptive system to ever go into production, and it flew for 47 years.