Direction of Rotation
Many designers in the past compromised the main rotor transmissions to obtain a "traditional” direction of rotation, although that mattered very little, while letting tail rotors turn in either direction even though that was an important decision. Experience gained during several recent development programs has proven that tail rotors should rotate with the blade closest to the main rotor going up to alleviate the instability and unsteadiness associated with the tail rotor vortex ring state in sideward flight (left sideward flight for helicopters with the "American” direction of main rotor rotation). A discussion of this subject will be found in Chapter 2. Figure 2.10 showed the dramatic improvement resulting from reversing the direction on the Lockheed AH-56. With the original—and wrong—direction, going from hover into left sideward flight not only required an unstable pedal travel but actually used up all the available control before reaching 15 knots. Not shown was the great unsteadiness that existed from 10 to 30 knots. When the rotation was reversed, the pedal displacement became stable, a good control
margin remained, and flight was much steadier. Similar experiences can be cited by engineers at Bell, Westland, Aerospatiale, and Mil. This main rotor/tail rotor interference phenomenon is not well understood, and as yet there is no analysis that will provide quantitative results.
If steadiness in left sideward flight is not enough reason for selecting a preferred direction, perhaps noise reduction is. Tests performed at Westland and reported in reference 10.9 have shown that noise in forward flight is lower if the tail rotor blade on top is going aft since it is not slicing through the main rotor wake quite so violently..