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 stick which he moves longitudinally to pitch the aircraft (longitudinal cyclic) and he moves 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 and drag on the main rotor, which lifts the helicopter and increases the engine power (the engine must overcome the aerodynamic drag on the blades to keep the main rotor spinning at the same speed).

Longitudinal Cyclic

The pilot moves the longitudinal cyclic forward to pitch the aircraft nose down (and pulls it aft to pitch the aircraft nose up), from the pilot’s point of view (POV).  This is accomplished by pitching the main rotor blades, but for cyclic controls the pitch provided to a blade varies as it revolves around the azimuth.  We’ll describe this in detail in the Cyclic Controls section down below.

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

Lateral Cyclic

The pilot moves the lateral cyclic right to roll the helicopter right side down (and moves it left to roll the left side down), from the pilots POV.   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 effective spins the helicopter counter clockwise (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 $$$$\theta (\Psi )=\theta_0 + \theta_C \cos \Psi + \theta_S \sin \Psi . \label{eq:mrcontrol}$$$$

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 aft rotor lift, 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).  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 roughly 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 an automatic flight control system (AFCS) 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, there are some unintentional off-axis responses.  We’ll discuss these below.

Collective

Raising the collective causes the engine to provide more torque to the main rotor, which imparts more reactive torque to the aircraft causing it to yaw nose right.  Pilots instinctively press the left pedal when increasing collective to counter this.  Also, in forward flight, raising the collective can increase lift more on the advancing side of the rotor than the retreating side.  This flaps the rotor up over the nose (reread the Cyclic Controls section if this sounds wrong) and results in a nose up pitch.   The peak flapping will not be exactly at $$\Psi =180^o$$ so some roll motion may occur as well.

Cyclic

As discussed earlier, the rotor usually reaches peak flapping roughly $$90^o$$ after peak pitch.  This “azimuth offset” varies slightly, effectively causing some pitch with lateral cyclic and some roll with longitudinal cyclic.

Pedals

The tail rotor is normally located above the aircraft’s center of mass.  This causes tail rotor thrust to create a roll moment.  Pressing the left pedal (increasing tail rotor thrust) will roll the helicopter - right side down - slightly.  It will also impart a slight lateral (rightward) motion.