Learning Objective: Identify the three primary flight controls and the basic control systems components.

Helicopter flight controls differ drastically from those found in fixed-wing aircraft. Helicopter flight controls consist of both cyclic and collective pitch control systems and the rotary rudder flight control system.

The hydraulically powered flight control mechanism, shown in figure 10-6, provides you with an example of systems common to most helicopters. These are the systems on which you will most likely be working. Fairly exact values, such as tolerances, pressures, and temperatures, are given to provide instructive coverage. When actually performing the maintenance procedures, consult the current technical publications for the latest information and exact values.

The cyclic pitch control system provides the means of controlling the forward, aft, and lateral movements of the helicopter. Movement of the pilots or copilots c yclic stick transmits through control rods and bell cranks. This movement is sent to the auxiliary servo cylinders, the mixing unit, and three primary servo cylinders. These primary servo cylinders control movement of the rotary-wing blades.

The cyclic system has a stick trim system that hydraulically operates the controls for automatic flight. During automatic flight, trim movements are controlled manually by the cyclic stick grip switch. The switch is overridden for major control changes by stick movement.

Moving the cyclic stick forward extends the aft primary servo cylinder and retracts the forward primary servo cylinder. Aft movement of the cyclic stick extends the forward primary servo cylinder and retracts the aft primary servo cylinder. In both cases, the helicopter will advance in the direction of stick movement. Movement of the stick laterally will move the helicopter right or left, corresponding with stick movement. This movement occurs by retracting and extending the left and right lateral primary servo cylinders.

The collective pitch control system provides vertical control of the helicopter. Movement of the collective pitch control stick is sent through control rods and bell cranks to the appropriate auxiliary servo cylinder. Movement is sent from the servo cylinder to the mixing unit. At the mixing unit, all vertical movements of the collective sticks are sent to the primary servo cylinders and the rotary-wing swashplate. At this point, the pitch of all blades increases or decreases equally and simultaneously. A balancing spring attaches to the control rods to help balance the weight of the collective stick. A friction lock on the pilots collective stick applies the desired amount of friction to the tube of the collective stick. The lock prevents creeping during flight. It also provides feel for the pilot when operating the controls. The friction is applied by rotating the serrated handgrip on the collective stick to its stop. The grip of each collective stick contains several switches that are labeled for the function they control. In the automatic stabilization equipment (ASE) mode operation, the collective pitch operation controls through the auxiliary servo cylinder.

The rotary rudder control system controls the pitch of the rotary rudder blades. The blades control the heading of the helicopter. The pedals control the system through a series of control rods and bell cranks. These units connect to the directional bank of the auxiliary servo cylinder and the mixing unit. See figure 10-6. At the mixing unit, a control rod operates the forward quadrant. This quadrant connects by

Figure 10-6.Flight control systems.

cable to the aft quadrant. A control rod from the rear quadrant connects to the control rods, bell  crank, and pitch control shaft. These parts are found in the  rotary rudder tail gearbox. A hydraulic pedal damper is located in the auxiliary servo  cylinder bank (directional). Its purpose is to prevent sudden movements of the control pedals. The damper prevents rapid changes in blade pitch, which might cause damage to the helicopter. As on conventional aircraft, the rudder pedals are adjustable for different leg lengths. The rotary rudder system operates by manual input or automatically by input from the ASE. The negative force gradient spring cancels feedback loads exerted by the rotary rudder during flight. It also cancels feedback loads when the auxiliary hydraulic system is off. When the rotary rudder is stationary, an initial force is required to move either pedal from its extreme position. With the auxiliary hydraulic system on, the effect of the negative force gradient spring is zero.

The negative force gradient spring is preloaded to 600 pounds. To prevent injury to personnel or damage to flight controls, carefully follow the maintenance instructions provided in the MIM.

The basic components of the helicopter flight control systems are the auxiliary servo cylinder, the mixing unit, the primary servo cylinders, and the swashplate.