## 1 Introduction
One drawback that would impact their use would be low visibility due to smoke and extremely harsh environments. Additionally, with high-rise buildings, traffic congestion, or water scenarios, these robots are not practical to deploy. Moreover, the operation range of terrestrial firefighting robots is generally limited. On the other hand, aerial vehicles, especially multi-propeller drones, can easily access these locations, so they can inject fire-extinguishing agents to suppress the flames. The world's first flying-type firefighting drones, Ehang 216-F (Ehang, Guangzhou, China) [11] and the ZHUN Walkera (Guangzhou Walkera Technology Co., Ltd., Guangzhou, China) [12], used firefighting foam and dry powder, respectively, for the task. They are specially designed for suppressing fires in high-rise buildings. However, a drone can only carry a certain amount of the firefighting agent, so it is not possible to work continuously and be ineffective in serious fire accidents. This can be overcome by continuously conveying the water from a ground hydrant or pump to the drone's nozzle via a flexible fire hose. Alternate systems such as Aerones (Riga, Latvia) [13] and Goufei Fire-fighting UAV (Goufei Aviation, Chongquing, China) [14] were designed to overcome this issue. One drawback is that the higher the drone flies, the heavier it becomes. The higher power demanded of the propeller drone and its battery's capacity constraint meant that the fire accident location affects both the flight range and the time of the drone. One other thing to consider is that the airflow generated by the propellers can also spread the flame to neighboring areas. More recently, flying-type firefighting robots that use water thrust force have been considered, especially for a fire that happens in water areas. The approach was inspired by water-powered flying devices such as flying boards [15; 16] and water jetpacks [17; 18; 19]. In [20; 21; 22; 23], the Dragon Firefighter system generates thrust by speeding up the water flow through a nozzle assembly and uses a servos actuator system to adjust the angle of nozzles, thus adjusting the force directions. This not only lifts the hose and the system, but the water jet actuator also directs the water flow to suppress the flame. Unfortunately, rotational motions are very limited with the system due to its structure. A later flying-type firefighting system in [24] uses four nozzles for maneuvering and a separated sprinkler to suppress the fire. The movement of the proposed system is carried out by changing the flowrate of each nozzle. The development of this system is still in the initial stage. The placement of the actuation system and the location of the water-conveying hose have not been finalized yet. Furthermore, these abovementioned water-powered firefighting systems are generally in a stage of development and completion. Thus, the conceptual designs and mechanical designs are investigated preferentially. The motion control methods, therefore, are fairly simple. In particular, a proportional controller with speed feedback is implemented in a Dragon Firefighter [20; 21; 22; 23]. The simulations and experiments are adopted, and it is indicated that this controller is not sophisticated and cannot control the system precisely [20]. The investigation of the latter mechanical approach helps it works better with the same controller [21; 22; 23]. However, a more advanced control technique is also initially implemented to evaluate the feasibility of water-powered flying-type firefighting [24]. A sliding mode control technique is proposed to govern the system motions. The simulation tests indicated the robustness in tracking performance. However, the control effort itself is predicted to bring a challenge for practical applications due to the controller's actions. Therefore, a flying-type firefighting robot that is water-powered, automatically controlled, and has a novel weight-shifting mechanism for maneuvering is proposed in this study.