Aerodynamic Effects for Reactive Navigation of Ducted Rotorcraft with Minimal Sensing

Abstract

Tiny multirotor vehicles critically rely on real-time onboard feedback to hover stably. While the small size and light weight are advantageous for accessing confined space, the limited payload and endurance impact their ability to carry powerful sensory and computing units for localization and mapping. This thesis reports passive and active aerodynamics-based strategies for ducted rotorcraft to avoid a wall collision and explore unknown environments using only the sensing system that is conventionally conceived minimally required for hovering. The two frameworks leverage the duct-strengthened proximity effect.

The proximity effect refers to the aerodynamic interaction of a propeller with the nearby surface, which has been investigated in the context of rotorcraft as the ground, ceiling, and wall effects. In this thesis, we focus on the wall effect of ducted propellers, where the aerodynamic interaction is influenced and strengthened by the duct. In this way, the flow interaction dramatically changes the thrust in both direction and magnitude when the ducted propeller is near the wall, enabling the rotorcraft to passively or actively detect the nearby wall.

Firstly, we focus on the aerodynamic interaction of a tilted propeller with a curved duct. The flow of an angled propeller is manipulated by the duct to strengthen its interaction with a vertical surface. The thrust noticeably changes in both direction and magnitude. Momentum theory is employed to elucidate the changes in horizontal and vertical components of the propelling thrust in the vicinity of a wall. The modeling and force measurements, when combined with the analysis of the flight dynamics, reveal an existence of a stable distance between the robot and the wall when only the attitude and altitude of the robot are controlled. Flight experiments were conducted to validate that the stable distance between the robot and the wall can be manipulated through the attitude setpoint or control gain, eliminating the need for position feedback. The outcomes enable a human operator to effortlessly fly the robot safely along a corridor without a collision in the absence of vision or other sensing instruments.

We further investigate the proximity effect of a propeller with a regular duct, when the propeller axis is vertical. Similarly, the flow interaction is substantially strengthened by the duct, tilting the thrust vector towards the nearby wall. In this way, the momentum theory-based model, when incorporated with the flight dynamics, allows the robot to robustly estimate the obstacle’s distance and direction without using a range sensor or visual measurements. To this end, we devised a flight controller and implemented reactive navigation methods for the robot to fly safely in unexplored environments. A series of flight experiments validated the detection and collision avoidance ability. The robot safely navigated staircases and evaded obstacles during proof-of-concept flights.

To sum up, we study the proximity effect of ducted propellers in two configurations. The flow interaction is influenced and strengthened by customized ducts to substantially change the thrust when the propeller is near a wall. Then the proximity effect is modeled, characterized, and incorporated into the flight dynamics for the robot to interact with a nearby wall safely.

Date of Award	5 Dec 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Pakpong CHIRARATTANANON (Supervisor)