Design for the Operating Environment - Ring-Fin Tail Rotor Program

Recent increased interest in prolonged helicopter operation in close proximity to the ground has indicated the need for more positive directional control. Since 1975, Bell Helicopter Textron (BHT) has been investigating a helicopter tail rotor/fin improvement by models and through full-scale ground, wind-tunnel, and feasibility flight testing. The new concept consists of an airfoil-shaped cross-section ring that replaces the fin. The rotor blade tip vortices educt air around the guard ring by external mixing to produce increased thrust for the same power input and a larger maximum available value. Comparisons are presented on the thrust/torque/pitch angle relationships of 16-inch diameter models as well as fullscale test data for the open rotor alone, with a standard or simulated BHT Model 206B fin, and with the thrust-increasing ring-fin. Acoustic data under static thrust conditions show reduced noise levels up to 8 dBA by the use of the thrust ring and lowered operating tip speeds. Improved aircraft handling qualities are also indicated by feasibility flight test data. Potentials for applications of the concept are also reviewed.