Friday, August 19, 2011

Control Mixing Unit

The mixing unit is required to ensure that all control inputs are transmitted to the main rotor without loss of input or one affecting the other.
The mixing unit will provide inputs to the main rotor actuators, which are derived from a number of different control movements, for example, the inputs that may be transmitted to the main rotor are:
  • Collective pitch
  • Fore and aft cyclic
  • Left cyclic 
  • Right cyclic
In single rotor helicopters there are three actuators that control the main rotor, and it is essential that each receives the correct input to ensure that the aircraft response is in the correct sense and proportionate to any pilot/co-pilot inputs.

There are a number of variations of mixing unit design that will be found in various helicopter types, but in its simplest form the mixing unit comprises of 3 bellcranks mounted on a central shaft, the shaft itself is mounted on bearings that allow all 3 bellcranks to be moved. The following diagram shows a typical mixing unit and the axes of movement for cyclic and collective inputs.

Monday, August 8, 2011

Collective Pitch Control Lever

The collective pitch lever is used to increase or decrease total rotor thrust. The lever is fitted to the left hand side of the pilot and co-pilot crew seats, like all flying controls its operation is instinctive, pulling the lever upwards increases the pitch on all main rotor blades by the same amount at the same time, and therefore increases total rotor thrust. Pushing the lever downwards will decrease the pitch angle on all main rotor blades by the same amount at the same time, decreasing total rotor thrust.

As its name suggests the collective control is usually in the form of a lever, that is a bar or pole secured at one end and operating through an arc, however, there are variations. One helicopter for example uses a lever mounted vertically, rather than horizontally which operates with a push/pull action, another has a collective control mounted vertically through the cockpit floor moving vertically up and down. Although the term collective pitch lever is widely used there are variations between helicopter types, other common names for this control are simply Collective, Thrust Lever or Thrust Control.

The collective control is usually mounted onto a torque tube, sometimes referred to as a lay shaft. The torque tube is fitted into bearings secured to the structure which allow it to rotate as the collective pitch levers are moved up and down. This type of installation provides two main benefits firstly it allows both collective pitch levers to be joined together, ensuring that they both move whether operated from the pilot or co-pilot position. Secondly, a torque shaft will convert the rotary into linear movement, providing a pushing or pulling movement onto the control system.

Push/pull tubes are connected to a lever fitted onto the torque shaft, these transmit collective lever movement into the collective pitch control system. These movements are fed to the controls mixing unit, before being sent to the main rotor control actuators or servos.In most helicopters a switch box is fitted onto the end of the collective lever, which houses switches and controls for important systems, such as rescue hoists, searchlights and engine trim controls. 

This ensures that these important controls are always readily to hand and can be operated without the need for the pilot of co-pilot to remove their left hand from the collective pitch lever, especially during critical phases of flight, such as hovering.

However, there will be times when the flight crew must remove their left hand from the collective lever, to prevent the lever from lowering and thus reducing total rotor thrust a friction device is fitted. The friction device may be a simple clamp device fitted to the torque tube