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How Transmission Systems Work in Aircraft?

In aircraft, engines generate the power needed for the many accessories used during flight. Depending on the specific aircraft, the components of the transmission system will vary, but the general purpose of these systems is to adjust rotor speed across systems. While the engine operates at high speeds, not all systems across the aircraft should operate at matching rate because this can cause damage and prevent smooth operations. For your better knowledge, this blog will look specifically at the various components of helicopter transmission systems, and how the power from the engine is used in different parts of the aircraft.

Components of Helicopter Transmission Systems

The main components of the transmission system are the main rotor transmission, tail rotor drive system, clutch, and freewheeling unit. The main rotor transmission serves the purpose of reducing the engine’s rotation per minute (rpm) to an optimum rotor rpm. This is because the engine functions at a much greater rotation speed than the main rotor should, often 6 to 9 times faster. Additionally, the main rotor transmission can change the axis of rotation in horizontally mounted engines. A rotor rpm indicator is used to monitor these speed adjustments during clutch engagement.

Clutch Systems

One of the main differences between the transmission system in helicopters versus other aircraft is that a helicopter’s engine is not permanently connected to its propeller. Due to the greater propellor to body weight ratio of helicopter designs, the rotor must be gradually syncopated with the engine through the use of a clutch. In a typical reciprocating helicopter, two clutch types are used: a centrifugal clutch and a belt drive clutch.

The centrifugal clutch works with an inner assembly and an outer drum, while the belt drive clutch works with a pulley device; however, both clutch types work to slowly engage the rotors with the engine. A centrifugal clutch connects directly to the engine’s driveshaft, and centrifugal forces cause the inner assembly to gradually cause the outer drum to rotate at a speed that matches the engine. Meanwhile, the engine pulley system works with an upper pulley attached to the transmission input shaft and a lower pulley attached to the engine. As the engine runs, tension on the belts gradually increases, again working to superimpose the rotor and the engine. Advantages of this system include vibration isolation, simple maintenance, and the ability to start and warm up the engine without engaging the rotor.

Freewheeling Unit

A final part of a transmission system to be familiar with is the freewheeling unit, that of which  allows the main rotor transmission to drive the tail rotor drive shaft during autorotation. Also known as the autorotative clutch, this part automatically disengages the engine from the main rotor when engine rpm is less than main rotor rpm, allowing the main rotor and tail rotor to continue turning at normal in-flight speeds. As a useful part type for ensuring safety during the case of engine failure, a freewheeling unit is an important component for your aircraft transmission system.


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