# Helicopter Controls

Helicopters include four primary flight controls: collective, longitudinal cyclic, lateral cyclic and pedals. Though not discussed here, some helicopters provide throttle control for the engine and toe brakes for the landing gear.

The four primary controls are shown in the image below.

## Collective

The collective controls main rotor thrust. As you can see in the image above, it's located to the left of the pilot's seat. Pulling the collective up increases thrust, e.g. to ascend or arrest a descent. Lowering the collective reduces thrust. Let's see how it works.

How does the collective increase thrust? Raising the collective increases the pitch angle of the main rotor blades. To distinguish blade pitch from aircraft pitch, people refer to blade pitch as feathering.

The image below shows the tip of the blade rotate (front edge up, aft edge down) as feathering is increased due to increased collective. The collective control feathers all main rotor blades by the same amount, as opposed to cyclic controls described below.

The feathering angle impacts blade aerodynamics. Specifically, higher feathering—associated with raising the collective—increases lift and thereby main rotor thrust. Increased thrust mostly pulls the helicopter up. As a side-effect, aerodynamic drag also increases with collective/feathering. This increases the engine power and results in an off-axis yaw response.

## Longitudinal Cyclic Control

The longitudinal cyclic controls pitch and airspeed. The cyclic "stick" is located in front of the pilots seat, between the pilots legs, as shown in the diagram above. It is gripped by the pilots right hand and pressed forward to pitch the helicopter nose down / pulled aft to pitch the nose up. Nose down pitch accelerates the helicopter to higher speeds, while nose up pitch slows it.

Like the collective, the longitudinal cyclic works by feathering main rotor blades. However, it is more complicated for the cyclic. Cyclic controls add feathering that changes cyclically as the blades spin around the azimuth. Pushing the cyclic forward increases the feathering angle when the blades move aft— blades on the left side of a rotor spinning counterclockwise (as viewed from above). It decreases feathering when blades move forward (on the pilot's right side). So each blade has a different feathering angle, and that angle constantly changes as the blades spin around the azimuth.

Cyclic feathering is accomplished by tilting the main rotor's swashplate. We’ll describe cyclic behavior in more detail in the cyclic controls section below.

As the main rotor tilts forward, thrust pulls the helicopter forward like a propeller. Of course, this means less thrust is acting vertically so that the helicopter will accelerate down as a side-effect. 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 cyclic control right to roll the helicopter right side down, and moves it left to roll the left side down. This too is accomplished by cyclic blade feathering, further described in the section cyclic controls below. Lateral cyclic is accomplished using the same control stick as the longitudinal cyclic above, but moving it left/right rather than fore/aft.

Moving the control right, and rolling the right side down, causes the aircraft to accelerate right. 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 yaw or heading of the helicopter. When a 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 feather the tail rotor blades. It's analogous to the collective control, in that it feathers all blades on the rotor uniformly (unlike cyclic). The pedals are essentially a collective control, but for the tail rotor.

Tail rotors on American helicopters typically produce thrust to the right. This follows from the fact that main rotors spin counterclockwise (viewed from above). When the left pedal is pressed the tail rotor feathers to increase thrust. This thrust pushes the tail right, which effectively turns the helicopter's nose to the left. Likewise, pressing the right pedal decreases tail rotor feathering and turns the nose right.

## Cyclic Controls: More Detail

Cyclic controls on a helicopter can be confusing. Here we attempt to explain them in detail. We start with some notation that will better enable us to explain afterward.

In accord with the diagram above, we let $$\Psi$$ denote the azimuth angle of a blade within a rotor revolution. $$\Psi=0$$ when a blade is over the tail of the helicopter, $$\Psi=90^o$$ when the blade is to the right of the aircraft, and $$\Psi=180^o$$ when the blade is over the nose of the helicopter.

We let $$\theta$$ denote the feathering angle of a blade (otherwise known as the pitch angle of the blade). Collective increases $$\theta$$ for all blades, independent of $$\Psi$$. Cyclic controls, however, make the 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 "feathering pattern" is accomplished using a device called a swashplate, described here. Since forward longitudinal cyclic pitches the nose down, you might expect it to govern $$\theta_C$$—increasing feathering (and hence lift) of the main rotor over the tail and decreasing it over the nose. However, it turns out that $$\theta_C$$ primarily increases with left lateral cyclic. Let's elaborate.

Increasing $$\theta_C$$ does increase blade lift aft, over the tail. However, the blades are flexible and able to move up/down relative to the fuslage. This up/down movement of the blades is called flapping. Increasing lift on a blade over the tail accelerates it upward to a peak flap around $$\Psi=90^o$$. (The exact $$\Psi$$ depends on the rotor design as described here.)

Flapping at $$\Psi=90^o$$ tilts main rotor thrust to the left, creating a left wing down roll moment. For many rotor designs, this lateral flapping will also create a left rolling "hub moment." Both of these moments are what the pilot expects from left cyclic. As it turns out, left cyclic increases $$\theta_C$$.

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 helicopter and results in peak flapping over the nose. This has the effect of tilting the thrust aft and increasing aircraft pitch.

## SAS, SCAS, and AFCS

Helicopters are not as stable as fixed wing aircraft and can require significant concentration to maintain attitude. Many helicopters include SAS or SCAS systems to ease the control workload.

Large, modern helicopters have automatic flight control systems (AFCSs). These perform a number of useful functions including increasing stability, holding attitude/altitude/airspeed, and even coordinating turns.

Some helicopters have “flight director” modes that fly the helicopter along a desired path. These are all accomplished by changing the same four controls mentioned above. Rarely, the AFCS may also pitch the horizontal stabilizer (e.g. in the Bell UH-1Y).

## 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 produces unintentional off-axis responses.

Collective movements can yaw the helicopter and the pedals can induce roll, just to name two. For more information, see our article on off-axis responses. In some helicopters, the AFCS can minimize or eliminate off-axis responses.

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