What Are the Components of a Helicopter Transmission System?

In the realm of mechanics, a transmission is simply a machine placed within a power transmission system which is capable of managing the application of power. When placed within a vehicle, the transmission system is tasked with transforming the output of an internal combustion engine into a force that can be harnessed by wheels, rotors, and other such assemblies. In regard to helicopters in particular, the transmission system is implemented for the means of transferring power from the engine to the main rotor, trail rotor, clutch, and freewheeling unit. In this blog, we will discuss the primary components of a typical helicopter transmission system, allowing you to better understand how each assembly section and its related components work together to help the helicopter achieve and maintain flight.

Before power from the engine can be supplied to the main rotor of the helicopter, it first must go through the main rotor transmission. With such an assembly, the output RPM of the engine can be reduced as necessary to meet the optimal RPM value for the rotor. This value will vary depending upon the helicopter in question, and different reduction ratios are present in different systems. The main rotor transmission also serves to adjust the axis of rotation between the engine and rotor shaft, unlike airplanes that have a shaft or propeller that is directly connected to the engine crankshaft.

For a majority of helicopters, a dual-needle tachometer may be used to measure the engine and rotor RPM, or a percentage of both. Such measurements are crucial for maintaining safe RPM limits, and it is important to understand that the failure of the main rotor transmission will have the rotor rotate at a fixed speed alongside the engine. While conventional gauges may provide indications through the use of needles and dials, glass cockpits will often have a color-coded vertical scale. As the main rotor RPM is paramount for efficient lift, pilots should always know how to properly read tachometers.

While a variety of aircraft feature permanently connected engines and propeller assemblies, helicopters differ with the implementation of clutch systems. This is due to the fact that the weight of the rotor is not properly balanced with the power of the engine, thus disconnection is required when the starter is engaged. With the use of the clutch, the rotor load can slowly be picked up over time. Centrifugal clutches and manual clutches are the most common types, and their implementation will vary depending upon the aircraft and its design.

In some instances, a belt drive clutch is used to transfer power between the engine and transmission, and such assemblies utilize a lower pulley connected to the engine and an upper pulley affixed to the input shaft of the transmission. The belt may either sit loosely over the pulley or use varying means of applying tension. With a centrifugal clutch, an inner assembly is connected to the engine driveshaft and features lined shoes. As the engine increases its speed, the shoes will slide against the outer drum due to centrifugal force and the transmission input shaft will start rotating. As friction increases between the drum and clutch shoes, the rotor will turn faster and denote changes on the tachometer. Once the two needles are superimposed, the engine and rotor will have become synchronized.

For the tail rotor of the helicopter, power is derived from the engine with the use of a shaft and gearbox that is placed near the tail boom. With one or more long or short shafts connected with flexible couplings, the drive shaft is capable of flexing with the tail boom. As the tail rotor is primarily relied on for the means of balancing flight through anti-torque measures, the pilot will be capable of varying pitch with the use of anti-torque pedals.

With the proper maintenance and repair of transmission system parts and components, helicopters can continue to operate efficiently and reliably for the benefit of flight operations. Limitless Aerospace is an online distributor of aircraft parts, offering customers competitive pricing on driveshaft components, gear reduction parts, torque converter products, and much more. Take the time to explore our diverse offerings at your leisure, and our team of industry experts are readily on standby 24/7x365 to assist you through the purchasing process as necessary. At Limitless Aerospace, we are more than just a reliable distributor; we are your strategic sourcing partner.


October 6, 2022
August 18, 2022

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