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Why Is The PW JT8D So Loud?  
User currently offlineAirWillie6475 From United States of America, joined Jan 2005, 2448 posts, RR: 1
Posted (8 years 11 months 2 weeks 2 days 16 hours ago) and read 18055 times:

After witnessing a rocket, or should I say an MD80 takeoff at BURs short runway, and amidst the ringing car alarms left and right, I wondered why the md80 engines are so loud. I have always associated the loudness with old engine technology but I found last night that the average age of the MD80 is the same as the 737-300. The 733 is not loud at all. What makes the MD80's engines so loud?

30 replies: All unread, showing first 25:
 
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 1, posted (8 years 11 months 2 weeks 2 days 16 hours ago) and read 18048 times:

One simple (partial) answer is in the bypass ratio of the engines. The JT8D is a relatively low bypass engine. That is, a higher percentage of the air going in the intakes actually goes through the engine core and exhausts at very high temperatures and velocities.

On the 737-300, the CFM-56 engine has a higher bypass ratio, so a greater percentage of the air entering the intakes just goes around the engine through the fan bypass ducts. This has the effect of wrapping the hot, high-velocity exhaust in a layer of relatively cooler, less disturbed air.

I'm sure there is more, even without considering hushkits etc. but I think this is a fair, simple answer.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineAC21365 From United States of America, joined Jul 2005, 402 posts, RR: 0
Reply 2, posted (8 years 11 months 2 weeks 2 days 14 hours ago) and read 17999 times:

One word:



Turbojet


User currently offlineAccess-Air From United States of America, joined Sep 2000, 1939 posts, RR: 13
Reply 3, posted (8 years 11 months 2 weeks 2 days 13 hours ago) and read 17982 times:

Actually,
the P&W JT8 engine is a 1st generation low bypass turbofan engine. The model of engine that powers the MD-80 series of aircraft is actually an updated version. Like the guys stated above the bypass fan diameter was increased to provide more thrust and increase the amount of bypass air flowing around the core to help silence the engine. Also notable but hidden by the huge clamshell thrut reverser buckets is a daisy petal exhaust nozzle that also helps deaden the sound. Believe it or not the MD-80 DOES meet Stage 3 noise regulations but it has been known under certain circumstances to actually slip into Stage 2 noise levels. The difference between the newer JT8Ds on the MD-80s noise-wise and the older JT8Ds on say a DC9 or 737-100 or 200 is that there is more crackle in the engine sound and more rumble.
The cooler air that bypasses the the core in the JT8D 200 series muffles the rumble and crackle.
The older JT8Ds have been hushkitted the best that they can be but they all still crackle.... Smile
What I have noticed is that the 737-700 and 800 engines on take off are Louder than engines on the 737-300/400/500. They 700 and 800 seem to rumble a lot louder....

Anyway, thats all I have to add...

Access-Air



Remember, Wherever you go, there you are!!!!
User currently offline113312 From United States of America, joined Apr 2005, 574 posts, RR: 1
Reply 4, posted (8 years 11 months 2 weeks 2 days 13 hours ago) and read 17969 times:

It was a lot quieter than the engines that came before like the JT3, JT4, CJ805 or the Rolls Royce Conway.

User currently offlineNKP S2 From United States of America, joined Dec 1999, 1714 posts, RR: 5
Reply 5, posted (8 years 11 months 2 weeks 2 days 12 hours ago) and read 17945 times:

Quoting AC21365 (Reply 2):



Quoting AC21365 (Reply 2):
One word:



Turbojet

Wrong. TurboFAN, just a lower bypass one.


User currently offlineAeroWeanie From United States of America, joined Dec 2004, 1610 posts, RR: 52
Reply 6, posted (8 years 11 months 2 weeks 2 days 12 hours ago) and read 17923 times:
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Quoting AC21365 (Reply 2):
Turbojet

No, the JT8D-200 on the MD80 is a moderate bypass ratio turbofan. The JT8D-7 or -9 used on a DC-9, 727 or 737-200 is a low bypass ratio turbofan. The -200 is a "refan" version of the original engine.

Quoting SlamClick (Reply 1):
One simple (partial) answer is in the bypass ratio of the engines. The JT8D is a relatively low bypass engine. That is, a higher percentage of the air going in the intakes actually goes through the engine core and exhausts at very high temperatures and velocities.

