Helicopters - Exhaust or not?

SharpByCoop

SharpByCoop

Enjoying the discussions
Knifemaker / Craftsman / Service Provider
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Oct 8, 2001
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I was disturbed once again by a REALLY LOUD helicopter that was flying fairly low over our subururban neighborhood (1500 ft?). It was painted white and didn't lok military. The sound of the exhaust coming at me was deafening and pervasive. "Doesn't the FCC regulate the engine noise on these things?!"

As it went away from me, the sound was noticeably quieter....

The thought occurred to me: Was this the exhaust, or was it multiple sonic booms from the props?

Educate me.

Coop
 
The engines can be loud, but are typically drowned out by the rotor blades when in the air (causing the whop whop whop sound).

Fly a teathered weather balloon from your house. Rotor jocks love weather balloons! Just kidding!!! Please do not do this without permission from the appropriate authorites.
 
Coop, it's the danged news choppers that piss me off - it's dawn and the traffic report is more important than a citizen's ability to sleep with the bloody thing hovering a thousand feet above one's house...:grumpy:

What I'm really suprised about is nobody actually gets pissed off enough to walk outside and send a little buckshot their way!!! :mad:


:D
 
actually the sound you are hearing is the compression of the air between the main rotor and the tail rotor and the tail boom. As the air is "caught" between the two points, it compresses and then releases as the rotors pass. Sort of like an imaginary paper bag being popped every time. The sound change has to do with the doppler effect and the movement of the air as it releases.

Remember Bay watch? they always had the USCG flying the Aérospatiale
HH65 Dauphine's They had an enclosed tail rotor which drastically reduced the sound signature, When a hh65 flies over, you often can not hear it until it is directly overhead, A blackhawk or a jetranger are often heard at least two miles away.

There are new designs which use ducted air like a Harriers control vents to counter the torque of the main rotor spinning. NO TAil Rotar, or NOTAR designs are very quiet when compared to older designs, to the point of almost being stealthy when flying over head.

http://www.helis.com/Since80s/h_h65.php

If you are getting repeated awoken by low flying choppers, call your local FAA office. There are standards that these corporate choppers must maintain and a lot of time no one monitors them until they get a complaint.

We live near a National headquarters for a huge medical company and they have choppers flying in and out all the time. They used to fly like a crow from the offices to the local airport where they had civil jets waiting for them. After all the neighbors signed a complaint to the FAA, the choppers were rerouted away from residential areas and now fly down a hiway corridor and then over an industrial park, a much more acceptable course for us. The new routing adds 3 minutes to the previous flight times.
 
It's rotor noise, but not sonic booms (the rotor tips are not breaking the sound barrier).

There is no easy way to quiet the rotor like putting a better muffler on the engine. Ever notice your window fan is pretty loud when it's on high? And yet it's not powerful enough to lift its own weight, let alone a helicopter.... Some designs are noisier than others, but anything that powerful is bound to make a lot of noise.
 
Thanks guys. That clarifies things. No, not worth complaining about because it is infequent, but still it's notable.

Coop
 
SharpByCoop said:
I was disturbed once again by a REALLY LOUD helicopter that was flying fairly low over our subururban neighborhood .... Doesn't the FCC regulate the engine noise on these things?!
Click to expand...

Probably not the FCC, the Federal Communications Commission, no.
 
I might be the oddball here but I live not too far from McChord AFB, Fort Lewis and one mile from a small rural airstrip. I love the daily airshows that go on around me. I get to see all sorts of experiemental and ultralight aircraft as well as a fine collection of military. It is music to my ears and I have to go outside to see what's going overhead each time.

The other night the stealth bomber was flying around along with the newest army choppers that you can not hear until they are right over you. FREAKIN' COOL!!!!! The private airstrip has a P-51 Mustang that fly out of it on nice days. I can hear that Merlin engine fire up before he even takes off 1 mile away.

It's all a matter of perspective. They don't fly enough to be annoying but just enough to keep it interesting.
 
Here in Tucson we have the opposite problem: Police Helicopters at night. They fly low and slow and wave their immensely high powered spot light around. Just what one needs late at night when they are trying to fall asleep.

Chad
 
I also live near a very popular light aviation airport and do not mind that traffic at all, during the summer when the b-25's and Hellcats are flying around that is cool. The Migs and the czech trainers i can do without. The migs burn lube oil, as well as fuel, and the residue coats everything left outdoors when they fly over.

