Physical Modeling for the Gradual Change of Pitch Angle of Underwater Glider in Sea Trial

A gradual change of the pitch angle was observed for a glider in a sea trial in August 2014. It took place when the glider moved in both upward and downward glides between the sea surface and a depth of 1200 m and there was neither any internal mass movement nor buoyancy engine regulation in the steady glide. In this paper, this phenomenon is validated via a nonlinear dynamic model, which is restricted to the vertical plane. The model is based on Kirchhoff's equations of motion for a submerged rigid body and takes the seawater pressure, temperature, and density, and the deformation of the pressure hull of the glider into consideration. While the variations of aforementioned four parameters influence many physical parameters, including added mass, buoyancy, center of buoyancy (CB), hydrodynamic forces, and torques, their effects on glide velocity appear to be the primary cause for the gradual change of pitch angle observed. To minimize the gradual change of pitch angle, two effective approaches are proposed and validated, namely increasing the lift/drag ratio and decreasing the hydrodynamic torque coefficients.