Investigation of Fan-In-Disk Aerodynamics in Edgewise Flight

An analytical investigation into the aerodynamic characteristics of a “fan-in-disk” configuration was conducted and results were compared with wind tunnel test data. The fan-in-disk configuration consists of a thin circular duct (disk-like) body, a center cabin, and a fan located between the center body and the circular disk duct. An exit vane system installed on the bottom of the duct exit was used to create the control forces and moments in different test conditions. Wind tunnel tests were conducted in the Glenn L. Martin 7.75- by 11-foot wind tunnel in the University of Maryland. Data acquired included six-component aerodynamic forces and moments, and pressures along cross sections of the body. An analytical model of the wind tunnel test model was developed that utilized a modified panel method to represent the body and center cabin and used blade element model to represent the fan. Interactions between the body, cabin, and fan were included in the prediction of aerodynamic forces acting on this configuration. Both wind tunnel test results and analytical predictions indicated that the presence of the duct improved the aerodynamic efficiency of this configuration in hover even though the depth of the duct is relatively thin (disk-like). Both experimental and analytical results also demonstrated that a large inherent nose-up pitching moment resulting mostly from the significant pressure suctions on both the upper surface of the front body and the lower surface of the aft body in edgewise flight. In addition, wind tunnel test data showed that the mass flow through the fan disk generated a large drag force in edgewise flight, referred to as “momentum drag”. The modified panel method, including a flow separation correction, correlated with the total drag measurements, but significantly under-estimated the magnitude of the drag. The experiment showed that the exit vane control system evaluated was not able to trim the large nose-up pitching moment and drag that are inherent to a fan-in-disk configuration.