There is no doubt that a helicopter is a loud machine. MedEvac, police, news, and military helicopters seem to always be in the sky around us, and even inside our homes we can still hear them, but why are they so loud? Surely there must be ways for the manufacturers to reduce their noise!?

The loudest noise on a helicopter is produced by aerodynamic vortices created by each main rotor blade as it rotates through the air. When the following blade hits those vortices’ it creates a loud slap or snap sound. The tail rotor & engines also add to the noise signature of each helicopter.

All around the world, there is ongoing research to make the latest generation of helicopters quieter but in the meantime what is it that creates so much noise in these machines?

What Makes Helicopters So Loud?

There are three main areas on a helicopter that create noise and depending on where you are in relation to the helicopter will depend on which part you hear the loudest:

• Stood Close By – The engine will be the noisiest part of the helicopter
• Helicopter Approaching Overhead – The main rotor will be the noisiest part of the helicopter
• Helicopter Overhead – The main & tail rotors will be the noisiest parts of the helicopter
• Helicopter Passed Overhead – The engine will be the noisiest part of the helicopter
  Source: Airbus Helicopters

To understand how each area creates the noise lets break each area down:

How Do Helicopter Main Rotors Make Noise?

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“Almost everything you hear from a helicopter is aerodynamic noise. Reduced pressure on the top of the main rotor blade draws air upwards producing a vortex. The blade tip vortex is then directed downwards. When other rotor blades subsequently come into contact with these vortices, the ‘chopping’ or throbbing noise that is characteristic of helicopters is produced.” –
Karen Mulleners, DLR Institute of Aerodynamics and Flow Technology

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The DLR Institute of Aerodynamics and Flow Technology has been doing some incredible research in collaboration with many agencies, manufacturers, and universities around the world in noise reduction in helicopters.

Some of their research has really helped to pave the way for designers to create helicopter parts that dramatically reduce the noise they produce.

As noted, the main noise producer on a helicopter is the main rotor. We have all heard the deep thumping sounds of the Bell Huey or Boeing Chinook well before we see them, but what is that sound created by exactly?

As a rotor blade passes through the air is spills vortices off the back and tip of it. Each rotor blade creates two areas of sound:

Blade Vortices:

The vortices created off the back of each main rotor blade produce an aerodynamic condition called BVI – Blade Vortex Interaction. As the vortices leave the trailing edge of the main rotor blade they get hit by the next rotor blade coming round. When the next blade hits each vortex it creates a snap or slap sound.

Because vortices are created down the length of the blade there is a wall of vortices in the path of the next blade. Each vortex that gets hit creates a sound. The culmination of this sound increases its volume.

The fewer main rotor blades there are on a helicopter, the larger each blade has to be to create enough lift to enable the helicopter to fly. These larger blades create larger vortices behind each blade, therefore as each vortex is intercepted by the next blade the louder, and deeper the slap.

This is the reason why the large blades on Huey’s (with only two blades) and Chinooks (with only 3 blades per rotorhead) create such deep sound that travels for miles.

Blade Tip Vortices:

Blade tip vortices are just what they sound like. They are the vortices that are created around the very tips of each main rotor blade. These can be easily seen on a helicopter as it flies in humid air:

These tip vortices create many problems for helicopter designers but the two main sound-producing problems occur when:

1. The tip vortices are intercepted by another main rotor blade, like the BVI mentioned above, and when they interact with the tail rotor.

Below shows a digital simulation of how the tip vortices interact around the helicopter:

2. When the helicopter has long main rotor blades, a main rotor system that rotates fast, or has a high cruise speed, the blade tips can begin to reach the speed of sound on the advancing side of the helicopter. As the advancing blade tip reaches the sound barrier it creates a loud snap, amongst many other aerodynamic problems!

This loud snap is another noise-producing area helicopter designers face in preventing. It is not as common as BVI because designers purposely try and avoid the blade tips reaching this speed because of the high loss of lift it creates when it reaches the sound barrier, but as helicopters are designed to go faster and faster, this problem becomes more applicable.

