A Study on Propeller & Rotor Noise and Reduction of their acoustic intensity

Summary:

Basics of propeller propulsion
Basics of lift by rotar wing
Study of general engine noise
Noise study of the aircraft’s aerodynamic profile (fuselage, wing)
Propeller noise study
Considerations of reductions and modifications
Conclusion: How to lower the noise?

Introduction:

Let’s start by listening to this video, and observe the radical change in the sound after the two helicopters pass.

During the passage of a propeller aircraft or a helicopter, it is a valid question to ask why these types of aircraft emit a noise so powerful and over such a long distance! Without a doubt, we are in fact witnessing a combination of aerodynamic phenomena and complex engineering parameters. We are going to carefully analyze all these phenomena to better understand them, from the study of the motor to the interactions between the pure aerodynamic flow of the aircraft, as well as different existing “filters” that modify the sounds we hear.

While we will certainly concentrate on helicopters, we will also look at some cases of propeller aircraft for which the noise is similar. Also, when we say “noise,” it refers to the acoustic intensity of the aircraft as it is heard by an observer on the ground, and not by a passenger.

The basics of aircraft propulsion by propellers:

In order to move through the mass of air around it, an aircraft must balance its drag with its traction.

We are going to concentrate on propeller-only aircraft, omitting aircraft equipped with jet engines, which we discussed in a previous article.

A propeller linked to an axle or a transmission is made of metal or wood and is oriented according to a very precise angle: the propeller acts like a wing by creating high and low pressures, but is aligned with the axis of the aircraft. The angle of the propeller in relation to the axle is called “pitch.” Think of the pitch as a kind of gear box for aircraft. For example, the neutral position for a propeller is referred to as “feathered” (zero pitch) and it does not generate any traction nor any drag: the propeller is aligned with the axis of the wind.

As with a car motor, adjusting the pitch also allows to adjust the traction force. And, as it is not recommended to start a car in 5th gear, the same is true for trying to take off in an aircraft with too much pitch! For this reason, fixed-pitch propellers are a compromise between low-speed performance and a maximum speed that is not too low. The risk for a fixed-pitch propeller is over torquing of the motor. In a more technical way, the pitch is in fact the distance traveled by the aircraft in a single rotation of the propeller. In the feathered position, for example, the aircraft does not move forward and this stalled position corresponds to the neutral position of a car’s transmission!

The basics of aerodynamic lift by rotor wing

An order to fly, a helicopter must generate lift. This lift is not generated by the airflow over the wings but by the airflow around the main rotor! Whereas an airplane must lift its nose in order to increase the angle of attack of its wings, the helicopter is able to climb while keeping its attitude (and not its altitude, attitude refers to its orientation within the 3 axes). To do this, the pilot moves the collective lever which serves to increase the angle of attack collectively (hence the name) on all rotor blades. This results in the rotating blades acting in the same way as fixed wings: they generate more and more lift and make the helicopter go up.

Now, for the transition, we modify the angle of attack during their rotation. By modifying the angle of the advancing and retreating blades, we modify where the lift is generated so as to create torque where we want it to be. The cyclic lever pushed forward will generate more lift towards the rear and tilt the nose down to move forward. The same principle applies for all 360 degrees of direction. Note the variation of angle of attack through the entire rotation.

To adjust the yaw, we modify the blade angles of the tail rotor. We use the collective effect on these smaller blades to simply pull “more” or “less” on the tail of the aircraft! This creates a lateral force that either makes the helicopter turn in circles or move by the inertial force of the main rotor.

Now that you have the basics, you will better understand why we will discuss the angle of attack of rotor blades and propellers!

The role of piston and turbine engines in general aircraft noise

Reminder: in the case of a typical jet engine serving to generate thrust (a dedicated article on different types of jet engines can be found here) the sound comes from the turbulent flow of exhaust gas and from its potential supersonic speed. Indeed, the gas does not have a smooth, laminar flow and the supersonic speed produces whistling and cracking that is a characteristic of a jet engine. The engines that we are interested in here are those which are non-jet propulsion: they turn a rotor or a propeller which then creates thrust or lift.