Jet noise (as opposed to inlet noise) is caused largely by the shearing of the high speed air coming out of the engine and the outside air. As you lower the speed of the air coming out of the engine, you lower the noise produced.

Turbojets accelerate a little air a lot to produce thrust. Low bypass ratio engines accelerate more air by a a lesser amount to produce the same thrust. High bypass ratio engines accelerate a lot of air by even less to produce the same thrust.


User currently offlineN766UA From United States of America, joined Jul 1999, 8310 posts, RR: 23
Reply 7, posted (8 years 11 months 2 weeks 2 days 11 hours ago) and read 17893 times:

Because it's freakin' awesome that's why!  silly  I love when the Mad Dogs, DC-9s, 727s, and 732s depart!


This Website Censors Me
User currently offlineCRJonBeez From United States of America, joined Nov 2004, 317 posts, RR: 3
Reply 8, posted (8 years 11 months 2 weeks 2 days 8 hours ago) and read 17811 times:

can't get enough of those lower bypass ratio fans! another personal favorite, though still ridiculously loud for being current, is the JT15D5's found on the beechjet...from the front of that engine comes a shrill like you wouldn't believe!

User currently offlineAC21365 From United States of America, joined Jul 2005, 402 posts, RR: 0
Reply 9, posted (8 years 11 months 2 weeks 2 days 7 hours ago) and read 17798 times:

Yall sure know how to flame someone  thumbsup 

User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 10, posted (8 years 11 months 2 weeks 2 days 3 hours ago) and read 17763 times:

The JT8D is a low Bypass Turbofan with approx Bypass ratio of 1:1.
The CFM56 is a high Bypass Turbofan,with a Bypass ratio of 5:1.

Quoting AC21365 (Reply 2):
Turbojet



Quoting Access-Air (Reply 3):
the P&W JT8 engine is a 1st generation low bypass turbofan engine.

The JT8D is a Low bypass Turbofan.

Quoting Access-Air (Reply 3):
Like the guys stated above the bypass fan diameter was increased to provide more thrust and increase the amount of bypass air flowing around the core to help silence the engine

Any Link/Details on this Diameter Increase.

regds
MEL



Think of the brighter side!
User currently offlineNKP S2 From United States of America, joined Dec 1999, 1714 posts, RR: 5
Reply 11, posted (8 years 11 months 2 weeks 1 day 22 hours ago) and read 17721 times:

Quoting AC21365 (Reply 9):
Yall sure know how to flame someone thumbsup

Well this isn't the civil av forum. If you make a declarative statement, you'd better know of which you speak. Call it "know-it-all burn-out".

If you don't know....feel free to ask questions ( after using the search function of course Wink )

[Edited 2005-11-15 14:59:20]

User currently offlineSinlock From United States of America, joined Dec 2000, 1650 posts, RR: 2
Reply 12, posted (8 years 11 months 2 weeks 1 day 20 hours ago) and read 17692 times:

Quoting HAWK21M (Reply 10):
The JT8D is a low Bypass Turbofan with approx Bypass ratio of 1:1.

The JT8D-200 series found on the MD-80 has a bypass ratio od 1.74 to 1 The other "older" JT8D modals have a bypass ratio of .96 to 1.



My Country can beat up your Country....
User currently offlineOkie From United States of America, joined Jul 2003, 3103 posts, RR: 3
Reply 13, posted (8 years 11 months 2 weeks 1 day 20 hours ago) and read 17685 times:

Quoting AeroWeanie (Reply 6):
Jet noise (as opposed to inlet noise) is caused largely by the shearing of the high speed air coming out of the engine and the outside air. As you lower the speed of the air coming out of the engine, you lower the noise produced.

Turbojets accelerate a little air a lot to produce thrust. Low bypass ratio engines accelerate more air by a a lesser amount to produce the same thrust. High bypass ratio engines accelerate a lot of air by even less to produce the same thrust

Just add a little more here, it takes less energy to accelerate more air at less velocity hence, the core (the largest noise producer) can be smaller to produce an equal amount of thrust.

Okie


User currently offlineSovietjet From Bulgaria, joined Mar 2003, 2616 posts, RR: 17
Reply 14, posted (8 years 11 months 2 weeks 1 day 15 hours ago) and read 17629 times:
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JT8Ds are pretty decently loud. But you should go hear some NK-8, NK-86 and D-30 Series 3s to experience some raw power and sound.