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How Do Helicopter Tail Rotors Make Noise?

Tail rotors create noise just like the main rotor with each blade leaving vortices and the next blade impacting them. This tail rotor BVI creates noise but at a higher frequency because of the higher rotational speed and smaller length of each tail rotor blade.

The tail rotor is also subjected to disruption from the main rotor vortices as they descend backward and downwards from the main rotor. The vortex simulation model above show’s how the main rotor vortices flow into the path of the tail rotor blades which causes them to have secondary BVI on each tail rotor blade, causing noise.

To help combat this noise there have been several very unique designs to helicopters over the decades that all aim to increase tail rotor performance and cut noise levels. These are the most noticeable designs:

Airbus/Eurocopter Fenestron:

The Fenestron is a trademarked Anti-Torque system used on many Eurocopter, now Airbus Helicopters.

This anti-torque system works by placing a multi-bladed fan type of tail rotor system within a duct in the tail of the helicopter. The duct is integrated into the tail boom and is usually made of a fiberglass skin.

The fan unit is made up of between 8-18 blades, depending on the aircraft model, and is of a much smaller diameter than a conventional tail rotor system.

The Fenestron design allows for several improvements over a regular tail rotor by:

• Creating smaller tip vortices on the ends of the fan blades to reduce the amount and size of each vortex, resulting in quieter snaps as vortices are intercepted by the next fan blade
• Having more blades, the length and width of each fan blade can be reduced thus creating smaller vortices for the following blade to interact with
• The shrouded design helps to channel the main rotor vortices away from the Fenestron fan resulting in fewer vortices being hit by the Fenestron fan and creating noise

MD Helicopters NOTAR

The patented NOTAR (NoTailRotor) system from MD helicopters does away with the conventional tail rotor altogether.

The NOTAR system comprises of an engine-driven, variable pitch fan that is mounted perpendicular inside the tail boom to produce large volumes of low-pressure air that exit through horizontal slots along the right-hand side of the tail boom and via a rotatable ‘Jet Thruster’ at the end of the tail boom.

As power is increased when the collective is raised, the pitch on the fan blades is increased to produce more airflow to counteract the increased torque.

This system uses utilizes the Coanda effect to create an anti-torque to maintain yaw control of the helicopter. This system is very similar to the Airbus Fenestron as it uses a multi-bladed fan for creating airflow, the added benefits to this design are that the fan is installed inside the tail boom allowing it to stay completely isolated from any main rotor vortex interaction and helps to muffle or reduce the noise generated by the fan BVI.

Bell Helicopter EDAT

The EDAT or Electrically Distributed Anti-Torque system is the latest design development from Bell Helicopter.

The 4 fan unit uses independently controlled motors that adjust their speed to control the thrust they produce. Supplied by two transmission-driven electrical generators, the motors adjust the speed of each 4 bladed-propeller to match the anti-torque required by the helicopter.

This EDAT system helps to reduce noise in the following ways:

• 4 small blades on each motor create small vortices for the next blade to interact with
• Constantly changing speed changes the size of each blade vortex, thus reducing is the volume of snap when intersected
• Minimal propeller rotation at low anti-torque requirements creates fewer vortices produced
• Shrouded fan casing helps to divert main rotor vortices away from the 4 propellers

How Do Helicopter Engines Make Noise?

There are two types of engines used in helicopters. The piston engine, used in smaller helicopters up to around 4-5 seats, and then the Turboshaft Gas Turbine engines which are used in all sizes of helicopters.

The piston engines generally do not produce much noise pollution. With attached mufflers, they are not much different to an older car when stood next to a running helicopter.

The gas turbine engines are a whole different animal. When a helicopter is running and about to lift off, the noise levels can be well over 100db which can really do damage to a person’s hearing, even for a short period, let alone a pilot with decades sitting inside of one!

Because of this, when I fly I wear:

• A pair of custom-molded earplugs from Westone (AviationSurvival.com Link)
• My Evo Helmet (AviationSurvival.com Link), or
• My Bose Headset with Active Noise Reduction (Amazon.com Link).

