It would appear up to last year the Herc was doing lower circuits

http://www.raf.mod.uk/rafbrizenorton...AA4264C911.pdf

This makes very interesting reading, Dec 2012

West Oxfordshire Planning Policy Consultations - Draft Local Plan October 2012 - View Comment


RAF Brize Norton - Environment

Start-up to take-off
 

...Still haven't seen a plausible answer to HALTONAPP's original post: why does the C130 'noise' go on so long? Other threads at other times have mentioned ground runs for maintenance/test purposes. Is that what he can hear?

As for start-up to take-off, I notice that commercial airlines (generally with newer kit than RAF) don't start-up until they are being pushed back. Contrast with RAF (albeit with VC10 and Tristar) who seem to start-up well before push-back. Maybe out Voyagers don't start until being pushed back?

Regards
BATCO

Right BATCO here is a theory from a thick civi pilot.

I think the C130 engines are single shaft or direct shaft engines.

This means that the whole lot spins at a constant speed all the time. The props will be in the region of 1500-2000rpm and the turbine will be at about 21000rpm.

A jet aircraft at idle will be at about 21000rpm.

Most of the noise off a turboprop is the props thrashing the air into submission and it has quiet a large angular section which it radiates out from as the fan isn't contained.

The jet fan is contained and has a more restricted cone of noise.

Now the frequency of the noise comes into play.

The props will be pumping out in the 25-35hz region and the Jet will be in the 350hz region.

Now the lower the frequency the better it can diffract round objects and reflect off them. So noise abatement walls etc won't be as effective as the sound will bend round them and also reflect off them with minimal power lost.
Also as well the lower the frequency the more efficient the air is of transporting it.

You can relate this to music with bass frequency's travelling further and being more surrounding than the higher frequency's sometimes you can hear only the bass with none of the rest of the music.

You will also get loud zones where the noise has been reflected off an object and an interference pattern has been set up so the noise seems to drop then get louder again. Again this is more likely to happen with low frequency's.

So I reckon its a mixture of the cone of noise being larger around the aircraft virtually 360 degrees. The frequency which its at being better at travelling. The frequency allowing better diffraction and reflection. And I presume most of the noise abatement constructions being designed for jet frequency's of noise.

As a suggestion they could look at dykes of earth with enough width to reflect upwards that frequency instead of diffracting round they need to be flat topped as well otherwise the sound will curl round over the top of them and the backside soft so as not to transmit.

"As a suggestion they could look at dykes of earth with enough width to reflect upwards that frequency instead of diffracting round they need to be flat topped as well otherwise the sound will curl round over the top of them and the backside soft so as not to transmit."

Much like the 'Bund' at Lyneham then!

IIRC the Tri* does normally pushback and then start BATCO, depending on where parked at Braise Mutton; the VC10 parking was not always nose into the 'waterfront' so pushback was not necessary

Think you will find its to allow the poor old tug to make headway, if I remember correctly Concorde if you started more than one would push the tug, not the other way around.

Isn't it a case of making certain that they can start all of the engines before commiting to loading?

I've beat the crap out of a Ten Engine to free off the Airstart Valve utilising the jacking handle off the safety raiser, to look up and see all these worried faces in the windows, we didn't always start them all on the front, but two when pushed back.

Engineering Acoustics/Outdoor Sound Propagation - Wikibooks, open books for an open world

The first maths part is a bit of a mind screw but lower down its not to bad. Decibels was always a bit of a bitch when I did it at Uni.

The interesting figure is figure 2

At 62hz the Atmospheric attenuation coefficient dB/km at 10 deg C 50% humidity is 0.16 and at 250 hz its 1.05 and 500hz its 2.28

So its no wonder people think its noisier because it will be. Also as well the 25-35hz is more in the normal voice range than 250hz which is a shrill woman level. And people are attuned to hear things in the human voice range.

You grow accustomed to certain types of noise. When I lived in the Officers' Mess at Brize we had over 20 VC10s stationed there (10 Sqn and 101 sqn) but only a handful of Tristars. As a result, we heard many more VC10 movements and so became used to them. I could sleep through the ZeroDarkThirty Dulles and Deci departures (on the days when I wasn't driving either) but would be woken by the much quieter Tristars because they were a relatively rare sound.

