Rotating swash plate Control rod Stationary swash plate Pitch link Drive link Inner and outer parts turning at same rpm Outer turning much faster than inner Figure 4-11.  Stationary and rotating swash plate. Figure 4-12.  Freewheeling unit in normal drive position and  freewheeling position. Note that in the top example, the engine  output shaft (inner part) drives the rotor shaft (outer part) at the  same speed (normal flight). In the bottom example, the rotor shaft  (outer part) breaks free under autorotation, as it turns faster than  the driver shaft (inner part). Figure 4-13.  Antitorque rotor produces thrust to oppose torque.  $_{ }$B $_{l}$a d e r o t a ti o n B l a d e r o t a t i o n Tail rotor thrust to compensate for torque T o r q u e T o r q u e Freewheeling Unit Antitorque System 4-7 The stationary swash plate is mounted around the main rotor  mast and connected to the cyclic and collective controls by a  series of pushrods. It is restrained from rotating by an anti- drive link but can tilt in all directions and move vertically.  The rotating swash plate is mounted to the stationary swash  plate by means of a uniball sleeve. It is connected to the mast  by drive links and must rotate in constant relationship with  the main rotor mast. Both swash plates tilt and slide up and  down as one unit. The rotating swash plate is connected to  the pitch horns by the pitch links. Since lift in a helicopter is provided by rotating airfoils,  these airfoils must be free to rotate if the engine fails. The  freewheeling unit automatically disengages the engine from  the main rotor when engine revolutions per minute (rpm)  is less than main rotor rpm.  [Figure 4-12]  This allows the  main rotor and tail rotor to continue turning at normal in- flight speeds. The most common freewheeling unit assembly  consists of a one-way sprag clutch located between the engine  and main rotor transmission. This is usually in the upper  pulley in a piston helicopter or mounted on the accessory  gearbox in a turbine helicopter. When the engine is driving  the rotor, inclined surfaces in the sprag clutch force rollers  against an outer drum. This prevents the engine from  exceeding transmission rpm. If the engine fails, the rollers  move inward, allowing the outer drum to exceed the speed  of the inner portion. The transmission can then exceed the  speed of the engine. In this condition, engine speed is less  than that of the drive system, and the helicopter is in an  autorotative state. Helicopters with a single, main rotor system require a  separate antitorque system. This is most often accomplished  through a variable pitch, antitorque rotor or tail rotor.  [Figure 4-13]  Pilots vary the thrust of the antitorque system to  maintain directional control whenever the main rotor torque  changes, or to make heading changes while hovering. Most  helicopters drive the tail rotor shaft from the transmission  to ensure tail rotor rotation (and hence control) in the event  that the engine quits. Usually, negative antitorque thrust is  needed in autorotations to overcome transmission friction.