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Development of the Hybrid Underwater Glider Petrel-II
A Propeller-Driven Glider for Oceanographic Research

Fang Liu

Yanhui Wang

Shuxin Wang

Main configuration of Petrel-II: 1) buoyancy regulating unit, 2) attitude regulating unit, 3) control center, 4) vertical tail fin, 5) antenna duct, 6) propeller unit, 7) emergency unit, 8) horizontal wings and 9) pressure hull.
The Autonomous Underwater Glider (AUG) first conceptualized by Henry Melson Stommel in the 1980s is a novel oceanographic research platform. An active buoyancy regulating unit combined with a set of fixed wings is used to enable AUGs to move vertically and horizontally in a sawtooth trajectory. Due to the special forward propulsion, AUGs can work continuously for several months at a horizontal speed of about 0.5 knots and with a long flight range, low speed and low noise. However, the maximum horizontal speed of traditional AUGs is lower than 0.5 knots. The risk of mission failure could occur when the AUG operates in areas with strong currents, where the speed of the current exceeds the maximum forward speed of the AUG. The low maximum speed of AUGs not only strictly limits the platform's application, but also makes it less reliable. Therefore, hybrid underwater gliders (HUGs) are proposed for oceanic observations with high speed requirements.

A HUG, Petrel-II, that was developed by Tianjin University in 2012 combines behaviors of traditional AUGs and those of AUVs. It is equipped with an active buoyancy unit, a compact propeller unit, an attitude (pitch and roll) regulating unit and a pair of fixed wings. It has two working modes, buoyancy-driven gliding and propeller-driven level flight. Petrel-II can automatically switch between the two modes according to commands or environmental circumstances. Its maximal gliding speed reaches 0.8 knots, and its propelling speed can reach up to 3 knots. It is particularly suitable to perform survey missions in strong currents. In addition to the improvement on the speed capacity, Petrel-II is characterized by a low drag, a high depth rating, light weight and high cost-efficiency. It has experienced two lake trials and three sea trials in 2013. The experimental results verify the maneuverability and effectiveness of the vehicle.

Components and Specifications of Petrel-II
Petrel-II consists of two parts: the mechanical part and the electrical control part. The mechanical part can be functionally divided into five subsystems: the pressure hull, the buoyancy regulating unit, the attitude regulating unit, the propeller unit and the emergency unit. Besides the above subsystems, the wings and the tail antenna are important mechanical accessories of the vehicle. The electrical control part of Petrel-II has the following subsystems: the processing unit, the navigation unit, the energy unit, the communication and position unit, and the scientific sensor suites. The communication and position unit consists of wireless, Iridium and GPS modules.

For convenient transportation and launch, the weight of Petrel-II is limited to 68 kilograms. Petrel-II's main body is 2.2 meters long with a maximum diameter of 0.22 meters. The wing span is 1.2 meters, and the antenna mast is 1-meter long. The wings and the antenna mast can be easily removed, and the entire vehicle can be loaded into a small box for transportation. In order to expand the operational field, the maximum work depth is 1,500 meters. The maximum forward velocity is 3 knots in propelling mode. The range is roughly 1,200 kilometers and is related to operational depth, glide angle, survey mission and battery life.

Mechanical Configuration
Petrel-II's main body consists of two wet sections and one sealed pressure hull, to which the wings and a tail antenna are attached. The conventional low-drag, torpedo-shaped body is adopted to improve hydrodynamic performance. The fore section of main body, which is critical to low drag design, has a comparatively long ellipse shape to hold laminar boundary layer flow. In addition, the size, shape and placement of the wings and the fixed fins are optimized by using the computational fluid dynamics software. The wings are located at the aft of the pressure hull and swept at 45 degrees. A customized conductivity, temperature and depth (CTD) sensor from Sea-Bird Electronics (Bellevue, Washington) was selected as an attempt to reduce drag. To continue this article please click here.

Fang Liu received a bachelor's degree in mechanical engineering from the University of Science and Technology of Beijing in 2009. He joined the school of mechanical engineering at Tianjin University in 2009. He has been engaged in developing AUVs.

Yanhui Wang received bachelor's, master's and Ph. D. degrees in mechanical engineering at Tianjin University, where he is an associate professor at the school of mechanical engineering. He has been involved in the research of various underwater vehicles.

Shuxin Wang received a bachelor's degree in mechanical engineering from the Hebei University of Technology in 1987 and master's and Ph. D. degrees in mechanical engineering from Tianjin University. He is a professor at Tianjin University and has been active in various aspects of the research and design of ocean vehicles since 2000.

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