Experimental Investigation of Aerodynamic Interactions During Shipboard Launch & Recovery of Unconventional UAVs

A series of tests has been conducted for an unconventional Unmanned Air Vehicle (UAV) configuration - a flying-wing, tail-sitter with proprotor propulsion; and a generic ship model in the 8- by 10-ft Subsonic Wind Tunnel at the Naval Surface Warfare Center Carderock Division, USA. This configuration is similar to that which was investigated for DARPA’s Tern program. The major objective of this test was to understand the various mutual interactional aerodynamic phenomena when an unconventional UAV is operating near a ship deck. During testing, the time-varying wing and proprotor aerodynamic loads were measured separately with independent six-component load cells. Tufts and smoke techniques were used to visualize the air flow patterns around the ship deck, wing, and proprotor. To make the data set representative and comprehensive, test parameters included tunnel wind speed, proprotor rotational speed, wing pitch and roll angles, as well as wing locations relative to the ship deck and hangar. Several interesting mutual interactional aerodynamic phenomena between the ship airwake and aircraft model were observed and investigated. The most profound phenomenon revealed was the distinct bi-modal transient behavior exhibited in the wing rolling moment, which is characterized by reversals in the sign of mean rolling moment from positive to negative, or vice versa, in an instantaneous, irregular, and random manner while the test conditions were held constant. Time synchronizations of wing loads and flow visualization of ship airwake have confirmed that this aerodynamic reversal directly resulted from a bi-modal ship airwake phenomenon, for which vortices shed from the hangar side edges experienced a reversal in rotational direction at apparently random times, at zero-degree ship heading. This distinct bi-modal ship airwake and resulting bi-modal wing rolling moment could be a significant challenge in control for an unconventional UAV with large wing when operating in close proximity to a naval ship deck.