Let’s first concentrate on propeller aircraft. There exists two large engine families: piston engines and turboprop. Even though we want to discuss only the engine itself, separating the noise of the engine and the noise of the propeller is complicated.

The principle function of a piston engine is very similar to those we find in our cars. However, the differences remain between the two, such as using magnetos instead of spark plugs or the RPMs that are much higher in aircraft than cars (aircraft engines can turn between 2300 and 500 RPM). Contrary to a car engine, the turboprop is composed of a gas turbine that drives a propeller. Beyond being more expensive, requiring more fuel, and being more complex, the advantage of a turboprop is its performance! It allows propeller aircraft to travel at speeds between 300 and 800km/h. Beyond these speeds it is preferable to use jet engines because the propeller begins to lose its efficiency due to transonic flow (we will get to this later).

A piston engine installed on an aircraft is generally considered 1.5 to 2 times louder than a car engine! This intensity is due to the large fuel consumption of the engine in operation (25 L/h for a Lycoming 0-235-L2A with 188hp) and also because the “silencers” (mufflers for aircraft) are expensive and heavy, so they aren’t always installed on small, general aviation aircraft. In addition, the silencers are large and produce additional drag that needlessly reduces the performance of the aircraft, which is virtually silent once it is stabilized in cruising altitude. Even if these engines seem noisy, the majority of the noise from light aircraft comes from its propeller. They make almost no noise in comparison to the other family of propeller engines: turboprops.

The turboprop is, in fact, a gas turbine, similar to a jet engine. The turbulent hot gas is ejected behind the engine (generating a tiny amount of thrust, contrary to typical jet engines) in a non-laminar flow. Taking into account all sources of noise (turbulent hot air exiting the exhaust, high-speed air in the intake turbine, and mechanical parts in rotation) we get the characteristic whistle of turbines.

As a private pilot, I was able to hear the difference between a piston engine or a turboprop during the approach of these aircraft on the tarmac. Whereas a light aircraft can be heard purring on its final approach, the turboprop can be heard during its downwind leg, base leg, final approach, and deceleration! Except for when we cross the geometric plane of the propeller (we will see why later), we can hear the powerful whistle continuously roaring. The human perception is also different. Where the piston engine emits a sound relatively tolerable at low to mid frequencies, the turbine emits a sound that is both loud and high-pitched, frequencies less agreeable to hear.

One thing to add, even though we will speak of this more later, is that the propeller of a turboprop or a piston engine is not streamlined in the same way as a jet engine exhaust nozzle, for example. In the same way a noise barrier along a freeway helps to reduce the sound intensity, a duct can help make turboprop engines quieter.

Of course, in a turboshaft (the type of engine characteristic of helicopters as you can see in the image below) we use a gas turbine. However, the ejected gas is not used for propulsion!

As with a turboprop, the goal is to set the propeller in rotation, with the small difference that a rotor is mounted on a free wheel that serves to let the blades turn by inertia if the engine is shut down. Indeed, without its engines, an aircraft glides via the air mass flowing around its wings, but a helicopter with a rotor that doesn’t turn will fall like a rock. The rotor is therefore free to rotate and the air that passes around the blades sets the rotor in motion (like a windmill). The energy stored by the rotor during this rotation is then used to slow down the decent by varying the pitch with the collective. This technique is referred to as “autorotation.”

These gas turbines have a very high rotation speed and the transmission serves to reduce the RPMs transferred to the rotor. To give you an example, the engine of a SA342 Gazelle (Airbus Helicopters) spins at 43500 RPMs but the main tri-blade rotor spins at 387 RPMs!

These mechanical movements are loud enough, whether it is the transmission itself or the gear belts, that they add to the overall noise of the powertrain.

