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Explain This Noise Supressor

Thu Feb 05, 2004 12:17 am

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Explain this noise supressor.
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RE: Explain This Noise Supressor

Thu Feb 05, 2004 1:12 am

From what I see it looks like the tailpipe of a JT8D installation on a 737-200 (I´ve done my apprenticeship on them looong ago  Smile/happy/getting dizzy )
TheJT8D engine is a bypass engine, this means the fan blows a sizeable amount of cold air around the core engine through a concentric bypass duct. The star shaped unit in the middle is the mixer, which is sited at the exhause of the core engine in the hot air flow. It mixed the hot air with cold fan air, cooling it down. This mixing process is supposed to reduce the exhaust noise (not by much!). At the end of the tail pipe you´ll find the bucket ( or rather guilotine type thrust reverser, a dangerous piece of work!)

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RE: Explain This Noise Supressor

Thu Feb 05, 2004 1:26 am

The whole hush kit includes an inlet, exhaust, aft engine mount and insulation. The new inlet moves the guide vanes forward I believe to improve flow. The extended exhaust is basically a mixing chamber allowing core and bypass air to mix quieting the exhaust. The insulation is used to protect the hydraulic lines running to the thrust reversers. And finally the new aft engine mount is stronger to carry the added weight of the hush kit.

I know someone else will help me out and explain this a little better.
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RE: Explain This Noise Supressor

Thu Feb 05, 2004 7:16 am

The picture shows the mixer that is installed in the exhaust of a basic JT8D engine, those models up to a -17AR. As MD-11 engineer pointed out, its purpose is to mix the hot, high velocity flow from the core of the engine with the cool, lower velocity fan flow to reduce the overall velocity of the exhaust gases. Since the primary component of jet noise is the shearing action of the high velocity exhaust gases against the ambient air, any reduction of the exhaust gas velocity will reduce the noise. Although it has been frequently stated on that the mixers reduced thrust, a lot of testing was done to show that installation of the mixer did not alter the thrust-EPR relationship or cause a thrust loss. As ATLamt pointed out, hushkitted engines also have the inlet guide vane case spaced forward to eliminate the fan blades cutting through the wakes in back of the inlet guide vanes (kind of like how a siren works) that are the second loudest source of jet noise. Although it may not sound quieter to the ear, when measured on a gage, the installation of the hush kit did quiet the engine.

The picture shows the engine is installed on a 737, so there was no need for the tailpipe extension. Some of hushkits on DC-9s and 727's do have an extension, but it is not required on a 737. That is a standard 737 installation with the thrust reverser installed. This configuration also did not require the installation of the supplemental aft bracket. The supplemental aft bracket was developed for use on those JT8D engines that were going to have the Nordam developed 737 hushkit that included a suppressor and another mixer that were very heavy.

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