I am literally under the initial point for landing so everybody flies over me, the helo's i was complaining about were flying in and out 30 times a day sometimes and that gets to be a bit much. Also, for some reason, the S76 bizz chopper is LOUD, annoyingly so, can;t hear the TV loud. Same with the jet rangers.

PS when i lived in the city, for a while they were flying police choppers all the time over us. Neighbor got fed up with it and started to fire 12 ga rescue flares at them. A guy down the street was arrested after launching some 4 in mortar style fireworks at them. The chopper flights got a lot higher after that. I have no problem if the choppers are actually doing work at night, IE chasing a felon, but just patrolling that bothers me, besides, with FLIR or low light gear, it would be better to just sit up high and watch, and then direct ground units into place with out telegraphing their position. In my opinion, artificial Sun spotlights are irresponsible and just wanna be's wasting taxpayer money unless actively used to direct officers.
 
Surely down to the FAA !

Flight rules normally stipulate a minimum height at which they can fly................
 
ChadHahn said:
Here in Tucson we have the opposite problem: Police Helicopters at night. They fly low and slow and wave their immensely high powered spot light around. Just what one needs late at night when they are trying to fall asleep.

Chad
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Yeah, with the AFB to the south, we're all used to the sound of stuff flying overhead during the day. I've been flashed a couple times by the police copters at night riding my bike around. You'd think they'd know better than to startle a guy on a bike. I should crash next time they do it and sue them.
 
Do you live near a hospital? The Life Star (MediVac) helicopter flies very low very fast over our area because, I assume, it has permission from the FAA due to it's special situation of needing to get patients to the hospital fast. They are also painted red and white to be identified easily.
 
Guntotin_fool said:
actually the sound you are hearing is the compression of the air between the main rotor and the tail rotor and the tail boom. As the air is "caught" between the two points, it compresses and then releases as the rotors pass. Sort of like an imaginary paper bag being popped every time. The sound change has to do with the doppler effect and the movement of the air as it releases.
Click to expand...

How in the world did you come up with the explanation that rotor noise is a result of compression of the air between the main rotor and the tail rotor and the tail boom? I used to teach helicopter aerodynamics so I've just got to ask.
 
That is the description given to me by a PHD level aeronautical engineer who works on making Helo's stealthy. He works in the same building as my wife and we discussing why some thump and some do not. He is currently working on the next gen replacement for the apache, the one after the cancelled comanche. I may have not completely described the process, but that was basically how it was described to me, the rotor makes no whomp whomp noise until it is place in proximity with a fixed object which then creates a disproportionate area of high pressure air and low pressure air. the low pressure area above the airfoil and the high pressure area below the attempt to equalize.

The sentence about the doppler effect was referring to how the pitch changed as the chopper flew over.
 
I may have simplified it too much, but if you read,
Blade-vortex interaction (BVI) noise
BVI occurs when a rotor blade passes within a close proximity of the shed tip vortices from a previous blade. This causes a rapid, impulsive change in the loading on the blade resulting in the generation of highly directional impulsive loading noise. BVI noise can occur on either the advancing or retreating side of the rotor disk and its directivity is characterized by the precise orientation of the interaction. In general, advancing side BVI noise is directed down and forward while retreating-side BVIs cause noise that is directed down and rearward. It has been shown that the main parameters governing the strength of a BVI are the distance between the blade and the vortex, the vortex strength at the time of the interaction, and how parallel or oblique the interaction is (Hardin 1987, Malovrh 2005). and combine it with

A tail-rotor which is recessed into the fairing of the tail (a fenestron) reduces the noise level directly below the aircraft, which is useful in urban areas. In addition, this type of rotor typically has anywhere from 8 to 12 blades (as compared to 2 or 4 blades on a conventional tail rotor), increasing the frequency of the noise and thus its attenuation by the atmosphere. This type of rotor is in general much quieter than its conventional counterpart: the price paid is a substantial increase in the weight of the aircraft, and the weight that must be supported by the tail boom. For example, the Eurocopter EC-135 has such a design.
For smaller helicopters it may be advantageous to use a NOTAR (from NO TAil Rotor) system. In this yaw-control method air is blown out of vents along the tail boom, producing thrust via the Coandă effect.

you can see that the interaction of turbulence, vortex streams coming off the end of the blades, and the reaction of the main rotor and the tail rotor both operating in each others wake that causes a majority of the noise.
 
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