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Design of longitudinal controller based on acceleration command with flight test

Ordinary autopilots take the attitude angle control system as the inner loop. On account of the complex relationship between velocity and height, especially in the stage of turning, the compensation for pitch attitude and throttle is not determined in theory. To track height and velocity accurately, acceleration control with zero steady-state error is designed as the inner closed loop of longitudinal controller in the article. The compensation of normal and longitudinal acceleration, which is calculated from attitude angles in real time, is added to the acceleration command, offsetting the error made by gravity and thrust. Normal acceleration and pitch angular rate which are measured in the body coordinate system from accelerometers and gyros respectively are used in the state variables feedback control. The methods of pole-placement and feedforward control are adopted to achieve the desired flying qualities, and to realize the altitude hold and velocity hold at the stages of turning and climb. The article makes a design of height controller and velocity controller, and then simulates on the nonlinear six-degree-of-freedom model. Analyses of results of flight tests are made in the end, which shows that this method works very well.