The primary purpose of a vertical stabilizer or fin is to provide stability in yaw. While the tail rotor itself provides considerable yaw stability, the vertical stabilizer may also be required to provide sufficient aerodynamic side-force to offset the tail rotor thrust in forward flight and to provide sufficient anti-torque to allow continued flight in the event of the loss of the tail rotor – see Horst & Reschak (1975). This side-force can be provided by using an airfoil section with a relatively large amount of camber. Alleviating the tail rotor thrust in high-speed flight by means of a side force on the fin is usually desirable to minimize tail rotor flapping and cyclic loads and to maximize component fatigue life. With sufficient forward speed and some side-slip angle, the side-force can be great enough to allow continued flight without the tail rotor, although this flight condition is difficult to achieve in practice.
The vertical stabilizer also forms a structural mount for the tail rotor. Because the flows will strongly interact, the tail rotor can be considered to be an integral part of the fin and empennage assembly. The size of the vertical stabilizer directly and adversely affects tail rotor performance. A smaller stabilizer will reduce the adverse effects on tail rotor efficiency, but this must be balanced against the effects on yaw stability and other design requirements including sideward flight. Because of the need for side forces on the fin to be low when the helicopter is operating in hovering flight with a crosswind, the trailing edge of the fin may often be blunt because this tends to help reduce adverse fin forces at large angles of
attack. The special complications associated with the design of the tail rotor itself means the aerodynamics of the tail rotor and the interactions with the fin must be discussed separately – see Section 6.9.