Why do they make that particular noise?
It isn’t as though other propeller driven aircraft make that noise.
It isn’t the engine-type.
What is peculiar to helicopters that causes the distinctive noise?
The prop-speed?
Bigger prop at lower rpms?
Not all helicopters sound the same and it also depends on where you are relative to the blades. Sitting in the open door of a jetranger preparing to lift it sounds very much like a regular prop plane, but a bit slower and deeper, with the higher-pitched turbine sound in the background.
Standing on the ground as the local news chopper flies 1000 feet overhead and all I hear is that “whop whop whop” sound.
First, helecopters for the most part do not have propellors. Rotary wing aircraft have rotors.
So what noise are you referring to? The heavy, popping “chop chop chop” usually comes from two bladed rotors. Rotors with more blades tend to make a smoother, queter sound. MCSO operates a McDonnel NOTAR helicopter that has a five bladed main rotor and uses engine bleed air for yaw control instead of a tail rotor which makes a far less staccato sound than say a Bell 212. Some piston engine helicopters make a chriping noise from rubber drive belts but those are less common now adays.
Why do Italians hate helicopters?
BTW, changing the speed of the rotors changes the pitch of the sound. When foley artists add the sound of a helicopter to a film, they sometimes work with a 1 second sample of an actual helicopter that is looped as many times as necessary to fill up the proper time space. When the on-screen helicopter changes altitude or banks, they alter the speed of the playback to create the desired difference in the sound.
Most of the noise from helicopters comes from the tail rotor. The tail rotor turns at high RPM and makes a lot of noise. That’s why the NOTAR is so quiet. No tail rotor.
The “pop pop pop” sound that is sometimes heard with helicopters is “rotor slap” or “rotor chop”. It occurs when the helicopter is in a shallow descent, and comes from the main rotors.
I think I might know what the OP is getting at and these answers don’t quite nail it.
The rotors move in a smooth rotary motion. They are not jerking around, like the second hand on a watch. So why do you hear a chop-chop sound? I can see that if you are very close underneath the rotors that you would hear a chop sound every time a rotor passes overhead, but you hear this even when the copter is quite a distance up. Is there some sort of interference pattern in the sound waves eminating from the rotors, like beats or something that causes this? I think this could turn into a physics-of-sound question moreso than a mechanical one.
Are you sure?
The differences between a 2-bladed main rotor and a 2-bladed propeller are usually diameter and rotational speed. These two factors will determine the tip speed. In each case the blade tip has to pass through the wake of the previous blade, which is what causes the noise.
Unfortunately, I don’t know enough specifics about helo rotor blades to calculate tip speed and compare to that of a typical propeller. Anyone?
Im just gonna throw this in…
My radio controlled helicopter has no such noise, more like a fast buzz
Im sure its rotors spin much faster per scale size though.
Because there’s two of them, they’re quite large, and they’re rotating fairly slowly. Main rotor RPMs are only in the hundreds, while regular plane propellers spin at thousands of RPMs. There’s enough time between each blade’s rotation to hear them individually.
As another person pointed out, the more blades a helicopter has the ‘smoother’ the sound the main rotor makes.
Another factor:
The rotor blades’ angle of incidence changes, possibly during each full rotation. It might contribute noise mechanically or through changed air displacement. Could that contribute to the noise?
To answer questions:
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I’m fairly sure other prop-driven aircraft never make that intermittent/staccato noise.
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Italians have no problems with helicopters.
Thankyou all so far and CookingWithGas for
Wanna bet? Go watch some air racing sometime if you can. The T6 Texans have extremely loud prop noise no doubt in part from tip speed that is far higher than was ever intended. The monsterously hotrodded unlimited warbirds are very quiet in comparison because they have sufficient gear reduction to make sure the big props never have excessive tip speed. Strega and Rare Bear don’t even sound like their st raining but a pack of T6s sound like really angry bees.
Woo hoo, a question I am qualified for. As I am currently finishing my doctoral dissertation on reducing exactly the phenomenon you are describing, I feel confident in my ability to answer.
First, the noise depends on flight condition. In standard criuse the primary noise sources are the tail rotor and transmission. The tail rotor makes so much noise due to its high tip speeds and being parallel to the freestream. Thus the wake flows directly through the disk plane resulting in high impulsive changes in the blade loading which produces noise. This is greatly lessened by ducting the tail rotor as was to be done on the Comanche. The NOTAR system, as has been described before, eliminated the problem by removing the tail rotor entirely. Ways to reduce transmission noise are also being developed.
What you are talking about though is the noise you hear primarily in low speed descent (when the helicopter is coming in to land). This is when the helicopter makes its distinctive “whop whop” noise. This results from the tip vortices produced by preceding blades passing through the disk plane and interacting with later blades. The term “blade slap” is archaic and it is now referred to as BVI, or Vlade-Vortex Interaction.
To understand this, recall what happens when an airline takes off on a humid day. You will see these white spining vortices coming off the wing tips. They are alwasy there, you just see them under certain circumstances. They are a byproduct of lift and therefore all wings, including rotor blades, make them. In a fixed wing aricraft they simply fly behind the aircraft and are only a problem to another smaller aircraft that may pass behind. In a helicopter, however, the can be impact, or pass in close proximity to, other blades. When this happens it makes a sharp change in the pressure on the blade, and this is what makes the noise.
Two consequences of this have been discussed. First, the faster the tip speed, the greater the noise (and higher the frequency). Faster tip speend mean more lift. Since the amount of noise is directly related to the strength of the vortex, and the strength of the vortex is directly related to the lift on the blade when it is produced, greater tip speed means more noise. However, you never intentionally change your tip speed, you try to hold it constant. Second, more blades means less noise. If you have more blades, then each of them has to carry less lift, and therefore their vortices are weaker.
The blade pitch does changes continously. This, however is slow enough relative to other forces that it does not directly create noise.
This is a different effect from the amplitude of the noise, it is the frequency of the noise. Humans do not hear all frequencies in our audible range equally well, the main rotor noise is right in our peak audible (and annoying) range, while propellors are far above. The main reason the propellors make less of a magnitude of sound pressure is that they are perpendiculr to the free stream velocity of the surrounding air and therefore their wake passes behind them rather than through them.
Absolutely right. The frequency is MUCH higher, and therefore out of the major annoying range (there is are weighting factors, the most common being A weighting, that take what is annoying to humans into account).
I get it. Ironically a 100% Italian friend of mine is writing the new version of the premiere helicpoter noise prediction code. The name of the code is WOPWOP. His father is disappointed in him.
In my haste to answer, there are several typos (Blade-Vortex Interaction), and I misrepresented one part. Increasing the tip speed does not necessarily increase the lift as you can always adjust it with a change in pitch.
I rountinely work with a Hughey, and it’s noise, when takiing off, primarily comes from the small jet just aft of the cockpit, I think. I’m just a lowly Bo’sun, but this thing packs a loud scream when gearing up.
If you are powering up it is all mechanical noise, not aerodynamic as the blade would be flat pitch so it did not produce lift. Was the noise likely the jet of air itself, or simply that the outlet offered a megaphone to the transmission?
Wow, thanks.
“Archaic”? I think you’ll find that more pilots will use “blade slap” than “Blade Vortex Interaction”. For example, we tend to say “settling with power” instead of “He entered a vortex ring state.”
Excellent dissertation, though.
So flight flight are you going to use the term “whop whop” in your doctorial dissertation? 
And can you briefly describe methods of reducing noise (other than what you mentioned)
Brian
Actually the punchline is, “Because the big propeller goes ‘Wop wop wop wop’ and the little propeller goes ‘Guinea guinea guinea guinea’”.