Sikorsky has used tail rotors canted 20° down to the left on both the UH-60A and the CH-53E, as may be seen in Appendix B. Reference 10.13 cites the advantages

of this concept as the more efficient use of hover power sfnce the tail rotor thrust vector has an upward component, the ability to tri^ with a center-of-gravity position behind the main rotor, and the alleviation of unsteadiness in the vortex ring state in sideward flight. The primary disadvantage is the development of a longitudinal response with directional control inputs. This can be mechanically compensated for in one flight condition, but not in all. In weighing the advantages against the disadvantages, reference 10.13 concludes: "Although the UH-60A control and Automatic Flight Control System (AFCS) design solved the coupling problems associated with the canted tail rotor, the feeling is that unless forced into an aft c. g. problem by other constraints, the canted tail rotor should not be considered.”

A horizontal stabilizer is not absolutely required on a helicopter. Helicopters designed before I960 were unlikely to have them but were nevertheless considered successful. A stabilizer does, however, make an order-of-magnitude improvement in the flying qualities in forward flight and is used routinely in modern designs.

The analytical methods of Chapters 8 and 9 can be used for guidance in choosing the area and fixed incidence that will provide acceptable static and dynamic longitudinal stability in high-speed forward flight at the most aft center – of-gravity position, but the stabilizer s effect on hover and low-speed flight should also be considered. The primary effect is the possibility of erratic longitudinal trim shifts when going from hover to forward flight if the main rotor wake impinges on the surface and produces a high download. There are three possible factors that might contribute to this problem: a large fixed incidence stabilizer located behind the main rotor wake in hover, a high rotor disc loading, and low control power. When one or more of these factors is significant enough to create a problem, designers have adopted three different approaches. One, favored by Bell, places the surface forward on the tail boom so that it is in the rotor wake in hover and thus does not experience a sudden change in download during the transition. The second method uses a T-tail with the horizontal stabilizer mounted at about the same height as the main rotor so that it does not feel the wake until high speeds are reached where the induced velocities are small. The third approach uses a movable incidence stabilator that can be aligned with the local flow during transition either by being permitted to float free or by being programmed as a function of flight parameters. A discussion of the development of the Hughes AH-64, which had both a T-tail and a stabilator during its development, can be found in reference 10.14. In this case, the incidence of the stabilator was programmed to be a function of air speed, collective pitch, and aircraft pitch rate, as shown in Figure 10.9. The air speed input is the primary means of changing incidence during the transition. The collective pitch input is used to reduce the upload in autorotation, and pitch

FIGURE 10.9 Stabilator Incidence Schedule for the Hughes AH – 64

Source: Prouty & Amer, “The YAH-64 Empennage and Tail Rotor—A Technical History,” AHS 38th Forum, 1982.

rate is used to make the surface into an "active control,” which allows its area to be significantly less for the same level of dynamic stability than if the incidence were fixed.

A similar stabilator schedule for the Sikorsky UH-60 is found in reference