The high-frequency noise and decibel levels created by gas turbine engines really do warrant hearing protection.

Helicopter engines produce the majority of their noise in three main areas:

• The Air Intake
• The Power Turbine
• The Exhaust

Air is drawn into the front of the engine by a compressor or series of compressors. The volume of air required by even the smallest gas turbine engine is phenomenal and the noise of the air going in, coupled with the noise of the compressor squeezing the air results in very high-frequency sound levels.

As air exits a helicopter engine it is passed through the Power Turbine. This draws the energy out of the passing airflow to turn the main transmission. These power turbines rotate at insane RPM. The power turbine in the AS350 B2 Astar that I fly turns at over 51,000 rpm at full power!

This is a major producer of high frequency noise from the engine!

The last noise-producing area is the gas exiting the exhaust system. The exhaust on a helicopter engine is larger and creates a lower tone in its noise pollution, but because of the huge quantities of gas exiting, it does create large amounts of noise.

Due to the nature and speeds of how the gas turbine engine operates designers are really at the end of the envelope of noise reduction. It is going to take the next leap in powerplant technology to reduce the noise from a helicopter engine!

How are Helicopters Being Made Quieter?

As pressure for quieter helicopters is coming from all angles the job of the designers to produce quieter machines is right up at the top of the specification list!

The quieter a helicopter can be made, the more markets it will appeal to leading to better sales. Here are just some of the ways in which helicopters have been designed to reduce their noise footprint:

More Blades

More main and tail rotor blades mean the blades can be smaller. Smaller blades produce smaller vortices that make less noise when intercepted by the next blade.

Shorter blades mean the tips travel slowly further reducing the blade tip vortices.

Slower Main Rotor RPM

Over the decades engineers have dabbled with slowing down the main rotor to reduce the size and amount of vortices being produced along the blade and especially at the blade tip.

The current development of hybrid/compound helicopters like the Airbus/Eurocopter X3 have been testing the slowing of main rotor rpm when at higher cruise speeds to prevent the main rotor tips from breaking the sound barrier.

As of yet, there is no helicopter in production that has a varying-speed main rotor. Varying the speed is a complex area that changes many aerodynamic and vibration conditions, but as technology and design evolve, this may be a feature we see in future helicopter designs.

Active Rotor Blades

An area that is getting some current testing and development is called ‘Active Rotor Blades’. One of the pioneers leading the way in this field is the DLR Institute of Aerodynamics and Flow Technology in Germany.

The design uses multiple swashplates to control the pitch of each individual rotor blade during its rotation around the helicopter.

In a conventional helicopter, there is only one swashplate and the rotor blades are controlled as a group, but by using a multi-swashplate system the engineers have been able to independently control each rotor blade leading to a noise reduction by 30% over the flight envelope.

This system is still in the development phase but it would not surprise me to see this or a derivative coming into production over the next few decades.

Blade Tip Design

You may have seen this already if you have a keen eye when wandering around airshows etc. Blade tip design has created some funky looking main rotor blades over the last few decades all with the aim of decreasing rotor tip vortices and their associated aerodynamic problems.

One of the major problems created by the tips of main rotor blades is called ‘Compressibility’ and it leads to increased noise, drag, vibration, power reduction, and blade twisting. By altering the shape of the blade tip engineers have been able to reduce the effect of compressibility to allow the blades to work more efficiently and produce less noise.

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If you wish to learn more about blade compressibility you can find a great article written by Phil Croucher Here at HelicoptersMagazine.com

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Tail Rotor Design

As mentioned above in the tail rotor section, helicopter manufacturers are constantly looking to improve the design of the anti-torque system on their helicopters.

Whether it be a ducted fan, electric motor, or no tail rotor system at all this area of helicopter design will always lead to some interesting innovations.

Flight Profiles & Procedures

One of the simplest ways to reduce helicopter noise is to alter how it is being flown. Certain flight maneuvers cause the noise produced by the main rotor to be extremely loud and this is actually one of the main reasons for noise complaints when flying over populated areas.

