Never completely understood this about helicopters

Thoughts while shaving...

Have you ever asked why a helicopter is so much louder when it approaches you then after it passes you?

Oh sure, I know the official answer is the Doppler Effect, but there must be more to it than this. The Doppler Effect would certainly account for frequency pitch changes, but why overall volume? Most know that helicopters are loud to begin with because their rotor tips are breaking the sound barrier. However, consider the following:

- On the ground, a helicopter is equally loud in all directions
- On the ground a turbine engine aircraft is dramatically louder in front than behind. However, that same engine in flight is far louder behind it than in front. An approaching aircraft in flight makes almost no sound at all until it passes overhead.
- If the rotor tips of a helicopter are breaking the sound barrier on one side, then the same rotor will break the sound barrier on the other side as it rotates.

Yet, the 'whomp-whomp-whomp' of a helicopter can really only be heard when it is approaching. When the helicopter is alongside, or passes directly overhead, the 'whomp' sound almost totally disappears and the only thing you hear after it passes is turbine noise.

Why is this?

Some background - I get to see a lot of helicopters at our place. They're usually flying the power lines south of us. As a result, I often get the pleasure of watching all sorts of pretty dynamic flying such as laterally, nearly backwards, hover and very slow flight. The FLIR inspections interesting to watch, but by far and away the most interesting flights are where they're dropping guys off on the energized lines either with, or without, the little roller carts they use for inspections. Sometimes the guys will just shinny up the lines on their knees. Some brave fellas, those guys are, but the pilots are awe inspiring with their precise control. In almost all cases though, the 'whomp-whomp' sound is only heard when they're approaching, not when they're stationary or departing. There's a little bit when they're stationary, but nothing compared to when they're approaching.

a reply to: Flyingclaydisk

Could it be from the way the rotor/blades tilt to move the helicopter forward?

a reply to: Flyingclaydisk

Look at how noise cancelling headphones work

a reply to: Flyingclaydisk

I was a helicopter mechanic in the Navy and I remember them being loud all the time!

It's definitely due to the doppler effect but it's even more complex with a helicopter then say a siren. If it's sitting on the ground as you said the blades have no pitch angle. When it is in flight the blades AND rotor head change pitch. The rotor head is fully articulated.

I'm sure someone smarter than me can explain how this would effect sound.

Edit found this

To understand the problem — and how it can be addressed — we need to begin with a look at sound itself. As you’ll (perhaps) remember from physics class in school, sound is a longitudinal, mechanical wave, and it must have a medium (a solid, liquid, or gas) to travel through. It cannot, therefore, travel through a vacuum such as in outer space. As a soundwave moves through a medium, it causes a displacement. This results in a variation in pressure: higher pressure with the crest of the wave, and lower pressure in the trough. 

When we hear sound, what we’re actually registering is a change in air pressure. These changes in air pressure cause vibrations in the ear, which are converted into electrical impulses that the brain recognizes as sound. Our hearing is attuned to a certain range of sounds; we can’t hear frequencies that are too low or too high. And at a certain intensity, sound can cause annoyance, what we might call noise.

What’s that noise?

Helicopters generate external noise through several areas. While the engine and transmission certainly generate sound, this is only really heard when the aircraft is up close. “The majority of noise the community is exposed to is aerodynamic in nature, made by the main rotor blades moving through air,” explained Juliet Page, a physical scientist at Volpe, the National Transportation Systems Center. In other words, it is the air itself that is making the noise. 

www.google.com...

a reply to: Flyingclaydisk

Maybe your senses are tricking you?

An approaching sound may be a possible threat so you will naturally pay more attention.
Once identified as non-threatening the senses will lessen their alert level.

Or someone's playing with the volume control when they fly around you?

It might be the sound barrier.

The forward speed of the helicopter adds to the rotor speed on one side and subtracts from the rotor speed on the other. I saw a diagram once that showed this. That is the main limit to forward speed as when the rotor breaks the sound barrier, it looses lift and only a small part of the rotor can break the sound barrier or rotor dammage will occur.

a reply to: FauxMulder

Heh, I'll bet you do think helos are loud all the time!! LOL!