The role of the fuselage and wings

What we call “aerodynamic noise” in the world of aviation is the noise caused by the relative wind passing around the aircraft. Indeed, the air will accelerate over the wing, become turbulent around any non-smooth surfaces of the aircraft, and finally converge, creating a vortex behind the aircraft. This source of noise may seem anecdotal, but they are not to be ignored! If we look at this graphic from the ICAO (International Civil Aviation Organization) we notice that the aerodynamic noise (airframe) is not insignificant, whether it is for takeoff or landing.

For example, when a glider flies overhead, we can hear a whistle that is soft and pleasant. This aerodynamic noise that seems majestic is in complete contrast to the sound of a fighter jet coming it at high speed or an Airbus with its flaps and air brakes deployed that quickly become very noisy.

Note: the data for “Fan” only refers to the forward ducted fan of a jet engine or turbofan and not the propellers which produce thrust.

The role of propeller blades in aircraft noise

Rotor blades provoke a characteristic noise that we can separate into several types of different noises: thickness, loading, broadband, vortex and blade interaction, high-speed impulse (HSI), and tail rotor noise.

All of these sources may seem complex, but once they are separated, these sources are very simple to understand! We will now switch over the the world of helicopters, because all the sounds of the main rotor can be compared with those of an aircraft propeller.

Thickness noise:

This source of noise is caused by the movement of a blade (and therefore its form) and can be represented as a displacement of air by the rotor blades. It is mainly guided in the geometric plane of the rotor, which means that we hear this type of noise better when we pass through the virtually infinite reach of the rotor’s horizontal plane. In other words, when a helicopter is coming toward or passes next to us. We hear clearly the sound of the blades during the startup of this Robinson R22, that makes a noise like the roar of wind.

Loading noise:

Loading noise is an aerodynamic effect due to the acceleration and distribution of forces in the air around the rotor blades, and this manifests mainly under the rotor. When we require the rotor to take on more force (low blade speed, high angle of attack to lift a heavy load…) the loading noise tends to increase.

Broadband noise:

Broadband noises are in fact from relatively random sources generated by the engine, such as turbulence ingestion through the engine or the intake of its own wake.

Vortex and blade interaction:

This is one of the principal sources of noise but also one of the most complex. When a blade passes through a vortex left behind by the precedent blade, it creates a variation of its load, generating a jerking force in the air and a sonar pulsation. The direction of the sound depends on the interaction with the blades. However, we can characterize two situations. On the “advancing” side of the rotor (where the blades move from back to front), the interaction noise is directed downward and forward of the helicopter (no matter its moving direction), and on the “receding” side of the rotor the noise moves down and back.

The main parameters of this noise are: the distance between the blade and the vortex (reduced by increasing the number of blades), and the vortex force (dependent on the angle of attack of the blade passing just before it) and the trajectory of the vortex in relation to the blade (dependent on the trajectory of the helicopter).

High-speed impulse:

As you know, to generate lift, the air coming toward the wing will accelerate when it passes over the upper surface of the wing, generating an atmospheric depression. The wing is then pulled from the top and pushed from the bottom.

However, it is this accelerated air that can pose a problem! Let’s take an example of the blade tip that advances at 800 km/h, attached to a helicopter that moves at 400 km/h. The receding blade moves at the speed of 800 - 400 = 400 km/h while the advancing blade reaches the speed of 800 + 400 = 1200 km/h. But in a standard atmosphere, the speed of 1234.8 km/h corresponds to Mach 1: the speed of sound! Considering that the blade rotation speed is constant at 800 km/h (on average), we have defined a “typical” sound barrier for helicopters at 400km/h. Indeed, when a blade approaches transsonic speed, the air accelerating over it risks passing the sound barrier, creating a supersonic shockwave and aerodynamic drag. To learn more about transsonic flow, I invite you to read my Twitter thread on this subject HERE

If a helicopter arrives at this speed, the passage of the blades between transsonic and supersonic speeds provokes small supersonic bangs! These “high-speed impulses” are in fact supersonic booms caused by the speed of the helicopter. In the case where we hear these impulses, it’s that the end of the blade has passed into transsonic speed, but also that the rotor has lost efficiency: the extremities of the blades generate a strong drag, vortices, and shockwaves that bring about vibrations and a loss of lift.

