Tuesday, July 26, 2011

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.

Monday, July 18, 2011

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.

Thursday, July 14, 2011

Helicopter Flight Controls

All aircraft manoeuvre by rotating about three axes: Longitudinal, Lateral and Normal.  Aircraft roll about the longitudinal axis, pitch about the lateral axis and yaw about the normal or vertical axis.  Like many other aircraft types, helicopters have duplicated control input devices in both pilot and co-pilot positions, although in some helicopters the co-pilots controls are designed to be easily removable to provide seating for passenger.
Helicopter Axes of Flight

To manoeuvre a helicopter three controls are used; a collective pitch lever, cyclic pitch stick and yaw pedals.

Movement of the collective pitch lever will increase or decrease the pitch angle of all main rotor blades, by the same amount and at the same time.  Increasing the pitch on all main rotor blades will increase the total rotor thrust, and decreasing it will have the opposite effect.

The cyclic pitch stick is used to tilt the main rotor disc, forwards, backwards and to the left or the right, or some combination of these.  This will provide a thrust in the direction in which the disc is tilted, and will cause the helicopter to move in that direction.  The tilting of the main rotor disc is achieved by independently adjusting the pitch on individual rotor blades causing them to move upwards or downwards. When cyclic pitch inputs are made the main rotor blades will be subject to an increase or decrease in their pitch angle as they rotate, and so the disc remains tilted in the direction selected by the pilot.

It is normal for modern helicopter engines to remain at a fixed flight idle speed, which is controlled by a fuel governor or computer controlled FADEC (Full Authority Digital Engine Control system), although some older helicopters and some of those below the 5700Kg MTOM(small helicops) weight category, provide a hand throttle twist grip on the collective pitch lever.  In governed or FADEC systems an increase or decrease in the power required is automatically achieved, in systems using a hand throttle it is necessary for the pilot to make adjustments to the engine RPM in response to control inputs, obviously the governed or FADEC systems are more accurate and relieve the pilot of the additional workload imposed by the need to maintain engine, and therefore rotor RPM.

The yaw pedals increase the pitch angle of the tail rotor blades, collectively only, as tail rotors do not require cyclic pitch inputs.the tail rotor is used to cancel out the torque reaction caused by the main rotor.  An increase in main rotor collective pitch will produce more torque, and will therefore require more thrust from the tail rotor to oppose the resultant torque reaction.

In addition to allowing the pilot to counteract torque reaction, the yaw pedals provide a means by which the helicopter can yaw,the nose moves to left or right.  To yaw the helicopter against the torque reaction more thrust will be required, therefore more pitch is applied to the tail rotor blades, producing the necessary thrust.  To yaw the aircraft in the same direction as the torque reaction, it will merely be necessary to reduce the tail rotor pitch and allow the helicopter to be rotated by the torque reaction force.
From this it can be seen that the helicopter controls are very interactive, for example in the hover an increase in main rotor collective pitch will cause an increase in torque, and therefore torque reaction and additional thrust will be required from the tail rotor to oppose any tendency for the torque reaction to yaw the helicopter.
Many helicopters, other than the most basic types, will incorporate electronic systems within the basic control systems to provide automatic stabilising and, in larger types automatic pilot.

Automatic stabilizing systems, sometimes referred to as AUTOSTAB, are used to ensure that the helicopter remains at a fixed height, heading and speed, regardless of any disturbing influences, such as wind gusts.  These systems ensure that the helicopter remains stable, without the need for continuous inputs from the pilot, thereby reducing pilot workload and fatigue.

Generally it is normal for flight control systems from the collective pitch lever and cyclic pitch stick, to the main rotor control actuators, to be of the push/pull tube type. Tail rotor control systems, from the yaw pedals to the tail rotor are normally cable operated systems, incorporating tension regulators, and push/pull tubes at each end, although some large helicopter types will use entirely push/pull tube systems for tail rotor operation
The main reasons for using control cables for tail rotor control, is that the systems are usually longer than those for the main rotor controls and cables provide a weight saving, and the ability to flex with the helicopter structure.

