# Configuration Design and Trans-Media Control Status of the Hybrid Aerial Underwater Vehicles
## 2 Configuration Design  
It is necessary for HAUVs to adapt to both the water and air mediums. This necessity creates unavoidable contradictions to the configuration design. Generally speaking, the design has obvious particularities in terms of weight matching, fuselage shape, wing layout, structural design, and power system. (a) In terms of the weight of the vehicle, the flight in the air should be as light as possible, and the underwater navigation should have considerable density and strength, which is conducive to diving. (b) Concerning fuselage shape, the different density increases the difficulty in configuration design. Based on the principle of Reynolds number similarity, there is a great difference in the speed of the same fuselage in water and air environments. (c) In the aspect of wing layout, the aircraft overcomes gravity by increasing the pressure difference between the upper and lower surfaces of the wings. When the velocity is constant, the wing area determines the lift. The submersibles can rise and dive by adjusting their own buoyancy. The wing surface is usually used to adjust the course and balance. In order to reduce the resistance of underwater navigation, the wing surface has a relatively short wingspan and a narrow \begin{table}
\begin{tabular}{c c c} \hline  & **Density (kg/m\({}^{3}\))** & **Dynamic Viscosity Coefficient (Pa-s)** \\ \hline Air & 1.225 & 1.789 \(\times\) 10\({}^{-5}\) \\ Water & 998.2 & 1.003 \(\times\) 10\({}^{-3}\) \\ \hline \end{tabular}
\end{table} 
Table 1: Physical parameters of the 15 \({}^{\circ}\)C air and 20 \({}^{\circ}\)C water. Figure 1: Water–air medium detections for underwater pipeline leakage. Figure 2: An imagined motion profile of an HAUV. chord. (d) In terms of structural design, underwater navigation has high requirements for both pressure and sealing. In flight, especially in low altitude flight, the sealing and pressure performance are hardly considered. Even the pressure hull used in high-altitude flight cannot be compared with the cabin in underwater navigation. In addition to this, the high-altitude flight requires overpressure in the cabin, while the submarine's skin is exposed to the inverse pressure: higher water pressure outside and lower air pressure inside. (e) In terms of the power system, the aero-engine relies heavily on air and cannot work underwater at present. The heavy underwater power system is also unsuitable for flight. It is challenging that HAUVs should accommodate both the fluid inertia and drag encountered underwater without compromising the weight and lifting area requirements of flight. [PERSON] and DARPA opened the prelude to the development of a manned HAUV, but these projects only demonstrated the preliminary design of the layout. Compared with a large-scale manned HAUV, researchers drew on the basic principles of bionics and turned their attention to a small-scale unmanned HAUV. Many animals, such as kingfishers, gannets, flying fish, and squid have the ability to cross the water-air surface [8; 9; 10]. The animals' habits aroused the researchers' interest, and they hoped to find the inspiration for the design of HAUVs from these animals. Two main kinds, multirotor HAUVs and morphing HAUVs, are being developed for the accommodation of air and water mediums. HAUVs that accomplish locomotion with changing air and underwater configurations could be defined as morphing HAUVs. [PERSON] proposed a variable-sweep HAUV which could dive into the water like a gannet [11]. The vehicle was used to research water entry impact acceleration through falling down experiments. [PERSON] and [PERSON] further designed a variable-sweep HAUV, powered by a \"squid spray\" water jet thruster which was based on compressed carbon dioxide. It achieved the water-exit process experimentally [4; 12]. More recently, the team designed a delta wing HAUV to improve flight stability after it left the water, as shown in Figure 2(a) [13].