The pressure change makes some sense, but you'd think this wouldn't be pronounced at slow speeds. Higher speeds I could understand, but the same characteristic seems to hold true if they're barely moving.

As for the pitch of the rotors, yes, I understand they are pitched more in flight than on the ground so this part is understood, but I'm still not sure I understand why the sound is different dependent on direction of flight.

ETA - Great info though! Thanks!

a reply to: beyondknowledge

But...all helicopters do it. Plus, this would mean that the helo had to be moving at the exact speed where one side of the rotors were at mach+ and the other side was at sub-mach. Consequently, if the helo sped up or slowed down it would have a dramatic impact on this effect. I had previously considered this concept, but ruled it out based on this.

a reply to: Flyingclaydisk

There's one that flies over our place very regularly. Going one way in the a.m. then back the other way in the p.m.
I like to joke that it's Noel Edmunds going to work!

I hear it coming but I can never locate it by the sound and when I do see it it's miles from where my ears tell me it should be. Very confusing.

a reply to: Flyingclaydisk

It might have something to do with where pressure waves are focused depending on direction, pitch, speed, etc.

I dont know, I can make em go but I couldn't tell ya the physics

a reply to: Flyingclaydisk

I think it's the Doppler effect.
You will notice the same thing with locomotives.

a reply to: Flyingclaydisk

I am not trying to be a dick tracy but as guy you go #1 standing up. As yer pocket buddy moves around doesn't the sound change also? Yes a chopper in in the air and toilet water is water but possibly similar?

a reply to: Flyingclaydisk
I haven't flown a helicopter in nearly 40 years and haven't thought about the physics of rotary wing flight but I'll give it a shot. The whop-whop sound is the "advancing blades" meeting the on-coming relative wind (air generated by forward flight) . It's the smacking of the wind by the advancing blade as the helo moves forward. The noise generated as the helicopter moves toward you is the sound of the "rotor plate" tilted down in front and up in the rear which projects the sound forward and down in front and rearward and up in the rear. This causes a louder noise as the helicopter approaches and quieter as it passes.

When the helicopter is sitting motionless on the ground, the "rotor plate" is flat and the sound is equal all around. As the helicopter is lifted into a hover, the pitch in the blades are increased and the "advancing blade" begins making noise which increases with forward movement.

Most rotor blades do not break the sound barrier and only very short, high RPM rotor blades or very long slow rpm rotors approach the speed of sound. The statement about blade tips are sonic on both sides of the rotor disk is not correct. The rotor system has an "advancing blade" and a "retreating blade" regardless of the actual number of blades. The advancing blade is the blade that is moving forward relative to the motion of the direction of flight. The retreating blade is just the opposite moving aft of the direction of flight. As an example, if the helicopter is flying at 100 MPH and the blade is being turned at 200 MPH the effective speed is 300 MPH and the lift is equal to that speed. On the retreating blade side the blade is still turning at 200 MPH but now the forward speed is subtracted and the lift is only equal to 100 MPH. The effect is called dissymmetry of lift. To counter this hinges are incorporated into the rotor head to prevent a roll-over in flight.

Please understand that I learned helicopter dynamics in 1971 and have forgotten much. Just like the military's infinite wisdom, they trained me to fly helicopters and I spent the next 21 years flying airplanes.

I have a small airport near my house, and the other day a chinook flew directly over it.
Wasn't on the flightradar app.
Nothing to add just thought it was cool lol.

I did actually hear it coming and ran outside just on time to see it go right over.

originally posted by: Flyingclaydisk
Thoughts while shaving...

Have you ever asked why a helicopter is so much louder when it approaches you then after it passes you?

Oh sure, I know the official answer is the Doppler Effect, but there must be more to it than this. The Doppler Effect would certainly account for frequency pitch changes, but why overall volume? Most know that helicopters are loud to begin with because their rotor tips are breaking the sound barrier. However, consider the following:

.......
- If the rotor tips of a helicopter are breaking the sound barrier on one side, then the same rotor will break the sound barrier on the other side as it rotates.

....

