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.

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