The noise created by Blade Vortex Interactions are really prominent during approaches to land so by adjusting the approach to have a steeper angle this can dramatically reduce a helicopter’s noise footprint. This is just one example.

Yet another testing program from the DLR Institute of Aerodynamics and Flow Technology in Germany has led to recommendations for helicopter manufacturers and operators to adopt:

Lower rotor noise thanks to a modified approach path – DLR on the way to the silent helicopter

To Finish

Helicopters are noisy machines and there is no getting away from that for the near future. Whether it is the noise created by the blade vortex interactions, blade tip compressibility, main rotor vortex interaction on the tail rotor, or the power turbine within the engine, the noise is there.

But by trying to reduce the noise bit by bit on every component an overall noise reduction of the helicopter is welcomed news to those who live under active helicopter flying areas!

Further Reading

If you would like more information on articles related to helicopters, please take a read of the ones listed here:

• How Do Helicopter Engines Work? Your Ultimate Guide
• Do Helicopters Have Autopilots? Depends If You Have $$$!
• Can Helicopters Fly In Snow & Ice? This May Surprise You!
• Can You Land a Helicopter Anywhere? What Will Get You Busted!
• What Is A Helicopter Blade Made of? It Used To Be Wood!

Flying for hours across the country in a helicopter can mean sitting in one position without being able to take hands and feet off the controls. By the end of the day, I practically flop out of the pilot seat! Oh, I wish my helicopter had a hands-off autopilot, and that leads to the question: “Are there any helicopters that have autopilots?”

Most large helicopters come equipped with automatic flight control systems to fly and navigate the aircraft automatically. Smaller, less complex automatic flight stabilization systems can be installed in smaller helicopters if space, weight, power, and budget allow.

A helicopter is a very complex aircraft to fly and because of the aerodynamics involved they are known as ‘Dynamically Unstable’. What this means is that when a pilot takes their hands off the cyclic (main control for pitch and roll), the helicopter will want to pitch and roll by itself, placing the helicopter into dangerous flight conditions.

To have an autopilot that can overcome this instability, fly the aircraft, monitor it, and navigate requires a very, very complex set of avionics, sensors, and flight control actuators. As you can imagine this is not cheap or lightweight!

Let’s take a look at all you need to know about helicopter autopilots…

Can Helicopters Have An Autopilot?

The systems required to allow an autopilot to be used in a helicopter are complex, take up quite a bit of room, and are very expensive. Because of this, helicopters of 4 seats or more are the only ones that usually qualify for having an autopilot system installed.

There are different levels of autopilot complexity and capability and the size of the helicopter and its primary usage will dictate what level of autopilot system will be installed when requested by the owner.

The most basic autopilot systems will give basic flight stabilization, hold a set altitude and heading and provide the pilot visual routing and approach guidance on a primary flight display (PFD) in the cockpit if coupled to a suitable GPS.

A great example of this is the Genesys Aerosystems HeliSAS Autopilot system fitted in the Robinson R44 & R66 helicopters. With an Aspen 1000H Primary Flight Display, this system can be installed for around $60,000.

When helicopters require more features and capability from an autopilot system they become integrated into very complex, whole-helicopter avionic systems that are very expensive, but they allow helicopters to be flown very precisely with very little pilot interaction.

These kinds of systems you will find in the majority of twin-engine helicopters like the Airbus H135 all the way up to the Sikorsky S92.

Although an autopilot system would be welcomed by many pilots, no matter the size of the helicopter, they are really only suited to helicopters that are large enough, powerful enough, and have digital avionics systems.

When Are Helicopter Autopilots Used?

