Frogs can keep an excellent aerial balance for landing and achieve consecutive jumps reliably. A safe landing requires an accurate body righting in the air. However, there is no systematic study on how the frogs adjust the aerial postures and body attitudes after jumping. The stretched long hindlegs swung quickly in the aerial phase, which revealed a clear relationship with the body attitudes. This study aimed to verify the function of frogs' hindlegs on aero body righting in the air. We captured the motions of both hindlegs and found the hindlegs adopted two movement modes, the bilateral parallel, and separated swings. The hindleg-induced torques by the two movements were negatively correlated with the body's angular accelerations on pitch and roll, respectively. Moreover, an analytical model was derived based on the conservation of angular momentum and verified by the dynamic simulations. Thus, we confirmed that the hindlegs are the dominant mechanism in aerial pitch and roll controls.We anticipate our achievements to inspire the design of air-righting tools.

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A novel water jet thruster which is capable of conducting two sequential jumps is proposed for the amphibious jumping robot. The proposed water jet thruster adopted a unique structural design with a double-layered tank. Liquid nitrogen was injected to the tank to pressure the propellant water and generate propulsive force, which is safe and effective for both aquatic and terrestrial environment. Then a theoretical jumping model was built to evaluate the performances of two jumping actions. According to the tethered thrust measurement and free jumping experiments, the estimation from the model met well with their experimental verifications. Moreover, CFD simulation was conducted to compare the emission process of inner and outer chambers in detail. We believe the demonstration of the novel thruster would drive the innovation of future amphibious robots that capable of leaping from both aquatic and terrestrial environments.

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