BATCO, 1021 was C130 prop RPM IIRC.
Yes single shaft driving a reduction gearbox. I'm sure someone can give you the compressor/turbine RPM. A driver only needed to know 100% RPM.

Many folks around LYE and RWB miss the C130 and the sound.

What we do not miss is the regular Fri pm VC10 doing circuits at LYE to avoid spooling up those "quiet" RR Conway by-pass engines abeam the O Mess at BZN and thus disturbing the 'elder statesmen' of the VC10 world enjoying their G & Ts at Happy Hour.

Normal start for Albert was one eng at a time. Each eng took about 1 min to on speed, 70% low speed, to keep the noise down on the ground! So 5 mins before taxy.

1066

Having spent most of my career split between BZZ and LYE I can say aircraft noise doesn't bother me one bit. I even bought a house in the area knowing fine well it was near an airfield and bank on the drone of a C130 engine to help me sleep! To the OP if you think the C130's loud just wait until the A400Ms arrive lol, you ain't heard nothing yet :)

Was forgetting those long blades would have to be slower.

And four by four of them must set up all sorts of harmonics bouncing around.

And as for these bad boys

http://www.aviationexplorer.com/Airb...o_Aircraft.jpg

Double the number of tips to produce vortex noise and I should imagine the counter rotating bit will generate some interesting interactions as well.

Mad Jock - you are quite correct, the locals will in the future look back on the halcyon days of the C130 with regret once the mighty (& noisy) A400M has arrived - although it won't be doing many circuits as the fatigue usage would be too great.

The C130 takes around 10 mins from start-up to taxi, noise on the ground is quite bad but once it's airborne I find it quite quiet (from the observer's perspective). When I lived in OMQs at LYE the noise from ac on the ground (15+ ship formations anyone?) could be horrendous but ac in the cct never bothered me.

Frankly I have little sympathy for those around BZN - they mostly bought their houses knowing the airfield was there & benefited to the tune of approx. 30% discount on the properties as a consequence. They seem to want to close down operations at the base which cannot be allowed to happen. I agree that LYE should never have been closed, a very foolish & shortsighted event for which a number of VSOs should be cashiered. Most of the locals around LYE would love the place to be re-opened but sadly that will never happen so the people of Oxfordshire will just have to get used to it.

I've heard from a few sources that one reason modern airliners are quieter is because their engines have a high bypass ratio. The blanket of ambient air that is bypassed shields the exhaust gasses that exit the combustion chamber. Its this blanket of bypassed air that also reduces or blocks the noise, as most of the roar is from the exhaust gases.
The Tornado GR4 RB199 engines have a low bypass ratio therefore sound like variable noise, constant air speed.

Sorry for the drift. Started my career with the sound of Gnats and ended with Typhoons. Why was the sound of two Hunters line abreast the only one that I lovingly remember? Nb. Had to be two or more, one was never the same sound.

The C130 noise goes on for so long due to maintenance engine ground runs which seem to drag on for ever.

Maintenance ground runs.

I can't speak for the J model! never worked on it! but, a K ground run post Prop/ engine change could last a good while. One run that lasted "forever" was a Pressurisation check where you needed to go to "safety valve" release. Even in the UK in summer! it would take a bit of extra throttle to get up there. Now that in itself is curious because the whole assembly, once in the flight range was running at 100%, regardless of throttle setting. Maybe a bit of aural naughtiness in the sumpies make up !

Smudge :ok:

Correct

It's where the high speed hot airflow impinges onto the cold slow moving air causing it to shear / rub against each other and creating the noise / crackle, by putting a warm moving layer between them, such as on a high ratio bypass fan engine this reduces the shear effect and hence the noise, on some of the earlier turbojets that has the likes of Stage 3 fitted, they basically added mixers that added more air into the jetpipe to stir it all up a bit and add a layer between the hot and cold.

Chevron exhaust nozzles increases the diameter of the jetpipe to reduce the speed of the high speed jet efflux. You will see those as the serrated edges on the new 787
See
Shelf truncated chevron exhaust nozzle for reduction of exhaust noise and infrared (IR) signature - General Electric Company

Though you will get a lot of high whine noise from the fan on a high ration bypass engine compared to a turbojet.

Noise noise noise
 

Having spent the odd week on the classic model Albert the good news is most k opperators are to some degree or other, hard of hearing, so unlike this thread are totally oblivious to the noise.

Double it and you're close. :8