Aerodynamic Loading Characteristics of a Model-Scale Helicopter in a Ship's Airwake

Abstract

Experiments have been conducted in a water tunnel using a specially designed Airwake Dynamometer (AirDyn) to characterize the aerodynamic loading of a helicopter immersed in the airwake of a generic frigate ship. The AirDyn is a 1:54 model-scale helicopter based on a Merlin AW-101, with a six-component force balance mounted inside the fuselage and a simplified spinning main rotor. The AirDyn has been used to measure the unsteady forces and moments imposed by the ship airwake at fixed locations along the flight path of a landing maneuver for a headwind and a 45 wind-angle. A region of ‘thrust-deficit’ in the headwind and a ‘pressure-wall’ in the 45 wind-angle were identified as ‘time-averaged’ loading characteristics caused by spatial velocity gradients in the airwake. The unsteady loading on the AirDyn in the headwind was compared with the 45 case in terms of the severity of the airwake disturbances and the stages of the landing maneuver at which they are significant. The causes of the observed AirDyn loading characteristics have been explained using unsteady Computational Fluid Dynamics analysis of the ship airwakes. The implications of the AirDyn aerodynamic loading characteristics for pilot workload and control input strategies during a real landing are discussed.