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Mathematical model and autopilot design of a twin engine jet plane

Atak, Hazal Cansu
Fighter aircrafts have very important place in military and aerospace industry. Lots of types of these aircrafts are used according to their primary missions like providing fire power from high above the ground and support for ground forces. Selecting the number of engines and their type are dependent to these missions. Turbojet, turbo prop and ramjet engines are most preferred types for these applications, and each has its own advantages and disadvantages. Turbojet engines have high performance as a means of propulsion and aircraft speed. Due to their small size and relatively small weights, it became convenient to use two turbojet engines. In this thesis, mathematical model and autopilot design of a jet plane with two turbojet engines is studied and simulations are done by using MATLAB / Simulink. First, a mathematical model of a turbojet engine is developed with Mach number and throttle setting as inputs, thrust and mass fuel flow rate as outputs. Next a jet aircraft model with relatively larger load capacity (due to having larger wingspan), with respect to the similar planes in use like F-18 (hornet) and F-22 (raptor) has been designed and analyzed by XFLR5 program. At this stage, placement of turbojet engines is done in such a way that sufficient yaw moment is created on the plane when their thrusts differ. Then, a mathematical model of the plane is constructed with the base of aerodynamics block that is feed with the aerodynamic coefficients coming from XFLR5, equations of motion block and turbojet engine blocks. In this flight dynamic model, engines are controlled independently to perform yaw moment. Elevator, rudder, aileron and two throttles for each engine are considered as the control parameters of the flight dynamic model and an autopilot is designed by using suitable cascaded PID controllers. Different modes of the autopilot and guidance are also discussed as part of our study. This work ends with simulation studies which are expected to show the importance of the approach presented here.