Helicopter Freewheeling Unit

The free wheel units are fitted so that the main rotor can not drive the engine (torque reversal) in the event of main rotor RPM over-running the engine  This occurs during flaring the main rotors and when the engines fail or have been throttled back for practice Autorotation during any critical situation.

There are two main types, the Roller and Sprag type.Some types can be manually selected for ground running without causing any rotation of the rotor systems.

Roller unit

This unit consists of an inner drive from the engine on which is mounted a cam ring and an outer drive to the rotor.  Circumferential interposed between the two are caged rollers which act as the driving medium.

When the transmission drive from the engine rotates, the rollers ride up the slopes of the cams and are jammed between the transmission drive shaft and the rotor drive forming a positive coupling between them.

Whenever the rotor overruns the engine the rotor outer drive of the free wheel unit is rotating faster than the inner engine drive thus releasing the rollers from their wedging action and making the rotor side of the transmission independent of the engine side.

will rotate without the engine.  The same would happen if the engine stopped.

Swash Plate Assembly

A Major part of a helicopter flight control system is the mechanism used to transfer control inputs from the non-rotating parts of the system, to those that are rotating. There are 2 commonly used methods used to achieve this:

• Swash Plate
• Spider Control

Of these, the swash plate is perhaps the most common device used for main rotors, and the spider for tail rotors, although a there are some helicopters that use spider control for main rotors too.

The swash plate assembly consists of a rotating and non-rotating plate, normally referred to as ‘stars’  because of their shape, they may be made from steel, titanium or light alloy, with the choice of material being dependent upon the in-service loads that the swash plate will have to withstand.

The non-rotating star is mounted onto the main rotor gearbox shaft by a large spherical ball, housed in its centre, sometimes referred to as a ‘Uniball’. This ball is free to run up and down a slider sleeve, fitted around the shaft. The rotating star is fitted to the non‑rotating star on a bearing, and so can freely rotate about it. The rotating and non‑rotating swash plates move as a single entity in the horizontal plane, and any change in the horizontal angle of the non-rotating swash plate, will be transferred to the rotating swash plate, which will move to the same angle.