Monday, July 18, 2011

Tail Rotor Control Systems


Tail rotor control systems can be of 2 types, control cable or push/pull tubes. In early helicopters cable systems were used because of the need to reduce the weight of the longer system, cable systems offering a weight saving of approximately 25-30% over tube systems. However, cable systems suffer from several disadvantages, they require strengthening of the structure because of the relatively high cable tensions, and the steel cables will expand and contract at a different rate to the light alloy structure.
Whilst the latter was overcome with the use of cable tension regulators, cable systems still required more maintenance, and were prone to developing faults. Many modern helicopters, especially the larger types, now use push/pull tube systems for tail rotor control.

Cable systems

The majority of cable systems use push/pull tubes from the yaw pedals to a cable quadrant, from here control cables are used to transfer control inputs through the fuselage and tail boom structure. In very early helicopters the cables were wound around a cable spool, usually 1½ to 2½ turns, which was connected directly to a mechanical screw-jack that turned the motion through 90° and provided the movement to the tail rotor, via a spider mechanism, although this provided a purely manual control system and was generally only used on the light helicopter types.
An alternative to this, especially where hydraulic controls were used, was to position another quadrant just before the tail rotor, and connect it via a push/pull tube to the tail rotor control mechanism, or hydraulic actuator. One of the quadrants would be a cable tension regulator, ensuring consistent cable tensions.
Cables used in a system that used a cable spool could either be of the ‘continuous loop’ type or would have nipples swaged on to the ends of the cable. Those systems using 2 quadrants would comprise of 2 cables, having swaged nipples at each end to ensure positive retention at the quadrants.

Push/pull tube system

In this system, the tubes transfer control inputs from the yaw pedals right through to the tail rotor control mechanism or actuator. Push/pull tube systems are more difficult to route than cables, and are comprised of many more components, many of which could potentially develop faults, but there is less possibility of lost motion developing rapidly within the system, as could be the case if control cables lost tension.

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