Freewheeling turbine engines do not require a separate clutch  since the air coupling between the gas producer turbine and  the power (takeoff) turbine functions as an air clutch for  starting purposes. When the engine is started, there is little  resistance from the power turbine. This enables the gas- producer turbine to accelerate to normal idle speed without  the load of the transmission and rotor system dragging it  down. As the gas pressure increases through the power  turbine, the rotor blades begin to turn, slowly at first and then  gradually accelerate to normal operating rpm. to the pilot on digital screens and vertical scale instruments.  The bottom right gauge in  Figure 4-18  replicates a vertical  scale instrument. The dual tachometer shown displays rotor  rpm (NR) on the left and engine rpm (NP) on the right side  of the vertical scale. Corresponding color limits are present  for each component parameter. In helicopters with horizontally mounted engines, another  purpose of the main rotor transmission is to change the  axis of rotation from the horizontal axis of the engine to the  vertical axis of the rotor shaft.  [Figure 4-20]  This differs  from airplanes, which have their propellers mounted directly  to the crankshaft or to a shaft that is geared to the crankshaft. On reciprocating and fixed turbine engines, a clutch is  required to enable engine start. Air, or windmilling starts,  are not possible. The two main types of clutches are the  centrifugal clutch and the idler or manual clutch. How the clutch engages the main rotor system during engine  start differs between helicopter design. Piston-powered  helicopters have a means of engaging the clutch manually  just as a manual clutch in an automobile. This may be by  means of an electric motor that positions a pulley when the  engine is at the proper operating condition (oil temperature  and pressure in the appropriate range), but which is controlled  by a cockpit mounted switch. Maintaining main rotor rpm is essential for adequate lift.  RPM within normal limits produces adequate lift for normal  maneuvering. Therefore, it is imperative not only to know  the location of the tachometers, but also to understand the  information they provide. If rotor rpm is allowed to go below  normal limits, the outcome could be catastrophic.  In a conventional airplane, the engine and propeller are  permanently connected. However, in a helicopter they are  not. Because of the greater weight of a rotor in relation to the  power of the engine, as compared to the weight of a propeller  and the power in an airplane, the rotor must be disconnected  from the engine when the starter is engaged. A clutch allows  the engine to be started and then gradually pick up the load  of the rotor. Some helicopters utilize a belt drive to transmit power from  the engine to the transmission. A belt drive consists of a lower  pulley attached to the engine, an upper pulley attached to the  transmission input shaft, a belt or a set of V-belts, and some  means of applying tension to the belts. The belts fit loosely  over the upper and lower pulley when there is no tension on  the belts.  [Figure 4-21] Belt Drive Clutch Clutch Structural Design Figure 4-21.  Idler or manual clutch.  Figure 4-20. The main rotor transmission reduces engine output  rpm to optimum rotor rpm.  Upper pulley V-Belts Lower pulley Gearbox Main transmission To engine Main rotor Antitorque rotor 4-12