Hush Kits? How do they work?

[Highbypass]

Hi all.

Can anyone enlighten me as to how a hush kit works on engines like the JT8D and the like?

Thanks...................................H.:cool:

[gaunty]

In simple terms the "noise" from a jet engine is usually produced by the very steep velocity gradient between the efflux and ambient air which is translated to turbulence and thus noise.

The very early low bypass engines were designed primarily to encase the hot high velocity flow off the core with a shield of lower velocity air from the bypass fan, usually the front compressor extended into the bypass duct.
This was to allow a slower mixing of the hot high speed core flow from the ambient airstream by producing a lower velocity gradient relative to the ambient flow.
Other means were tried by using a convoluted nozzle sytem, again desigend to mix the two airstreams in a way that reduced the velocity gradient.
Further research showed that this also had the effect of increasing the overall efficiency and the higher the bypass the better the result.
Which is why now over 75% of the thrust is produced off the fan which has evolved from a high speed large number of blades to a low speed wide chord low number of blades fan which 'resembles' a shrouded propellor. The core is there basically to drive the Fan and I guess the more advanced developments will be ever higher fan thrust as a percentage of residual core thrust.
It is simple physics in regard to the transmission of energy, low mass/high velocity ~= high mass/low velocity, which mitigates the velocity gradient therefore noise.

There is a fundamental difference however in the way they produce the thrust. And that resides in the way a propeller works v a fan.

You can make a JT8D quieter but you can't make it as quiet as a high bypass fan in the same way you can convert a sedan to a stationwagon by sending it to the panelbashers but it produces a far better result if you build it that way from scratch,:D

[john_tullamarine]

Gaunty identifies one of the principal sources of jet aircraft noise.

Long time since I did any work in acoustics but, if the memory serves me correctly, jet efflux noise is related to something in the vicinity of velocity to the 8th power ... so any reduction in mean velocity is a good thing for the unfortunates who, in droves, continue to buy houses near airports ..... taking advantage of lower costs due to the noise .. and then set up an active noise lobby to get rid of the airport .......

However, it is a case of looking at ALL the sources of noise (jet velocity is but one significant contributor) and treating those which are responsible for the higher energies and nuisance frequencies (as the ear's response to sound is very frequency dependent ... one of the reasons that we measure sound pressure levels against frequency weighted scales, typically dB(A) or dB(C) for jet noise). As a result, we see considerable effort directed towards the front of the engine as well as the back ... acoustic liners are but one example.

For no reason other than I had the URL to hand, the ABS DC9 Program (http://www.abshushkit.com/) site might be of interest .... especially as it shows the noise measuring sites ... many of the more sensitive runways have permanent monitoring sites set up to measure the noise generated by each takeoff ....

... and while the great unwashed of the suburban noise lobby might like quiet aircraft ... there is nothing quite so refreshing as the delightful crackle of a non-kitted B737-200/B727/DC9 scuttling over one's head on a crisp morning. I acknowledge that many who have not flown these lovely beasties might disagree ... but I guess that we have to tolerate such people ....

[Woomera]

Not to mention the delightful VC10 and of course the venerable B707/DC8 only to be upstaged by a Concorde in heat:p.

[QNIM]

I remember a VC10 departing Adelaide for Melbourne early one morning after diverting the previous
everning we could slill hear it 20 minutes later.
:eek:

[john_tullamarine]

Mention of the Concorde brings back a memory ... as an undergrad, our mob trooped out to a well-known airport which was then in the throes of rebuilding. Pulling strings and calling in favours, we managed to get ourselves not too far from the runway edge as the bird blasted off .. with the emphasis on blast ...

Many may be aware that LF audio will resonate body viscera and has been used as an instrument of torture in the past. I can still recall, now 30-odd years later, the extremely uncomfortable feelings in the gut ... but the display of mach bands in the exhaust up close made up for it all ...

I guess that security concerns these days would prevent current students an opportunity to experience that sort of thing ....

Ahh ... the delights of LOUD aircraft doing their thing ....