Analysis of Helicopter Downwash/Frigate Airwake Interaction Using Statistically Designed Experiments

A research program to investigate helicopter downwash/frigate airwake interaction has been initiated using a statistically robust experimental program featuring Design of Experiments. Engineering analysis of the helicopter/frigate interface is complicated by the fact that two flowfields become inherently coupled as separation distance decreases. The final objective of this work is to develop experimental methods to determine when computer simulations need to include the effects of a coupled flowfield versus using a simplified representation by superposing the velocity fields of the individual flowfields. The work presented was performed in the Old Dominion University Low Speed Wind Tunnel using a simplified 1/50 scale frigate waterline model and traverse mounted powered rotor with thrust measurement. Particle Image Velocimetry (PIV) velocity surveys were used with rotor thrust coefficient measurements at locations of identified interaction to help understand the underlying flow physics. Initially, PIV surveys of the frigate model landing deck in isolation and the rotor in isolation were performed to provide a baseline flow understanding. Next a designed experiment was devised yielding a response model for thrust coefficient as a function of vertical and longitudinal distance from the hangar door (base of the step), both with and without the rotor. This first experiment showed that thrust coefficient could be measured with enough precision to identify changes due to location using an advance ratio of 0.075 (V∞ = 5.14 m/s and ω = 5000 rpm). A second designed experiment determined the practical spatial resolution for mapping the thrust coefficient response along the frigate\u27s longitudinal center plane. Finally, a third designed experiment directly compared rotor thrust measurements between airwake and no-airwake cases and successfully identified regions that differed with statistical significance. Lastly, a qualitative comparison study was performed to investigate the limit to direct superposition of rotor downwash and frigate velocity fields using PIV results