This paper explains the mathematical modelling and flight controller design for autonomous Vertical take-off and landing (VTOL) Tri-Tilt rotor hybrid Unmanned Aerial Vehicle (UAV). A tri Tilt rotor UAV is a combination of vertical flight performance of a helicopter and forward flight capability of an aircraft. The front two rotors are used to tilt from the horizontal position to the vertical and vice versa, and the third middle rotor is placed in the aft of centerline of fuselage with a lesser angle. UAVs can be classified into two main types, i.e., fixed-wing UAVs and VTOL UAVs. The mathematical model of the Tri Tilt rotor UAV using force and moment equations are derived for vertical take-off to horizontal flight and vice-versa using MATLAB/SIMULINK. The development of self-guided and fully autonomous UAVs would result in reducing the risk to human life. Civil applications include inspection of rescue teams, terrain, coasts, border patrol buildings, police, and pipelines. A Proportional-Derivative control approach is used to stabilize the altitude and attitude of the UAV. The results obtained from the simulation reveals that the proposed controller achieves robust stability, good adaptability and robust performance in the transition corridor.