Helicopter pitch control

A helicopter controls its pitch by altering the pitch angle of individual rotor blades. The system that controls this is called 'cyclic'. Also called "cyclic pitch", cyclic is a vital part of a helicopter's control system, used to convert the lift produced by the rotation and pitch of its main rotor blade(s) into lateral motion (i.e. forwards, backwards, left or right). The helicopter pilot operates this system with the main control stick.

Cyclic is the rough helicopter equivalent to the engines, elevators and ailerons on a fixed-wing aircraft. Pushing the control stick forwards will cause the helicopter to go forwards if stationary, or to slow down if travelling backwards. Conversely, pulling the control stick back will cause the helicopter to go backwards if stationary, or to slow down if moving forwards. Pushing the control stick to the left or to the right will cause a side slipping motion (think strafing) if the helicopter is stationary and a motion similar to a bank if the helicopter is moving forwards or backwards (there's also issues of altitude involved in applying cyclic control, but let's not get carried away just yet).

The simplest way to imagine achieving this would be to literally tilt the main rotor shaft(s) in the desired direction, producing lift that is biased towards a particular direction. This is not practical because of the stress it would exert on any universal joint used, as well as the issue of limiting the tilt travel of the rotor shaft. In practice, helicopter design achieves the same effect by using swash plates. Swash plates tightly control the amount of lift each rotor blade generates at particular points in its rotation, producing the same effect as physically tilting the entire rotor disc without actually having to tilt it.

Before describing further how this operates, a description of the mechanical setup is pertinent. To understand this it is important to note that the pitch of helicopter rotor blades is variable; they can be rotated along the root axis to pronounce or flatten their angle of attack.

There are two main components in the operation of the cyclic pitch system:

A thick metal ring that surrounds the rotor shaft - you can see it if you look at the complicated bit of a helicopter underneath the centre of the main rotor. Sometimes it is square but usually it is circular. It can tilt in all directions but does not rotate. It is supported from below by four vertical rods attached by ball joints, one every 90°. These rods can move individually up or down to alter the pitch of the lower swash plate. At 'rest', the lower swash plate in profile looks roughly like this:

  Rotor Shaft-->|         |
 ___            |   ___   |            ___
| A |--------------| A |--------------| A |
|___|--------------|___|--------------|___|
 | |            |   | |   |            | |
 | |-Rod        |   | |-Rod            | |-Rod
_|_|____________|___|_|___|____________|_|_
|                                          |
|             Helicopter Body              |

The swash plate is represented by dotted lines and each 'A' represents a ball joint. The rods are actuated in response to the main control stick, making the swash plate pivot accordingly.

This is almost a mirror image of the lower swash plate (hereafter termed 'LSP', upper swash plate termed 'USP'). Again it is a metal ring surrounding the rotor shaft. It is attached to each rotor blade by a hinged rod and rests on the LSP via a bearing surface, which means its angle follows that of the LSP. If the LSP tilts, the USP will tilt in the same way.

Each rotor has a small arm pointing out from its root (where it joins the rotor hub) called a 'pitch horn'. This is attached, by a hinge, to the rod from the USP. When the USP moves vertically it will transmit a force to the rotor blades through the rods and pitch horns, causing them to alter in pitch. This crude diagram shows how it fits together:

 ---------Direction of rotation---------->
                         _______ _
Pitch Horn----------->  /_______|_| <---Hinge
       ________________//       | |
      /               //\       | |       
    /  Rotor blade   //  \      | | 
  /                      |      | |
/_______________________/       | | <---Rod to Upper Swash Plate
                                | |
                               /\/\/

The arrangement differs across individual models but the purpose is always the same.

Using the rotor blade shown above, if we apply a downward force to the rod, it will in turn exert a downward force on the pitch horn which will make the rotor blade rotate clockwise along its longitudinal axis, pitching downwards. If we apply an upward force to the rod, the rotor blade will pitch upwards. These forces are transmitted to the USP by the movement of the LSP underneath it, and depending on the position of the two in relation to each other.

