This study focuses on the analysis of the ground effect in counter-rotating coaxial rotors. To investigate the aerodynamic performance of a coaxial rotor system, the aerodynamic loading is measured for different rotor vertical spacing, rotational speed, and height above the ground. To link aerodynamic loading with flow topology, velocity fields in the rotor slipstreams are measured with particle image velocimetry (PIV). A semi-empirical model is additionally proposed to complement existing ground-effect theories from the literature by accounting for the effects of rotor spacing, ground proximity, and rotor-to-rotor aerodynamic interactions. The results of the performance analysis show that the ground effect is more pronounced in coaxial configurations than in single rotors, especially at minimum spacing and height, where the thrust increases about twice the corresponding value of single rotors. The analysis of the PIV velocity fields reveals how the inflow to the bottom rotor accelerates the downstream flow, increasing the flow rate and further reducing the induced velocity near the ground. As the rotor spacing increases, these interactions weaken, causing the aerodynamic loading to converge to that of a single rotor at a spacing around 90 % of the rotor radius. The proposed model inspired by experimental data provides a robust framework for predicting coaxial rotor performance near the ground. It also allows integration of the ground effect model into UAV control strategies for improved flight stability and safety.

Flapping wing Micro Air Vehicles (FWMAVs) hold great potential for real-world applications but are currently still hard to model. In this article, a simplified analysis of the equilibrium state of a tailless FWMAV in forward flight is presented. The definition of the equilibrium state complements previous dynamic and stability analysis, adding new information about the flight behavior of FWMAVs. A new aerodynamic decoupled model has been used for the analysis, considering separately the thrust force generated by the flapping movement and the lift and drag caused by the forward velocity. The aerodynamic forces are included in a dynamic model of the FWMAV, and the equilibrium state is derived. The formulation obtained is explicit in terms of the pitch actuator deflection, thus allowing its use for control corrections, and provides an estimation of the flight velocity. The thrust needed to maintain height is also formulated, demonstrating that forward flight is more efficient than hovering. The results are validated experimentally for the pitch angle, showing good agreement with the analytical results. Then, the dynamics of the FWMAV are simulated, comparing the results with experiments where the FWMAV goes from hovering to a specific pitch reference while maintaining its height. Additional simulations are performed with basic control considerations, showing how considering the equilibrium state for a feed-forward control significantly improves the flight behavior compared to PI and PID controllers, reducing the convergence time.

This paper analyses the methods and technologies involved in flapping-wing flying robots (FWFRs), where the actuation of the flapping wing produces thrust and lift force that mimics birds’ and insects’ flight. The focus is on the evolution of the flapping-wing technology and the challenges in prototyping, modeling, navigation, and control. The mechanism for flapping production, frequency control of the flapping, and wing/tail control for positioning the robot are important topics for successful prototyping. The article includes the study of the dynamics and aerodynamics of the FWFR. Using the combination of flapping and gliding has led researchers to seek more energy savings through this hybrid-in-nature dynamic system, which benefits from the wind, a natural and free energy source. The paper reviews the dynamics, design, and categorization of flapping-wing systems; it also includes control and onboard intelligent functionalities, particularly environment perception for positioning and guidance, as well as obstacle detection and avoidance.

On-site inspection of large-scale infrastructure often involves high risks for the operators and high insurance costs. Despite several safety measures already in place to avoid accidents, an increasing concern has brought the need to remotely monitor hard-to-reach locations, for which the use of aerial robots able to interact with the environment has arisen. In this paper a novel approach to aerial manipulation is presented, where a compact manipulator with a single degree-of-freedom is tailored for the placement and retrieval of sensors in the environment. The proposed design integrates on-board sensing, a high-performance force controller on the manipulator, and a thrust-to-force mapping on the flight controller. Experimental results demonstrate the high reliability achieved during both placement and retrieval tasks on flat surfaces (e.g., a bridge wall) and cylindrical surfaces (e.g., tree trunks). A total number of 89 flight experiments were carried out to demonstrate the robustness and potential of the compact, bespoke aerial design.

A wide range of applications for which unmanned aerial vehicles (UAVs) are ideally suited rely on the development of manipulators capable of exchanging forces with the environment. One such application is the installation and retrieval of intelligent sensors for monitoring wide-spread areas and locations that are difficult to access by any other means. Within this letter, we report on both indoor and outdoor flights tests of a novel force controlled lightweight compliant manipulator that allows a UAV to carry out this type of task. Installation and retrieval are both demonstrated with different scenarios, indoors and outdoors. Key results include interaction forces up to 22 N exerted by a small-sized multirotor, placement and retrieval operations carried out on flat as well as cylindrical surfaces, and an analysis of the overall system. The results of multiple flight experiments clearly demonstrate the potential of this approach for the deployment of sensors and other force-related tasks.