Helicopter autopilot systems really shine when the flight path of the helicopter can be taken care of to allow the pilot/s to concentrate on other tasks like:

• Flying IFR in the clouds or at night
• Long-distance flights to alleviate pilot fatigue
• Provide smoother, accurate flight paths
• Allow pilots to alter navigation or communication settings
• Allow for stabilized departure and approach profiles in poor weather
• Maintain accurate route tracking for increased time and fuel management
• Maintain accurate positioning when flying in very busy airspace

Flying long distances, IFR, and at night are the main uses for using an autopilot on a helicopter. By allowing the helicopter to do the flying, it allows the pilot more mental capacity to monitor the flight and prepare and configure the helicopter for the next phase of the flight.

To give you some idea of what a complex helicopter autopilot system can do here is an example:

The pilot can lift the helicopter into a hover and begin to take off down a runway. At around 40-60kts (model dependent), the pilot can press a button and the helicopter autopilot system will take over. It will then automatically climb up to the requested altitude that was set by the pilot, it will accelerate to the requested speed, set by the pilot, and fly a complete routing as programmed into the GPS by the pilot only with a couple of button pushes!

When the pilot is ready for the approach and landing, they will input the instrument approach procedure into the navigation system, the autopilot will pick up the instrument approach, turn the helicopter onto it and begin the approach down to the runway.

With another click of a button, the helicopter will begin to automatically level off at 150ft above the ground and slow down so it flies at 60kts & 50ft over the runway. With another button press, it will automatically come into a hover and by pressing the thumbstick on the collective control the helicopter will land. No pilot control is needed on the controls!

Pretty impressive stuff!!

Many years ago I heard a rumor that some pilots flying out to the offshore oil and gas platforms in the North Sea were having an unofficial competition to see who could do a specific trip with the least amount of time with hands on the controls.

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From wheels up to wheels down the record was something around the 20-30 second mark for the entire flight!

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Are There Different Types Of Autopilot Systems?

There are three basic forms of autopilot systems that can be installed in a helicopter:

1. SAS – Stability Augmentation System
2. ATT – Attitude Retention System
3. CPL – Coupled System

SAS

The SAS type of system is the most basic. In its simplest form, SAS is a stabilization-only system. The pilot is still required to hand fly the helicopter but the system will help to smooth out the pilot or weather-induced oscillations.

The SAS system uses basic sensors and electronic processing to monitor the aircraft’s attitude and the speed at which the attitude changes. It then sends electrical signals to actuators (integrated electric motor & gears) to move the flight controls to help dampen and smooth out the flight path.

SAS will not provide any auto-flight capability or visual navigation guidance to the pilot.

This type of system will be fitted to the smallest helicopters, usually consisting of 4 seats or more, and controls the pitch and roll axis.

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ATT

Attitude Retention is a system that keeps the helicopter in its current flight state when activated. It also controls the pitch and roll axis of the helicopter.

If it is activated in straight and level flight it will keep the aircraft there. If it is activated during a 10° right-hand turn it will keep the helicopter in a 10° right-hand turn. If wind or turbulence try and move the helicopter away from the set flight regime it will move the helicopter back to it.

This is a hands-off type of flight control system but it also allows for simple overriding control by the pilot. Pilots can change the current flight condition by either:

• Pushing against the flight controls to set a new regime then letting go and the helicopter will return to its current state of flight albeit on a new heading or altitude, or
• Moving a ‘Trim’ button on the cyclic to set a new heading or pitch attitude, or
• The pilot can press a button on the cyclic which deactivates the ‘Force Trim’ holding system, adjust the aircraft s flight attitude, then release the Force Trim button to now set the new flight regime for the ATT system to maintain

This type of system was the earliest form of a helicopter autopilot. It allows for hands-off flying but has no auto navigation display guidance or flight control commanding. More complex autopilot systems use this ATT system as their foundation on which to build the fully coupled auto flight systems.

CPL

This is the system that many people think about when they hear ‘Autopilot’. It is the most advanced autopilot system and contains many parts to allow fully autonomous flight once configured and activated.

A Coupled autopilot system will take in information from many sensors and provide visual navigation guidance to the pilots who can either fly the aircraft manually following the visual guidance cues on instruments or digital fight displays or when activated, the autopilot system will autonomously fly the aircraft ‘coupled’ to the same navigation guidance.