There is something that I haven't yet factored in, which unsurprisingly adds yet another complication to the chaotic events that result in rotary flight: gyroscopic precession. This principle is behind those experiments in science museums where you hold a spinning bicycle wheel and it feels so wierd if you try to tilt it. The principle states that any force applied to a rotating object will manifest itself 90° "later" in the direction of rotation. Say you're holding a bicycle wheel that's spinning clockwise so the wheel is on its side, parallel to the ground. If you try to tilt it forwards - away from you - it will tend to tilt to your right: 90° further in the direction of rotation.

This complicates rotary flight, because the helicopter design must take account of it when control motions are applied. If we have a main rotor spinning anti-clockwise, and want to tilt it forwards, we must attempt to tilt it to the right. That way, the tilt will actually occur in the direction we want. All control inputs must be 90° "behind" the desired direction.

This is factored into rotor system design in the way that the rotor blades are attached to the USP. Each rotor blade is linked to a point perpendicular to its own axis. This point depends on the direction the main rotor rotates when operating. Let's take an example where the main rotor rotates anticlockwise - as do those of most modern helicopters - how are the rotors linked to the USP? 90° "later" in a clockwise direction. If we look at a highly simplified main rotor hopefully I can illustrate this.

Main rotor...
                  0°
                  _
                 | |
                 | |
                 |1|
                 | |
      ___________| |___________
270° |_____2_____ O _____3_____| 90°
                 | |
                 | |
                 |4|
                 | |
                 |_|

                 180°

...and the swash plate underneath.  Zoomed in like.
             _____
            /  2  \
            |4 O 1|
            \__3__/

The numbers on the swash plate correspond to the rotor blade that is linked to that position. As you can see, rotor blade 1, in the 0° position, links to the USP at the 90° point, rotor blade 2 at 90° links to the USP at 180° and so on. This means the linkage from pitch horn to the USP isn't just vertical but horizontal as well. This is roughly how it would appear in plan view, barring design differences:

 Hinge to USP linkage (USP not shown)
 |
 |    Linkage
 |    |
 v    V
 _ ________ _
|_|________|_|
           | |
           | |<-- Pitch horn
           | | ___________________________
         \_|_|/                          _\
     HUB  _|_|          ROTOR            \
         /    \___________________________\

How is the force exerted on the USP? It is transmitted to it by the movement of the LSP, via the aforementioned bearing surface. The USP and the LSP contact each other more or less directly, so when the angle of the LSP changes, the USP 'follows' it. As the rotor blades and the USP rotate - remember, they are attached - their elevation changes because the LSP is stationary relative to them. The rotor blades tilt up and down constantly as they rotate; this is the hardest part to get across in text. If we consider a situation where a helicopter's swash plates are tilted in one direction, if you were to plot a trace of the pitch of a single rotor blade as it rotates it would look something like this:

 P| *                                                 *
 I|    *                                           *
 T|      *                                       *
 C|        *                                   *
 H|         *                                 *
  |          *                               *
 A|           *                            *
 N|             *                        *
 G|               *                    *
 L|                   *            *
 E|                        *  *   
  +----------------------------------------------------
  0                        180                      359
               P O I N T   O F   R O T A T I O N

Obviously the trace would be positioned differently depending on the tilt of the swash plates, but the overall trace would not change. It would flatten or expand in the vertical as the tilt of the swash plate were increased or decreased, respectively. If the swash plate were not tilted, the trace would be flat.

I realise this is difficult to convey effectively with text and HTML's limited diagrammatic capabilities, so I will use yet another example: a single rotation of a set of rotor blades and the effect of the swash plate assembly on their individual pitches. In this example using a 2-bladed rotor, the control stick has been pushed forward, giving the effect of tilting the rotor disc forward. The following is a plan view of how the rotor blades are attached to the swash plate:

                       Forward
                          ^
                          |
                       _______     
                      /____||_\      
Upper Swash plate--->//    || \\         
    Pitch horn-------/---->||  \\    Leading Edge  
---------------------____ _||___---------------------
   Rotor Blade 1     ____|_|____      Rotor Blade 2
---------------------   ||      ---------------------
  Leading Edge      \\  ||     //
                     \\_||____//
                      \_||____/

Each of the following steps represents 90° of rotor blade rotation, starting with the rotors in the position shown above. Note that the blades rotate counter clockwise. Recall the earlier discussion of gyroscopic precession if, when looking at this, it doesn't seem quite right.