The rare helicopters that jump right out of this “sound barrier for helicopters” have “pusher configuration” propellers and small wings that bear this phase of flight, like the Eurocopter/Airbus Helicopters X3! (C) Airbus Helicopters

The tail rotor:

The case for the tail rotor is particular because the strength of the sound depends on the distance of the listener in relation to the helicopter. Seeing as it has a small diameter and and a high-speed rotation of the blades, this rotor generates a sound at a much higher frequency than that of the main rotor (whose sound frequency is placed directly in the range of frequencies for which humans can sense). If we stand next to a helicopter, the sound that the tail rotor emits is considerable. However, at a farther distance the high-pitch frequencies are muffled by the atmosphere.

This rotor also generates the same sources of noise that was mentioned above because it acts just like a main rotor!

Possible modifications

Numerous natural and artificial filters have come to modify the noise of helicopters and airplanes.

First, as we just saw, the atmosphere is a very good filter. Even though the air propagates sound, it also muffles it over a distance and filters out the most high-pitched frequencies of the tail rotor. One modification of the sound that we can hear very well (and is well-known) is the Doppler effect. The sound of the rotor seems more high-pitched and rapid before passing in front of an observer, followed by a low-pitch and slower sound after its passage.

Now that you know the sources of noise and the atmosphere filters, and before speaking about noise reduction itself, let’s go back to the video from the beginning of the article and try to detect the diverse sources: supersonic impulses, Doppler effect, the difference between the sound in “the plane of the rotor” (thickness noise) and the sound “outside of the plane” of the main rotor…

Did you find them? At the beginning (from 00:00 to 00:10) the rapid clacks are the HSI, the small supersonic bangs. Next, from 00:11 to 00:18 we start to detect the noise interactions between the blades and vortices, along with broadband and thickness noise. Around 00::20, when the helicopter passes, we no longer hear the shockwaves, but just the loading noise. After the passage, we only hear the interaction and thickness noises, coupled with the Doppler effect.

Now let’s look at helicopter noise reduction: how can we create a quieter helicopter without degrading performance?

One simple solution, in theory, is to increase the number of blades! Indeed: more blades allow for a slower rotation speed for the same lift, reducing the loading noise as well as the thickness noise since the blades turn slower. Also, the blades are closer to each other and require less angle of attack. By reducing the rotation speed, we eliminate the risk of encountering critical Mach on the blades and therefore eliminating high-speed impulses.

What’s more, a two-bladed helicopter, in spite of being less expensive and more simple, is subject to risks of “mast bumping.” Two blades also make the helicopter less reactive.

The difference can be blatant; between a two-blade Huey and a three-blade Gazelle, the rotor of the Gazelle produces much less noise!

While it is still noisy, the difference is noticeable! If we analyze the spectrogram of a Huey (top image below) and that of a Gazelle (bottom image below), we notice that the Huey emits sounds on lower octaves but also stronger in general! We also notice the impulses of the Huey two-blade system, while in contrast the Gazelle emits a sound more fluid and linear.

Nevertheless, while this solution may seem simple, the (very complex) mechanics of installing many blades on a swashplate increases its design and production complexity, as well as its price. We understand better why the civil helicopters are not effected!

The military conducts research in order to reduce the noise of their rotorcraft. It’s why we can see prototypes or models in service that have the latest technologies integrated, such as the Boeing-Sikorsky RAH-66 Comanche.

On this aircraft, the noise reduction and the performance reaches the key points of the project requirements: fuselage smoothed out by retractable landing gear, heavily modified blades, rotor and tail enclosed in a fenestron, etc. Let’s analyse a their effects a little!

