Browsing by Author "Babl, Martin Ludwig Dietrich"

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    Stall and spin recovery using optimal trajectory planning
    (Stellenbosch : Stellenbosch University, 2021-03) Babl, Martin Ludwig Dietrich; Engelbrecht, Jacobus Adriaan Albertus; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: Aircraft pilots and autopilots would greatly benefit from a system that assists in recovering an aircraft after a severe flight upset such as a spin or stall. This system needs to perform the aerodynamic recovery that recovers the aircraft to its normal flight envelope. Once the aerodynamic recovery is completed the pilot or existing flight control system should perform the attitude, flight path angle, and airspeed recovery. Therefore, a stall and spin recovery system for aircraft using optimal trajectory planning is designed, implemented,and verified. A deep stall is a condition where an aircraft is trapped in a nose-high stall condition. While in a deep stall the aircraft’s elevator control surface cannot produce enough nose-down pitching moment to recover the aircraft from the stall. Spin is a condition where an aircraft naturally starts to rotate about the vertical axis after having stalled and follows a downward tight spiral trajectory. The NASA Generic Transport Model (GTM) is used as the basis for the design andverification of the system. The aerodynamic model of the NASA GTM simulation modelis modified to exhibit deep stall and spin behaviour. Simulations are performed to show that the modified aircraft model can be pushed into deep stall and spin, and cannot be recovered using elevator actions only. The deep stall and spin recovery task is formulated as an optimal control problem and solved using an A* and an RRT search algorithms.These algorithms find the optimal sequence of control actions and the resulting optimal state trajectory to escape from the deep stall or spin.The deep stall and spin recoveries are verified in simulation using the NASA GTM aircraft model. Simulation results show that the recovery sequences generated by the algorithms successfully perform the aerodynamic recovery. The deep stall recovery sequence first commands the rudder to yaw the horizontal tail plane out of the aircraft’s own wake to regain elevator effectiveness, and then commands the elevator to pitch the nose of the aircraft down and recover from the stall. The spin recovery sequence first commands the rudder to reduce the rolling and yawing angular rates and then commands the elevator to pitch the nose of the aircraft down. A trajectory regulator, in the form of a linear quadratic regulator(LQR), and a controls witch are implemented. The trajectory regulator provides robustness against external disturbances and model uncertainty. The control switch determines when the aircraft enter sa stall or spin condition and also when the aerodynamic envelope is recovered. The controls witch transfers the control authority of the aircraft from the deep stall or spin recovery system to the existing flight control or the pilot once the recovery is complete.General deep stall and spin recovery strategies were identified from the calculated recovery sequences. During a deep stall or spin recovery, it is possible to command the appropriate general recovery sequence and thereby nullify the calculation time required to plan the recovery sequence. When commanding the general recovery sequence the trajectory regulator regulates the state trajectories of the aircraft to track the planned general recovery state trajectories. The developed recovery system is successful in performing the aerodynamic recovery for the NASA GTM from both deep stall and spin conditions.