System identification for longitudinal derivatives for a VTOL UAV

• Main Author: Liang, QiYun
• Other Authors: Basman Elhadidi
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Aeronautical engineering::Aerodynamics; Engineering::Aeronautical engineering::Aircraft
• Online Access: https://hdl.handle.net/10356/142408
• Institution: Nanyang Technological University

VTOL UAVs have the best of both worlds – good endurance as well as versatility in launching, manoeuvring, and recovering. The VTOL UAV involved in this project features an unconventional aerodynamic design and relies on its aerodynamic elements to generate lift in Forward-Flight Mode. This makes it imperative to identify its aerodynamic derivatives, which relate to its flight and control response characteristics. Doing so will enable accomplishment of a myriad of other tasks, such as designing an Augmented Control System or an Autopilot. This project aims to measure and identify the longitudinal aerodynamic derivatives of a prototype VTOL UAV system. Theoretical and experimental components were involved, utilising a priori theoretical knowledge in experimentally measuring the derivatives. A Dynamic Wind-Tunnel Experimental Methodology was developed for the purpose of this project and used to gather flight data for System Identification. A 50% scaled Test Model was fabricated and mounted on a Rig Structure in a wind-tunnel, complete with a control system and an Attitude and Heading Reference System (AHRS). The Rig was constrained to allow 1 Degree-of-Freedom (DoF) of the Model – the Model was able to rotate freely on its pitch axis. Control surface actuation was used to “excite” and initiate the test manoeuvres and the Model’s dynamic response (Pitch and Pitch Rate) was recorded with the AHRS. With the defined input and the recorded dynamic response output, Input-Output Transfer Function (TF) Models were estimated for each run using System Identification software and an average TF Model was acquired. From the TF model, the damping ratio and undamped natural frequency, as well as the longitudinal aerodynamic derivatives (M_α and M_q) were calculated and obtained, accomplishing the project’s objective. The aerodynamic derivatives were also converted to dimensionless coefficients for upscaling purposes. These will aid further development of the VTOL UAV, such as the design of the UAV’s Control Systems, an Autopilot system, etc. Recommendations were also made, such as the additional study of servo dynamics, incorporation of better System Identification software, usage of better Controller hardware as well as potential developments to upscale this project.