We’ll start with the blades: there are five because, as we have seen, increasing their number helps to make the helicopter quiet. However, the blade ends and the serrated trailing edge can have surprising effects! The blade ends that are angled like an arrow tip are inspired by the wings of supersonic aircraft. By angling the end of the blade, the blade tip acts like a swept wing, slowing down the shockwaves but also maintaining lift. Also, the profile of the blade (its cutaway view) is inspired by supercritical airfoil design: wings dedicated to the reduction of shockwaves, used, for example, by the A320!

The small “teeth” visible along the trailing edge of the blades help the overall aerodynamics of the rotor. With all these modifications, the rotor is much more silent all while keeping its performance. However, designing and manufacturing brings along a very high cost.

A lot of work has been made on the turboshaft nozzles installed on our modern helicopters. By reducing the turbulent exhaust gases from the engine, we reduce the overall sonar intensity.

One last method for noise reduction is to integrate the tail rotor into a fenestron.

Take, for example, the one pictured here below of the EC135. A fenestron allows for increasing the number of blades on the tail rotor (increasing the number of blades, again and always!) but also protects the blades from debris, protects ground personnel from the tail rotor, as well as many other advantages.

Did you notice the strange layout of the blades inside the fenestron above? With this non-linear spacing between blades, we get different harmonics and blade-vortex interaction, bringing about a new type of noise reduction.

On the other hand, the complex mechanics and the weight of the device leads to a cost increase, weight increase, and also the loading noise of the main rotor! By reducing the tail rotor noise, we risk increasing one of the components of main rotor noise.

Another novel and relatively quiet solution is to remove the noisy tail rotor! But, we have to find a way to keep the helicopter from spinning out of control around itself! While the Chinook or Ka-50 uses two main counter-rotating rotors, there exists yet another solution even more original.

For their MD500, Hughes Helicopters made the choice to remove the tail rotor and replace it with a system of directed cold air and vertical stabilizers. The air is taken in from above the helicopter by a fan, then ejected at the end of the tail to the left side. When the pilot wishes to turn, the amount of airflow is increased or decreased to change direction.

Known for its speed and stability (less drag thanks to the smooth tail and vertical stabilizers), the MD520 NOTAR (NO Tail Rotor) is also very safe, because taking out the tail rotor also removes risk for accidental injuries.

Some owners of the MD520 NOTAR have expressed confusion and declare “Why isn’t such a nice system more accepted and more developed? What’s more, in summer, we can turn on the fan and sit by the tail to enjoy the nice fresh air!”

Conclusion : Are we going to lower the volume?

We have seen that a helicopter is noisy in principal: to be held in the air under a rotating wing requires making a lot of noise, with varied sources and frequencies. Even though there exists ways to reduce the sound intensity of helicopters, the price, complexity, and the added weight are negative factors in their implementation. Numerous helicopters that are more silent than others continue to enter into service, mainly in the military (where discretion is a synonym for survival) or by civil defense and air ambulances (because flying over the neighborhoods around a hospital 24 hours a day is not pleasant for everyone!). Little by little, with the research and the weight reduction of parts, we can hope to see quieter helicopters for the general public in the years to come.

Noise reduction around public airfields has been, for private pilots, a key factor in tensions between local residents and flying clubs. Quieter engine starts, propellers, and aircraft will help to calm these tensions.

The H160 (civil) and H160M Guépard (military), the newborns of Airbus Helicopters, mixes the pairing of fenestron and new technology blades to achieve 50% less noise than that of similar helicopters. All of that paired with a fuselage and empennage that improves aerodynamic performance. And so, the Guépard who is scheduled for delivery to the three branches of the French military (army/air force/marine) could bring along an added military force all while consuming 20% less fuel than the competition and cutting the noise intensity of its helicopter fleet by half.

Did you notice the odd shape of the blade ends? They are inspired by the swept wing design of supersonic aircraft! Listen to the difference of a traditional blade compared to a “Blue Edge” as it is heard from on board and then on the ground.

Thanks for reading, Niels.

PS : Thanks to "Zia" for the translation!