Ball Bearings

These bearings are used where it is not practical to use plain bearings, and where a high degree of reliability and precision is required.

The advantages are:

·         Low frictional losses

·         Wide range of loads may be accepted

·         Simple lubrication requirements

Ball Bearings

The most common type of bearing used on aircraft.  The ball bearing has many variations in design allowing it to be used in a wide variety of situations/

They carry radial loads and moderate axial loads in both directions and where a high axial load may be experienced, the balls run in a deep groove in the races.Two types of ball bearings are in general use.  The caged type and the crowded type.

Caged Ball Bearings

In general use, for engine applications and for equipment with rotational speeds in excess of 100 rpm.  When used within engine/gearbox casings they are lubricated with engine oil supplied by jets or by splash.  When used outside casings, they are lubricated by the application of grease which may be applied at specified intervals by grease gun, or may be of the pre-packed type where lubricating grease is packed and sealed into the bearing on assembly.

Crowded Ball Bearings

This bearing has filling slots in one or both races and has no cage or separators.  The balls therefore touch each other during operation, hence the term ‘crowded’.  They are suitable only where slow rotation or part rotation (oscillations) are found, and are usually of the sealed or pre-packed type.

Angular Contact Ball Bearings

Accept radial loads, and axial loads in one direction where a single bearing may be used.  For axial loads in both directions  an opposed pair of bearings is often used.

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. 

 

Control Mixing Unit

The mixing unit is required to ensure that all control inputs are transmitted to the main rotor without loss of input or one affecting the other.

The mixing unit will provide inputs to the main rotor actuators, which are derived from a number of different control movements, for example, the inputs that may be transmitted to the main rotor are:

• Collective pitch

• Fore and aft cyclic

• Left cyclic 

• Right cyclic

In single rotor helicopters there are three actuators that control the main rotor, and it is essential that each receives the correct input to ensure that the aircraft response is in the correct sense and proportionate to any pilot/co-pilot inputs.

There are a number of variations of mixing unit design that will be found in various helicopter types, but in its simplest form the mixing unit comprises of 3 bellcranks mounted on a central shaft, the shaft itself is mounted on bearings that allow all 3 bellcranks to be moved. The following diagram shows a typical mixing unit and the axes of movement for cyclic and collective inputs.

 

Collective Pitch Control Lever

The collective pitch lever is used to increase or decrease total rotor thrust. The lever is fitted to the left hand side of the pilot and co-pilot crew seats, like all flying controls its operation is instinctive, pulling the lever upwards increases the pitch on all main rotor blades by the same amount at the same time, and therefore increases total rotor thrust. Pushing the lever downwards will decrease the pitch angle on all main rotor blades by the same amount at the same time, decreasing total rotor thrust.

As its name suggests the collective control is usually in the form of a lever, that is a bar or pole secured at one end and operating through an arc, however, there are variations. One helicopter for example uses a lever mounted vertically, rather than horizontally which operates with a push/pull action, another has a collective control mounted vertically through the cockpit floor moving vertically up and down. Although the term collective pitch lever is widely used there are variations between helicopter types, other common names for this control are simply Collective, Thrust Lever or Thrust Control.

The collective control is usually mounted onto a torque tube, sometimes referred to as a lay shaft. The torque tube is fitted into bearings secured to the structure which allow it to rotate as the collective pitch levers are moved up and down. This type of installation provides two main benefits firstly it allows both collective pitch levers to be joined together, ensuring that they both move whether operated from the pilot or co-pilot position. Secondly, a torque shaft will convert the rotary into linear movement, providing a pushing or pulling movement onto the control system.

Push/pull tubes are connected to a lever fitted onto the torque shaft, these transmit collective lever movement into the collective pitch control system. These movements are fed to the controls mixing unit, before being sent to the main rotor control actuators or servos.In most helicopters a switch box is fitted onto the end of the collective lever, which houses switches and controls for important systems, such as rescue hoists, searchlights and engine trim controls. 

This ensures that these important controls are always readily to hand and can be operated without the need for the pilot of co-pilot to remove their left hand from the collective pitch lever, especially during critical phases of flight, such as hovering.

However, there will be times when the flight crew must remove their left hand from the collective lever, to prevent the lever from lowering and thus reducing total rotor thrust a friction device is fitted. The friction device may be a simple clamp device fitted to the torque tube

Cyclic Pitch Control Stick

The cyclic pitch stick is positioned centrally in front of the pilot & co-pilots seats, and is used to tilt the disc, causing the helicopter to move horizontally in any direction. Like all other flight controls it operation is purely instinctive, moving the cyclic pitch stick forwards will tilt the disc forwards, and the helicopter will move forwards, moving the cyclic stick rearwards has the opposite effect. Movement of the cyclic stick to left or right, will cause the helicopter to move in that direction. 

The cyclic pitch stick is pivoted at its lower end and is connected to 2 push/pull tubes, one transmitting left/right (roll) movements and the other fore and aft (pitch) movements. A yoke assembly allows these movements to be made independently so that only roll or pitch inputs may be made without causing movement of the other, however, simultaneous roll and pitch movements can be made if required.

In fore and aft movement, one push/pull tube transmits movements to the control mixing unit, Side to side movements operate 2 push/pull rods which operate in opposite directions, when the cyclic stick is moved to the left one rod will move forwards and the other rearwards, when the stick is moved to the right the opposite will occur. This is required because there are 2 lateral (roll) main rotor actuators which must operate in opposition to achieve lateral control, whereas, fore and aft pitch movements are achieved by only one main rotor actuator, that uses the fixed or non-rotating scissors as a datum point about which movements are made.

Like the collective pitch lever, the cyclic stick grip will incorporate switches for operation of important systems, these are normally associated with control trimming, autoflight or autostabilisation, cargo release and communication systems.

Tail Rotor Control Systems

Tail rotor control systems can be of 2 types, control cable or push/pull tubes. In early helicopters cable systems were used because of the need to reduce the weight of the longer system, cable systems offering a weight saving of approximately 25-30% over tube systems. However, cable systems suffer from several disadvantages, they require strengthening of the structure because of the relatively high cable tensions, and the steel cables will expand and contract at a different rate to the light alloy structure.

Whilst the latter was overcome with the use of cable tension regulators, cable systems still required more maintenance, and were prone to developing faults. Many modern helicopters, especially the larger types, now use push/pull tube systems for tail rotor control.

Cable systems

The majority of cable systems use push/pull tubes from the yaw pedals to a cable quadrant, from here control cables are used to transfer control inputs through the fuselage and tail boom structure. In very early helicopters the cables were wound around a cable spool, usually 1½ to 2½ turns, which was connected directly to a mechanical screw-jack that turned the motion through 90° and provided the movement to the tail rotor, via a spider mechanism, although this provided a purely manual control system and was generally only used on the light helicopter types.

An alternative to this, especially where hydraulic controls were used, was to position another quadrant just before the tail rotor, and connect it via a push/pull tube to the tail rotor control mechanism, or hydraulic actuator. One of the quadrants would be a cable tension regulator, ensuring consistent cable tensions.

Cables used in a system that used a cable spool could either be of the ‘continuous loop’ type or would have nipples swaged on to the ends of the cable. Those systems using 2 quadrants would comprise of 2 cables, having swaged nipples at each end to ensure positive retention at the quadrants.

Push/pull tube system

In this system, the tubes transfer control inputs from the yaw pedals right through to the tail rotor control mechanism or actuator. Push/pull tube systems are more difficult to route than cables, and are comprised of many more components, many of which could potentially develop faults, but there is less possibility of lost motion developing rapidly within the system, as could be the case if control cables lost tension.