Design, modeling and control of a lightweight 3D printed fixed wing VTOL UAV

• Main Author: Govdeli, Yunus
• Other Authors: Basman Elhadidi
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Aeronautical engineering
• Online Access: https://hdl.handle.net/10356/143610
• Institution: Nanyang Technological University

This thesis covers topics in additive manufacturing (AM), dynamic modeling and control of fixed-wing vertical take-off and landing unmanned aerial vehicles (VTOL UAVs). For AM, a guideline for AM of aircraft wings is developed. Two design methodologies were considered, namely a conventional design using ribs-spar-skin and an unconventional design using skin-periodic cellular structures. For a conventional rib-spar structure, the effects of skin thickness, number of ribs, additive building as well as printing orientations on the wing strength and weight are investigated. Among the obtained wing structures, the best performing wing configuration, i.e., a combination of number of ribs, skin thickness, printing and building orientations, is selected based on weight and strength. This selected structure is manufactured and tested. To exploit the design freedom provided by AM, periodic cellular structures were investigated. The bending stiffness was experimentally tested for diamond honeycomb and 3D-Kagome structures with various infill rates and truss diameters. The experimental results favored honeycomb structures and hence a second wing model was manufactured and compared to the conventional manufacturing model. The benefit of periodic cellular structures achievable by AM was the reduction of weight of 35%. For mathematical modeling of fixed-wing VTOL UAV dynamics, the aerodynamic forces are modeled using two techniques. In the first technique, a tilt-rotor flying wing is modeled using the quasi-steady approximation where the aerodynamic forces are calculated from DATCOM and CFD data. This implies that a linearization technique is implemented for the controller via gain scheduling. Considering the different characteristics of the vehicle with increasing rotor tilting angles, the transition flight between the level flight and hovering phases is linearized at 10° intervals. For each linear model, linear controllers are designed by minimizing a cost function based on attitude and position error. To enhance the model further, a thrust model was implemented to account for the propeller wake over the flying wing surfaces. In the second technique, an unsteady aerodynamic model is presented for two quadplane models, a tilt-rotor and a pusher, to account for the unsteady forces generated by acceleration of the vehicle. In this model, a gain scheduling is not needed as the controller is applied to the nonlinear dynamics of the vehicle. It is observed that the unsteady model is able to represent the transient forces at the start and end of transition flight as well as in the cases of disturbances. The performance of the linear controllers designed is compared with a type-2 fuzzy neural network (T2FNN) based controller for level flight under disturbances, where the T2FNN based controller learns the effects of disturbances over time and reduces the tracking error. First, this is implemented for the flying wing vehicle configuration. To reduce the oscillations while switching from one controller to the other during transition flight, the tilting rate is manipulated by a fuzzy switching algorithm. When compared to a fixed tilting rate, the fuzzy algorithm reduces the total tracking error by 45.2%. Second, the T2FNN based controller is implemented for the quadplane configuration. Under disturbances, the altitude change during transition flight is reduced by 34.16% when compared to the PID controlled case. Finally, for the sake of completeness, full flight envelope control results for both quadplane configurations are presented. The results indicate that the altitude hold during transition is more challenging for the tilt-rotor configuration compared to the pusher configuration.