1. 0/360°
   -The leading edge of Rotor Blade 1 is raised, increasing its angle of attack and therefore its lift.
   -The leading edge of Rotor Blade 2 is lowered, decreasing its angle of attack and therefore its lift.
   
   
2. 90° - In this position, the rotor blades have rotated such that their actuator arms are parallel with the axis of the swash plate's rotation. This has the effect of returning both Rotor Blade 1 and Rotor Blade 2 to the 'rest' position, meaning each rotor blade has equal lift.
   
   
3. 180° - This position is a reversal of the 0° position:
   -The leading edge of Rotor Blade 1 is lowered, decreasing its lift.
   -The leading edge of Rotor Blade 2 is raised, increasing its lift.
   
   
4. 270° - This position is a reversal of the 90° position but the effect is identical: both Rotor Blade 1 and Rotor Blade 2 have returned to the 'rest' position, so again both have equal lift.

The cyclic system creates a flapping motion by the rotor blades as they rotate, without trying to overstate. I say this because the movement up or down by trailing edges of the blades is quite small - probably 1-2 inches at most depending on the individual helo. The effect is that the rotors create differing amounts of lift as they rotate, such that the rotor disc effectively tilts in the same direction as the swash plates are tilted.

Of course, this has an effect on the vertical situation of the helicopter as well. Recall that I mentioned ailerons and elevators. Since the cyclic system pitches the helicopter, this will affect how it moves in the vertical, for more reason than one. For example, if a helicopter were flying forwards and the pilot pulled back on the cyclic stick, this would not only slow down the helicopter but make it gain altitude, since it would pitch upwards. Similarly if the control stick were pushed forwards in the same situation the helicopter would lose altitude and gain speed, since it would tilt forward further.

Since the rotation speed of the rotor blades is constant, using cyclic "takes" lift that would otherwise be used to keep the helicopter aloft and translates it into motion in the desired direction. Thus, lift decreases in proportion to the amount of cyclic control being applied. This can of course be compensated for, to a point, with the helicopter's collective control, but that's another node.

In summary, the cyclic system uses a system of plates surrounding a helicopter's main rotor shaft(s). These can be tilted by a set of four actuator rods, corresponding to the two dimensions of lateral movement. These actuators are operated via the pilot's cyclic control stick. When said plates are tilted, they effect a change in the pitch of the rotor blades as they rotate, such that the rotors create more lift in the direction that the cyclic stick is tilted in and less lift in the opposite direction to the tilt of the cyclic stick. This in turn causes the helicopter to move in that direction.

This system is more or less the same in helicopters past and present, though the mechanism for actuating the lower swash plate may vary (thanks to The Custodian for informing me of that). Bell Helicopters created what seems to be the most significant modification of this design, by the inclusion of a stabiliser bar in the mid-1940s. This is a bar, weighted at either end, mounted on top of the rotor mast that spins with the rotor blades. Connected to the cyclic system, it uses gyroscopic inertia to keep the rotor blades generally horizontal regardless of the angle of the helicopter fuselage (within reason, obviously). The stabiliser bar acts as a damper of sorts and it led to a great improvement in stability; the rotor blades would not follow every tiny movement of the helicopter fuselage - produced by, for example, gusts of wind or up draught - making for easier control.

Of course, this all gets even more fun when the damn thing starts moving in the horizontal. /me cackles

• aerospaceweb.org; "Helicopter Theory - Cyclic and Forward Flight"; <http://www.aerospaceweb.org/design/helicopter/cyclic.shtml>
• (Author unknown); "Other Helicopter Components and their Functions; Swashplate Assembly"; <http://www.geocities.com/flyingmouse1/Chapter_5.html>
• Cantrell, Paul; "Gyroscopic Precession"; <http://www.cybercom.net/~copters/aero/gyro.html>