Elements of Underwater Glider Performance and Stability

Abstract

Underwater gliders are winged autonomous underwater vehicles (AUVs) that can be deployed for months at a time and travel thousands of kilometers. As with any vehicle, different applications impose different mission requirements that impact vehicle design. We investigate the relationship between a glider’s geometry and its performance and stability characteristics. Because our aim is to identify general trends rather than perform a detailed design optimization, we consider a generic glider shape: a cylindrical hull with trapezoidal wings. Geometric parameters of interest include the fineness ratio of the hull, the wing position and shape, and the position and size of the vertical stabilizer. We describe the results of parametric studies for steady wings-level flight, both at minimum glide angle and at maximum horizontal speed, as well as for steady turning flight. We describe the variation in required lung capacity and maximum lift-to-drag ratio corresponding to a given vehicle size and speed; we also consider range and endurance, given some initial supply of energy for propulsion. We investigate how the turning performance varies with wing and vertical stabilizer configuration. To support this analysis, we consider the glider as an 8-degree-of-freedom multibody system (a rigid body with a cylindrically actuated internal moving mass) and develop approximate expressions for turning flight in terms of geometry and control parameters. Moving from performance to stability and recognizing that a glider’s motion is well described in terms of small perturbations from wings-level equilibrium, we study stability as an eigenvalue problem for a rigid (actuators-fixed) flight vehicle. We present a number of root locus plots in terms of various geometric parameters that illuminate the design tradeoff between stability and control authority.