Hydrodynamic implications for submarine launched underwater gliders

Underwater gliders are a type of long range unmanned vehicle that use bouyancy control and lifting surfaces to travel in a sawtooth trajectory through the water column. These vehicles are typically employed by oceanographers for environmental monitoring and also show promise as a sensor platform in military applications. This paper presents investigations of vehicle hydrodynamics relating to the deployment of a Slocum Glider from conventional submarines, where the standard vehicle was fitted with low aspect ratio wings to enable it to fit into a sabot for deployment from a 21 inch torpedo tube. Several non-dimensional hydrodynamic models are presented and compared with measured results from field trials in terms of lift-to-drag ratio as a function of angle of attack. The results show good agreement between the model and empirical data of the standard glider, however the experimental data showed the modified glider significantly outperformed the corresponding model prediction. The standard glider was observed to have a maximum lift to drag ratio that was higher, and occurs at a lower angle of attack, than that of the modified glider. The modified glider was observed to have a superior lift to drag ratio for a high range of angles of attack that in practice, dominates the typical operating state. Field trials revealed the modified glider to have a 5% greater range than the standard glider. These results are contradictory to the authors´ intuitive expectation, confirmed by the performance models, that the extreme reduction in the aspect ratio of the wings would introduce a significant performance degradation. In fact, the modified wings have resulted in a slight performance increase. With respect to submarine deployment applications, it is concluded that for shallow water operation, both wing geometries have similar performance due to a sub-optimal control system. Further work, potentially involving servodynamic simulations, deep water trials, and further hydrodynamic- - modelling, would enable exploration of the significance of the modified wing performance in deep water.