A coupled system comprises of two main parts:

• Autopilots – Provides the aircraft stability (mentioned above)
• Flight Director – Provides navigation guidance

Within the Flight Director are many systems that allow for fully automatic flight:

• Sensors
• Pilot Selection & Activation Controls
• Central Processing Unit/s
• Flight Control Actuation

These systems work together and are commonly referred to as the AFCS – Automatic Flight Control System. In the next section, I will try and describe how one of these complex systems works to provide fully automated flight.

How Do Helicopter Autopilots Work?

When you are requiring a helicopter to have a fully coupled autopilot system you are going to need deep pockets as the systems I am about to go through are not cheap, especially when it comes to aviation! I swear manufacturers add on an extra zero the second it mentions anything aviation related!

For a full AFCS auto flight system to work seamlessly it will need the following:

Sensors

Sensors are placed all over the aircraft to gather information and send it to the central AFCS electronic processing unit/s. These sensors are used to monitor things like airspeed, altitude, GPS position, flight control positions, electric actuator positions, rate of climb, bank angle, and hundreds of other parameters.

These sensors give the central processing unit its data to know where the aircraft is and what everything onboard it is doing. These are like the senses we all have – sight, smell, taste, hearing, balance, touch etc.

Without this information, the AFCS would not know where the aircraft is to do be able to do anything with it.

Pilot Commands

The AFCS will do nothing unless it is activated and told what to do by the pilot/s. The way this is done is by having a series of buttons and numerical selector knobs in the cockpit.

These Guidance Controls Panels allow the pilots to activate the auto flight system called the Flight Director when required and to also tell it which flight parameters the pilots wish to be automatically controlled.

The most common flight parameters used by the pilots are:

HDG – Heading: When pressed, this is used to fly the helicopter on a set heading. Mainly used when receiving vectors from air traffic control. The pilot selects the required heading via a selector knob and the helicopter will do a Standard Rate Turn (3° Per Second) and roll out on the requested heading.

NAV – Navigation: This is pressed when the pilot wants the aircraft to automatically fly the planned routing via GPS waypoints, navigation beacons or published airport departure procedures. The routing is entered using an MCDU – Multifunction Control Display Unit.

ALTA – Altitude Aquire: When pressed, this is used to climb or descend the helicopter to a set altitude. The system will set the helicopter in a climb for instance and automatically level off to the altitude set using a numerical selector knob.

ALT – Altitude Hold: When pressed, this is used to maintain the current altitude of the aircraft. Once ‘ALTA’ has captured and leveled off it will switch to ALT mode. This is mainly used when in cruise and different altimeter settings are received from ATC or weather reports. The helicopter will auto-adjust its height to maintain the pilot selected barometric altitude.

APP – Approach Mode: When flying into an airport using a published instrument approach procedure the pilot can select APP mode to allow the helicopter to acquire the approach flight path and automatically fly the helicopter down to the runway via the published routing.

IAS – Indicated Airspeed Mode: When pressed, this will accelerate or decelerate the helicopter to the required airspeed set by the pilots. This is done using a selector knob/button to set the target speed and when the value is changed, the helicopter will automatically alter the speed to meet the desired setpoint

There are many other functions, but for this article, the pilot will generally want the aircraft to fly under NAV, ALT & IAS. Then when flying around an airport ALTA, NAV, APP & HDG are commonly used.

FMS – Flight Management System

The Flight management system is exactly what it sounds like. It manages the flight parameters requested by the pilots and is part of the Central Electronic Processing Unit.

Within the FMS there are usually two independent autopilot systems to act as redundancy. The first one activated will become the master, and the second one will be the slave. They work together as a pair but if one autopilot fails, the other will automatically take over to 100%.

Central Electronic Processing Unit

This is the brain of the automatic flight system. It takes in all the data from the sensors, FMS & cockpit buttons and processes it using complex software code and algorithms to issue digital commands to the pilots’ flight displays and electric actuators that move the flight controls.

