A controller is designed for an aircraft to autonomously fly formation during aerial refueling. Requirements for a refueling autopilot are stated. A six-degree-offreedom model is developed for an F-16 lead aircraft and a Learjet LJ-25 wing aircraft. Bare airframe stability of both aircraft is investigated, and stability augmentation is performed. A Matlab Simulink simulation is built to reproduce the sensor inputs that will be available to the wing aircraft in flight, including disturbances. Control frames are investigated to determine the optimum presentation of the error vector for control during the task of air refueling. Control laws are developed from the initial premise of proportional-plus-integral control on position error only, and made more complex until desired performance is achieved. Tanker flight profiles are designed for the lead aircraft, and simulations are accomplished to estimate controller performance. Stability and robustness are investigated through the addition of noise, turbulence, and time delays while exploring the capability limits during increasingly aggressive profiles. Modifications for flight test are described, and flight test results are reviewed from 7 formation flights of a USAF C-12 and a Learjet LJ-25 under fully autonomous control in an operationally representative refueling environment. Actual controller performance is analyzed and compared to predictions, and suggestions are made for future controllers.
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