This paper presents an investigation of the fluid–structure interaction (FSI) effects on the stability of a quadrotor attached to a flexible hose conveying and ejecting pressurised fluid from an onboard nozzle. In this study, an analytical solution is derived to obtain the time and spatial responses of the free end, which could affect the quadrotor’s stability. First, the flow-induced force model was simulated at the hose plane to find out the contributing disturbances prior to the physical connection with the unmanned aerial vehicle (UAV). Thereafter, the flow-induced forces were introduced to the UAV dynamics model as disturbances to study the FSI response during flight. Physical experiments were conducted to compare the analytical responses of the UAV prior to and during ejection. The presented findings of the perturbations due to the FSI effect from the pressurised fluid flowing through the flexible hose to the free end and the jet reaction at the UAV nozzle will be used for the employment of a combined feedforward-feedback (FF-FB) quadrotor control strategy for a stable ejection phase. The proposed strategy shows an average improvement of 61.14% (x-axis) and 22.46% (z-axis) in terms of active position compensation during ejection as compared to a standard feedback (FB) control loop only.

In this paper, a three-dimensional hybrid kinematics-force (HKF) model for fluid force estimation coupled with position optimization of an aerial robot capable of high-pressure fluid ejection is presented. Motivated by the archerfish's unique (water-jetting) hunting mechanics, the HKF model comprises of two sub-models (hybrid kinematics and fluid force estimation) to maintain a specific fluid force at the point-of-contact (POC) during high altitude fluid jetting for high-risk maintenance (cleaning) scenarios. Industries with high-risk cleaning applications seek to employ robots to assist cleaners in routine operations on infrastructures or equipment at a height such as photovoltaic e-glasses, impact-sensitive objects, and many others. For a fluid jet, the stand-off distance represents the length of the fluid trajectory between the nozzle orifice and POC, and is also a vital variable in fluid force control.If the stand-off distance between the ejection source and the target is large, the fluid stream loses energy along the trajectory and the force at the POC is weakened thus losing some of the jet pressure at the target surface. The HKF model exploits this by estimating and maintaining a specific force at the POC through position optimization of the onboard nozzle coupled with the knowledge of the fluid parameters.To innovate high structure cleaning, the HKF model can be employed on an aerial robot with fluid ejection capabilities for accurate fluid force control at the POC. For high-pressure fluid ejection (on horizontal and vertical structures), the HKF model is able to predict the (high pressure) fluid force at the POC with less than 10% error.The proposed HKF model is tested and verified through simulations and the experimentation results are validated by flying the physical prototypes in actual deployment scenarios.

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