As the commands are issued to the electric actuators it constantly monitors the data coming back from the sensors to ensure the aircraft is doing what it has told it to do and it automatically makes any changes required.

Just like most things in a complex helicopter, there are two of them that work together as a pair but if one fails the other automatically takes over to ensure control is not lost.

On the Leonardo AW139 these central electronic Processing units are called MAU1 & MAU2 (Modular Avionics Units) and are mounted in the nose of the helicopter. They are working on their own, monitoring the other to ensure the operation and syncing data between one another to ensure a seamless transition if one fails.

Electrical Actuators

The electrical actuators do the physical moving of the flight controls that the auto flight system requests. They turn the digital signals sent from the Central Processing Unit to operate motors and gears, called actuators, to move a shaft up and down or rotate it back and forth.

Most actuators used on a helicopter are linear and ‘Dual Type’ meaning that there are two actuators joined together in one unit and each independent autopilot system controls one part of the actuator at 50%. If a failure occurs in one of the autopilots, the second autopilot will begin moving the actuator at 100% to account for the other failed system.

Above, you can see that the pilot can move the Main Rotor Servo control rods via the Cyclic, and the Dual Linear Actuators can also move the main control rods via the connecting bellcranks.

This allows either the pilot or the automatic flight control system (AFCS) to control the helicopter, but it also means the pilot can override the automatic system at any time just by grabbing the controls.

The automatic flight control system works by having a Dual Linear Actuator connected to every control rod that goes to a flight control hydraulic servo on the AW139:

• One for Pitch
• One for Roll
• One for Yaw

By issuing commands to the dual linear actuators, the helicopter can be controlled on each axis. This is a 4 axis AFCS system. The fourth axis is height. This is when the Collective control is raised and lowered by the pilot or AFCS, both the Pitch and Roll actuators operate to command the hydraulic flight control servos to raise/lower the swashplate ‘collectively’.

Flight Control Servos

These are the muscles of the helicopter! Hydraulic servos are what connect to the swashplate and slider of the main and tail rotors to change the pitch of the rotor blades.

This is like the power steering of your car. Larger helicopters require huge amounts of force to overcome the aerodynamic loads placed on the rotor blades and the only way to control the helicopter is to use hydraulic servos.

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The Sikorsky S92 has a dual hydraulics system pressurized to 4000 psi / 275 bar!

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The push/pull flight control tubes that are linked to the pilot’s Cyclic, Collective & Pedals move pilot valves in the hydraulic system that tell the high-pressure hydraulic servos how to move.

Most helicopters have 3 hydraulic servos to move the main rotor swashplate and the larger helicopters usually have 6 – each servo doubled up for redundancy running off two separate hydraulic pressure systems.

The AFCS system is connected in line with the push/pull tubes to move the pilot valves in the hydraulic servos. No matter if it’s the pilot or the AFCS actuators moving the push-pull/tube, the hydraulic servo knows no different. Whenever the control tube moves the pilot valve, the hydraulic servo moves and alters the pitch of the rotor blades.

This is how the AFCS is able to control the helicopter to guide it when coupled to the Flight Director.

To Finish

Helicopters can be fitted with autopilot systems providing there is enough spare weight, space, and budget to install a system. There are many types of autopilot systems on the market and having an autopilot in the helicopter really can reduce the pilots’ workload to allow much safer flight, especially when flying IFR and at night, even more so if it is a single-pilot operation.

Having flown helicopters with AFCS and ones without, they both have their pros and cons, but I know that when I fly long-distance flights or the weather is getting worse, the option to activate the autopilot systems would be very welcomed!

Further Reading

If you found this article interesting and would like to keep reading, I highly recommend the following articles from my blog:

• How Do Helicopter Engines Work? Your Ultimate Guide
• How Do Helicopter Controls Work? Pilot Tells All!
• How Do Helicopters Turn? Easy Yet Complex!
• How Much Can a Helicopter Lift? 20 Helicopters Compared!
• How High Can You Go In a Helicopter? Can You Land On Mount Everest?