Micro aerial vehicles (MAVs) are increasingly being used in human daily life, with multirotor designs being prevalently adopted due to their versatile flight capabilities and simple mechanical structure. However, as the connection between humans and robots becomes stronger, the requirements for multirotor are becoming more complex. Researchers are therefore working to improve their energy efficiency and maneuverability to enable them to meet the increasing demands of complex tasks in challenging environments.

This thesis proposes a passively morphing mechanism to meet rising multirotor requirements. First, a reconfigurable joint design is developed. By using a pre-stretched elastic component and stoppers, the passively reconfigurable joint can not only passively transfer between two modes but also withstand external force in both modes. This design is then applied to enhance multirotor abilities in various ways, resulting in three novel quadrotors.

Second, a passively morphing design reorients propeller thrust direction with a simple structure. Elastic element preloading passively drives morphing, and adjusting joint structural parameters tunes elastic element torque to fit multi-rotors of different sizes and applications. Combining two passive joints with a quadrotor airframe enables rolling through gaps narrower than the robot's diameter, preventing aerial collisions. Passive joints redirect propelling thrust for terrestrial operation without extra actuators, keeping the robot compact and lightweight.

Third, a new quadrotor adopts four passively reconfigurable joints to tightly couple attitude and a compact rolling cage, precisely controlling rolling/turning with improved rolling efficiency. Experiments validate hybrid locomotion and demonstrate rolling/turning setpoint control for panoramic photography. Power measurements show rolling significantly reduces the cost of transport, extending the operational range 15-fold.

Finally, passively foldable airframes are incorporated into a modular quadrotor scheme. A generalized model controls n-unit multi-rotors with different joint angles, needing no extra controllers for varied configurations. Modules can fly as normal quadrotors and use passive reconfigurable arms to perch, extending operational time. Multiple docked quadrotor modules fold the lowest module motor arms to manipulate payloads and fully actuate six degrees of freedom. Payload capacity also increases by docking more modules.

In summary, this work's contributions include (i) a passively reconfigurable joint design framework, (ii) dynamics/control strategies for quadrotors with reconfiguration utilized in various ways, and (iii) enhanced quadrotor abilities like greater operational range, gap traversing, payload maneuvering, and more.