Representative Tail Rotor Designs
Examples of modem tail rotor assemblies are shown in Fig. 6.34. Some tail rotors may be very simple two-bladed teetering assemblies (e. g., Fig. 4.20), whereas others may be relatively sophisticated bearingless or hingeless designs. Common amongst all tail rotors is the lack of any cyclic pitch; only collective pitch is used because control of the tail rotor disk orientation is not required. Nevertheless, the tail rotor must be provided with flapping so that the blades may be allowed to respond to the changing aerodynamic environment. Lead-lag hinges are not used to save weight and reduce mechanical complexity. Instead, a large amount of <S3 or pitch-flap coupling is built into the tail rotor design. This provides a means of allowing the blades to pitch cyclically in such a way as to minimize blade flapping produced by the changing aerodynamic loads (see Section 4.12). In addition, a set of preponderance weights may be attached to the tail rotor pitch horns. These are a set of weights that lie out of the plane of rotation and rely on centrifugal forces to help keep the
(a) Four bladed tractor (UH-60) (b) Four bladed pusher (AH-64)
Figure 6.34 Representative tail rotor assemblies (a) UH-60 bearingless tail rotor, (b) AH-64 twin teetering tail rotor
control forces to within manageable limits. Preponderance weights are usually only found on tail rotors that use highly cambered airfoil sections so as to help minimize moments transferred to the pitch control system.