Sunday, July 10, 2011

Types of helicopter layout


The classical layout is also sometimes referred to as the ‘Sikorsky’ type.  This configuration is currently the most common, and has a single horizontally rotating main rotor to provide both lift and thrust, and a vertically rotating tail rotor to counteract torque reaction, and provide directional control.
Examples of this configuration are Sikorsky S61, S76, AS332 & AS225 Supe Pume etc.


Designed to overcome the operating problems associated with tail rotors, the Notar (NO Tail Rotor) helicopter uses the classical design layout, but has a ducted air supply and vertical stabilizers to counteract torque reaction and provide directional control.

A typical example is the MD 600N Explorer.


The tandem configuration provides two horizontally rotating main rotors, mounted at the front and rear of the fuselage, which in some helicopter types overlap or intermesh (the main rotor blades pass through the same arc, but in opposite directions).  No separate tail rotor is required as torque reaction is cancelled by the rotors turning in opposite directions.
Examples of this configuration are the CH 46 Sea Knight and CH47 Chinook


This layout is also characterised by having two horizontally rotating main rotors, except that these are mounted at the sides of the fuselage.
An example of this configuration is the V22 Osprey aircraft, although this is a compound aircraft that offers advantages of both helicopter and aeroplane flight modes.

Side-by-Side Intermeshing

This configuration is similar to the Side-by-Side, but the rotors are generally connected to a common single main rotor gearbox, and rotate at an angle to the fuselage, overlapping or intermeshing into each other.

Co-Axial or Contra-Rotating

This design has two main rotors mounted on separate shafts, but with a common axis of rotation, and the rotors are mounted one above the other, turning in opposite directions.

Saturday, July 2, 2011

Ice and Frost Removing Methods

Prior to flight the aircraft should be inspected to ensure it is free from deposits of frost, ice and snow and when necessary an approved de-icing fluid should be used.
The method of treatment will depend upon environmental conditions and whether the deposit is either ice, frost, snow or slush.

This is best removed using a product called Kilfrost ABC and can be either applied by spray or brush hand method, one application is usually sufficient and should be applied within 2 hours of flight.
Care should be exercised with glazed panels and new paintwork.
Alcohol based fluids are particularly prone to the ‘washing out’ of oils and greases from bearings resulting in an ingress of moisture which could subsequently freeze and jam controls.

Wet snow should be removed using a brush or ‘squeegee’ while light and dry snow can be removed using compressed air.
It is not advisable to use a hot blower owing to the possibility of wet snow freezing.
Moderate to heavy ice and residual frozen snow should be removed with a de-icing fluid and in some instances it may be necessary to spray the aircraft just prior to departure.
Care should also be exercised to prevent fluid from contaminating the aircraft windscreen and cabin windows.

This is the simplest method, however in severe conditions several applications are sometimes necessary, making the operation less cost effective.
The fluid is normally sprayed by hand from a container either pressurised by air or fitted with a hand pump.
This method is used mainly in emergencies or at small airports.

A static unit containing water and de-icing fluid is heated. This mixture is then pumped to a mobile unit which houses a tank, pump and a hydraulically operated boom mounted on a platform and several spray lances.
Hot fluid is pumped from the static unit to the insulated tank on the mobile unit, the proportions of water and fluid being adjusted to suit prevailing weather conditions.

Normal spray temperatures of 70'C and a pressure of 700 kN/mm2 are obtained.
This heat when transferred to the aircraft skin, breaks the ice bond and large areas of ice may be flushed away using the side of the nozzle. Fluid remaining on the aircraft skin being only slightly diluted is effective in preventing ice reforming.

This consists of a hot air blower and is only suitable for frost deposits, care must be taken to ensure residual water is removed on completion of the defrosting.