Yet, the 'whomp-whomp-whomp' of a helicopter can really only be heard when it is approaching. When the helicopter is alongside, or passes directly overhead, the 'whomp' sound almost totally disappears and the only thing you hear after it passes is turbine noise.

Why is this?

When I first went to work with NASA (more years ago than I care to think about) I spent my first couple of years as a researcher in the Advanced Rotor Systems Research branch. At that time, the particular whop-whop noise made by the Huey helicopter in forward flight was an active research topic, because nobody could completely explain it.

First, some background: When a helicopter rotor is turning, it is the tip that is moving fastest with respect to the air, so if there are going to be any noise effects, it is at the tip where they will probably occur first. When a helicopter is in hover or in pure vertical flight, the tip of any given blade will see the same velocity relative to the air at every location around the rotor disc. As the rotor enters forward flight, that is no longer true. The forward flight results in a relative wind blowing against the motion of the helicopter. On one side of the rotor disc, the blade will be advancing into the wind. US designed helicopters almost always have the rotor turning counterclockwise, so the advancing blade will be on the starboard side of the helicopter. European and Russian helicopter rotors turn clockwise, so their advancing blades are on the port side. Obviously, if a rotor is advancing into the wind on one side of the rotor disc, it is retreating on the other side. In other words, the rotor blade experiences a higher relative wind speed on the advancing side than it does on the retreating side. This also means that the rotor tip on the advancing side is the part of the helicopter that is moving fastest WRT the relative wind.

The amount of lift that a rotor blade will generate is proportional to the relative wind speed, squared. So, if you want to keep the lift on one side of the rotor disc approximately equal to the lift on the other side (to avoid an overturning moment) you increase the angle of attack on the retreating side and decrease the angle of attack on the advancing side. That's why helicopters have cyclic control.

As the helicopter flies forward faster and faster, the windspeed of the rotor tip on the advancing side is going to be the first part of the helicopter to approach the speed of sound (Mach 1). Most helicopters are designed with the intention that the wind speed flowing over the airfoil section of the rotor tip on the advancing side approaches but does not reach Mach 1.

Even though the rotor tip airfoil may not reach Mach 1, it is still in what is called the compressibility region. That means that its relative wind speed is fast enough that all the air molecules can't get out of the way quickly enough to avoid compressing the air in front of the rotor tip. Noise can be characterized as a series of pressure pulses radiating out from a source in all directions. Since the pressure pulse is greatest on the tip of the advancing rotor blade (and therefore lowest on the tip of the retreating blade) the compressibility noise is also greatest on the advancing side. It's also directional for the same reason. A lifting rotor in forward flight will always sound louder than a retreating rotor.

By the way, even though we all knew that the source of the whop-whop noise on a Huey had to be due to something like this, we never could definitively identify its source.

a reply to: 1947boomer

I find all this fascinating and the personal experiences of members is precious.

I once tried a helicopter simulator and on full manual I couldn't even get the thing off the ground. As a kid, I remember going to a local park where people flew RC helicopters... Big ones. newbies used to practice for hours, and that was just to get the choppers a few centimeters off the ground.

Anyway, this thread made me curious about stealth tech and silent helicopters. Apparently, the stealth part is less to do with sound and more to do with their radar footprint. At least until 2010 when new developments called "Bluecopter Technology" were being developed.

Here's the 2010 Wired Article that explains it.

And here is the audible difference between the conventional blade and the "Blue Edge" one.


God knows where they're at now after 11 years or even more advanced SAP programs.

a reply to: 1947boomer

Great info!

Thanks!!!

a reply to: buddah6

Also great info!!

Thanks!!

a reply to: 1947boomer

Okay, so after thinking about this for a minute...your explanation explains the origin of the sound, but I'm still not understanding the directionality of it. In other words, your explanation makes perfect sense, but why do you only hear these sounds when a helo is approaching you vs. going away from you?

Seems like the sound should radiate out in all directions equally. Doppler Effect may affect the pitch of that sound depending on whether it is approaching or departing, but shouldn't affect the overall sound radiation pattern.

a reply to: Flyingclaydisk

science class with Mr. Disk