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:
Fore and aft 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.