Review of hybrid aquatic-aerial vehicle (HAAV): Classifications, current status, applications, challenges and technology perspectives

In recent years, there has been a growing need for vehicles with air-water amphibious capacities for both military and civil applications, and the technological advances in unmanned systems make it more feasible to create such vehicles. As a result, the hybrid aquatic-aerial vehicle (HAAV), which integrates the locomotion capacity of aerial vehicles and underwater vehicles, is facing a vast developing opportunity, and plenty of related technologies have emerged. However, no current HAAV has simultaneously achieved the operational capacities of UUVs and UAVs in their moving domain without sacrificing performance. The exploitation of HAAVs is still facing several significant challenges, such as insufficient water-air compatibility, inefficient water-entry locomotion, and inefficient water-exit locomotion. The development trend of HAAVs could be more transparent, with the proposed technologies and design concepts being sorted out clearly. Therefore, a literature review of this novel vehicle is urgently in demand. We build this review on our previous survey published in 2015, which is regarded as the earliest review on HAAVs. In this paper, we put forward a novel systematic classification method of the fully functional HAAVs by considering the structure configurations and introduce the prototype development of each kind. Then, the research statuses of the drive solutions and trans-media solutions are analyzed respectively. After that, we analyze the demands of HAAVs both in the military and civil applications and evaluate the device performances of the various HAAVs. Furthermore, the applicability of each category is eventually obtained by comparing their device performances with the application demands of various applications. Finally, the challenges of water-air compatibility, water-entry locomotion, and water-exit locomotion are discussed. Moreover, the key technologies to overcome them, i.e., morphing blade techniques, morphing wing techniques, variable density techniques, structural buffer techniques, bionic water-entry techniques, surface dehydration techniques, and bionic water-exit techniques are analyzed. The research in this paper will provide more promising solutions for aquatic-aerial locomotion and inspire more practical studies in the HAAV field.