TM 1-1520-238-T-7
11–47
11–7.
SYSTEM DESCRIPTION (cont)
11–7
d. Longitudinal (Cyclic) Flight Control System.
(1) Purpose.
The purpose of the longitudinal (cyclic) flight control system is to provide mechanical inputs
to the main rotor for longitudinal control of the helicopter.
(2) System Operation.
(a) The pilot and CPG cyclic sticks (fig. 11–18) are moved forward or aft to control longitudinal
movement of the helicopter. The pilot and CPG longitudinal LVDTs monitor cyclic stick movement during DASEC,
CAS and BUCS operation. An electrical signal is developed proportional to cyclic stick movement and sent to the
longitudinal servocylinder to position the main rotor head. During normal operation, the SPAD is part of the
mechanical linkage. When jamming prevents normal cyclic longitudinal stick movement, a force of 30 foot-lbs. on
either cyclic control stick causes the SPAD shear pin to snap, severing the mechanical control linkage which
activates BUCS. The BUCS select trigger switch, on the CPG’s collective stick, must be closed to activate BUCS.
When the CPG’s shear pin snaps, BUCS is activated and the BUCS ON indicator on the pilot and CPG
caution/warning panels light. When BUCS fails, the BUCS FAIL indicator on the pilot and CPG master
caution/warning panels light.
M70-258
CYCLIC
STICK
LONGITUDINAL LVDT
LONGITUDINAL SPAD
Figure 11–18.
Cyclic Longitudinal Control System LVDT and SPAD Assemblies
(b) The longitudinal feel spring and magnetic brake assembly (fig. 11–19) are located beneath the pilot’s
cyclic stick. The longitudinal magnetic brake assembly is a 28 VDC controlled unit that engages to establish a
longitudinal trim position. The feel trim assembly is a spring loaded device that overrides the longitudinal magnetic
brake assembly when the cyclic stick is moved against the magnetic brake assembly. The feel spring assembly
allows cyclic stick inputs to override the longitudinal force trim setting. When the cyclic stick is not engaged
against the feel spring assembly, longitudinal flight trim maintains current trim position.
(c) The longitudinal mechanical control linkage (fig. 11–20) provides an increase or decrease in cyclic
movement to the longitudinal servocylinder. The longitudinal servocylinder, connected to the mixer assembly,
hydraulically assists the mechanical linkage by moving the main rotor swashplate forward or aft, which moves the
pitch horns and the blades of the main rotor head.
