# Controls

Most helicopters include four primary controls:  collective, longitudinal cyclic, lateral cyclic and pedals.  The pilot holds the collective control in his left hand and raises it to make the helicopter climb (and lowers it to make the helicopter descend).  The pilot’s right hand grips the cyclic control which he moves longitudinally to pitch the aircraft (longitudinal cyclic) and laterally to roll the aircraft (lateral cyclic).  The pilot’s feet rest on pedals which, when pressed down, yaw the aircraft.  Larger helicopters often have an automatic flight control system (AFCS) to assist the pilot with these controls and reduce workload.

## How the Controls Work

### Collective

In a traditional helicopter, raising the collective increases the pitch of the main rotor blades.  This increases the aerodynamic lift produced by the blades and the main rotor thrust.  Increased main rotor thrust pulls the helicopter up into a climb.  As a side-effect, this increases the engine power which results in off-axis responses.

### Longitudinal Cyclic Control

The pilot moves the longitudinal cyclic control forward to pitch the aircraft nose down, and pulls it aft to pitch the aircraft nose up.  This is also accomplished by pitching the main rotor blades, but is more complicated than the collective control described above.  This control adds a time-varying blade pitch using a swashplate.  We’ll describe this in more detail in the Cyclic Controls section down below.

Moving the cyclic control forward and pitching the nose down also causes the helicopter to increase airspeed.  As the main rotor tilts forward it’s thrust pulls the helicopter forward like a propeller.  Of course, this means less thrust is acting vertically so that the helicopter will descend as well.    Likewise, if already in forward flight, moving the stick aft decreases airspeed and causes the aircraft to climb.

### Lateral Cyclic Control

The pilot moves the lateral cyclic control right to roll the helicopter right side down, and moves it left to roll the left side down (from the pilots point of view, looking forward).   This too is accomplished with cyclic main rotor pitch and is described in the Cyclic Controls section down below.

Moving the control right and rolling the right side down causes the aircraft to gain lateral, rightward airspeed (or reduce leftward airspeed if moving in that direction).  This control is also used, in conjunction with the pedals, to turn the aircraft to a new heading in forward flight.

### Pedals

The pedals control the aircraft’s yaw / heading: when the pilot pushes down on the left pedal the helicopter’s nose turns left, pushing the right pedal turns the nose to the right.  Unlike the other controls above, pedals do not affect the main rotor.  Pedal movements change the collective pitch on the tail rotor.

Tail rotors on American helicopters typically produce thrust to the right (this follows from the fact that main rotors spin counter clockwise, as viewed from above).  When the left pedal is pressed the tail rotor increases (collective) pitch, which increases thrust.  This thrust effectively turns the helicopter's nose to the left.  Likewise, pressing the right pedal decreases tail rotor pitch and turns the nose right.

### Cyclic Controls - More Detail

In accord with the diagram above, we let $$\Psi$$ denote the angle of the blade within a revolution: $$\Psi=0$$ when the blade is over the tail of the aircraft, $$\Psi=90^o$$ when the blade is to the right of the aircraft and $$\Psi=180^o$$ when the blade is over the nose of the aircraft.  We let $$\theta$$ denote the pitch of a blade.  Collective pitch increases $$\theta$$ for all blades, independent of $$\Psi$$.  Cyclic controls, however, make the pitch angle $$\theta$$ change as $$\Psi$$ changes via the equation $$$$\theta (\Psi )=\theta_0 + \theta_C \cos \Psi + \theta_S \sin \Psi . \label{eq:mrcontrol}$$$$

This unusual pitch variation is accomplished using a device called a swashplate, described here.  Since forward longitudinal cyclic pitches the aircraft nose down, you might expect it to govern $$\theta_C$$ – increasing pitch (and hence lift) of the main rotor over the tail and decreasing it over the nose.  However, it turns out that $$\theta_C$$ is primarily governed by lateral cyclic:  $$\theta_C$$ increases as the lateral cyclic is moved left.  This is because blade aerodynamic forces are not transmitted to the helicopter as you might first guess.  Increasing $$\theta_C$$ does increase rotor lift aft, over the tail, but the primary effect is to accelerate the blades upward (relative to the fuselage) over the aft portion of the rotor.  This vertical movement of the blades relative to the fuselage is called flapping.  It turns out that $$\theta_C$$ flaps the blades to a peak height near $$\Psi=90^o$$ (the exact $$\Psi$$ depends on the rotor design as described here).  This flapping means that the main rotor thrust is tilted left, providing a leftward force and a rolling moment.  For many rotor designs, this lateral flapping will also create a moment about the rotor hub adding to the roll moment due to the tilted thrust.

By an analogous argument, moving the longitudinal cyclic aft primarily increases $$\theta_S$$.  Increasing $$\theta_S$$ accelerates blades upward over the right side of the aircraft and results in peak flap up position over the nose of the aircraft.  This has the effect of tilting the thrust aft and increasing aircraft pitch.

## AFCS

Helicopters are not as stable as fixed wing aircraft and can require significant concentration to maintain attitude.  Larger, modern helicopters have automatic flight control systems (AFCSs) to assist the pilot.  These may have several modes from augmentation of pilot inputs (SAS), to attitude hold modes and even “flight director” modes that fly the helicopter along a desired path.  These are all accomplished by changing the same four controls mentioned above (i.e. the AFCS does not use any other mechanism to move the aircraft).

## Off-Axis Effects

The four controls described above primarily induce motion in a specific axis: the collective governs vertical movement, the longitudinal cyclic governs pitch, the lateral cyclic governs roll and pedals govern yaw.  However, each control has some unintentional off-axis responses.  E.g. pedal movements can result in roll and pitch responses.  For more information about this see our article on off-axis responses.