This paper presents asymptotic properties of Linear Quadratic Gaussian with Loop Transfer Recovery controllers as applied to the design of flight systems. In many flight control problems output feedback design methods are needed when the states are not available for feedback. Optimal control based designs have proven to be useful in practice due to their robustness and performance. Their implementation using output feedback through a full order observer or Kalman filter can degrade stability robustness properties of these controllers. Recently, a new method of Loop Transfer Recovery was published that offers different asymptotic properties as compared to conventional Loop Transfer Recovery. This method asymptotically achieves positive real system behavior at certain loop break points for minimum phase systems, as well as for non-minimum phase plants where the number of output measurements exceeds the number of controls. This paper compares these approaches and highlights the design features that engineers can use to tune and achieve excellent robustness and performance in output feedback flight control systems. A design example using the X-45A Joint Unmanned Combat Air System pitch-plane dynamics is utilized to illustrate the control designs.
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