Shroud-fin integration shelf for a helicopter empennage structure

Abstract

A shroud-fin integration shelf for a helicopter empennage structure that includes a shroud embodying a ducted fan antitorque device, a vertical stabilizer contiguous with the shroud, and a horizontal stabilizer intersecting the vertical stabilizer. The shroud-fin integration shelf is structurally configured and dimensioned to eliminate adverse aerodynamic effects experienced by the vertical stabilizer in reverse thrust operating conditions wherein mass airflow exiting the ducted fan antitorque device may flow along the shroud and interfere with longitudinal mass airflow over the suction surface of the vertical stabilizer. The integration shelf, which provides an acute transition between the shroud and the vertical stabilizer, has an overall predetermined width that includes a first shelf width on the ducted fan antitorque device inlet side of the empennage structure and a second shelf width on the opposite side of the empennage structure. The first shelf width is greater than the second shelf width to ensure attached mass airflow over the shroud is sharply separated at the shroud - shroud-fin integration shelf. The first and second shelf widths have dimensional magnitudes in the range of about four percent to about sixteen percent of the diameter of the fan assembly of the ducted fan antitorque device. Preferably, the shroud edge defining the structural boundary between the shroud and the shroud-fin integration shelf is sharp, i.e., approximately ninety degrees. The empennage structure also includes an aft shroud closure having a generally 'V' shaped configuration to fix the separation points of mass airflow over the aft portions of the shroud.