Strategies For Tiltrotor Airframe Download Reduction Using Active Flow Control

A numerical study is conducted to assess the relative benefits of three active flow control (AFC) strategies for reducing the download (vertical drag) on a flapped NACA-64A223 airfoil placed at 90 degrees to the onset flow. Three types of unsteady “near-tangent” jets, located on the upper surface shoulder of the flap, are considered. Namely, a pulsed blowing jet, a pulsed suction jet and an oscillatory zero-net-mass jet. Predictions are made using a modified version of the NASA Langley CFL3D unsteady thin layer Navier-Stokes flow solver. Here, an unsteady transpiration boundary condition is used to emulate the temporal variations in the oscillating (or pulsed) jet velocity. Predicted flowfields that provide insight into the mechanics of the interaction between the different types of jets and the developing boundary layer on the airfoil/flap are presented. It is shown that the use of an oscillatory or a pulsed suction jet provide for the most effective means for reducing the vertical drag – primarily through reattaching the majority of the otherwise separated boundary layer flow on the flap. By comparison, the use of a pulsed blowing jet is shown to be less effective. Where applicable, comparisons with experimental data are made to assess the accuracy of the flow solver.