User currently offlineLightsaber From United States of America, joined Jan 2005, 13265 posts, RR: 100
Reply 15, posted (8 years 11 months 2 weeks 1 day 8 hours ago) and read 17538 times:
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Quoting SlamClick (Reply 1):
One simple (partial) answer is in the bypass ratio of the engines. The JT8D is a relatively low bypass engine. That is, a higher percentage of the air going in the intakes actually goes through the engine core and exhausts at very high temperatures and velocities.

On the 737-300, the CFM-56 engine has a higher bypass ratio, so a greater percentage of the air entering the intakes just goes around the engine through the fan bypass ducts. This has the effect of wrapping the hot, high-velocity exhaust in a layer of relatively cooler, less disturbed air.

I'm sure there is more, even without considering hushkits etc. but I think this is a fair, simple answer.

Did anyone think I wouldn't answer this?

Yes, it is due to the moderate bypass ratio. Also, the nacelle isn't that great a damper on the JT8D-200's. (Note, there is a kit to improve that, but it adds weight for the sound absorbing material.)

As to the physics. Specific noise energy is produced to the 7th to 8th power of the velocity of the exhaust gas. Yes, its very non-linear! That is why high bypass engines are so much quieter than low bypass engines. This is also why mixing the turbine stream gas with the fan gas helps. The turbine gas might exhaust to the same pressure as the fan gas which means they'll be at the same mach number. But... Since the turbine gas is hotter, it will be going faster. (The speed of sound goes as the square root of temperature.) Faster=noisier This is why some nacelles are "mixing nacelles." Its purely to minimize noise. (e.g., some Trents, V2500, etc.)

Quoting Okie (Reply 13):

Just add a little more here, it takes less energy to accelerate more air at less velocity hence, the core (the largest noise producer) can be smaller to produce an equal amount of thrust.

True! And I was trying to point out, the core exhaust (turbine exhaust) is faster and hence noisier than the fan air.

Lightsaber



Societies that achieve a critical mass of ideas achieve self sustaining growth; others stagnate.
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 16, posted (8 years 11 months 2 weeks 1 day 2 hours ago) and read 17497 times:

Quoting Sinlock (Reply 12):
The JT8D-200 series found on the MD-80 has a bypass ratio od 1.74 to 1 The other "older" JT8D modals have a bypass ratio of .96 to 1.

Agreed.I guess I was thinking of the JT8Ds -1 to 17Rs,You are correct on the -200.
regds
MEL



Think of the brighter side!
User currently offlineBR715-A1-30 From , joined Dec 1969, posts, RR:
Reply 17, posted (8 years 11 months 1 week 6 days 17 hours ago) and read 17372 times:

The JT8D-1 through 17 also have 2 Fan Stages as opposed to 1 fan stage on the JT8D-209 through 219. (Which means that they have 2 fans instead of 1). I guess that is to accelerate more air.

A Little Trivia, but before the BR715-A1/C1-30 was selected for the 717, It was originally going to sport JT8D-218/219 engines.


User currently offlineNKP S2 From United States of America, joined Dec 1999, 1714 posts, RR: 5
Reply 18, posted (8 years 11 months 1 week 6 days 17 hours ago) and read 17363 times:

Quoting BR715-A1-30 (Reply 17):
The JT8D-1 through 17 also have 2 Fan Stages as opposed to 1 fan stage on the JT8D-209 through 219. (Which means that they have 2 fans instead of 1). I guess that is to accelerate more air.

I don't know about that. The earlier JT-8's ( at least the dash 7's through the 17 anyway ) did indeed have 2 fan stages. I remember well the challenge of "dressing" the C-2 fan blades though the IGV's and C1 blades, and we used to R&R C-1 and C-2 discs regularly ( engine on wing / tail pylon ).

It's been about 5 years since I've worked on one. They could break your balls sometimes but they were/are good engines. I'm sure the 717/MD-90 would've been a good performer with the "big block" JT-8's but really, the design has long since reached it's developmental apogee that even my own nostalgia can't ignore.

[Edited 2005-11-17 19:11:26]

User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 19, posted (8 years 11 months 1 week 6 days 5 hours ago) and read 17261 times:

Why wasn't a JT8D upgrade to replace those on the B732s ever suggested,considering the Fuel burn.
regds
MEL



Think of the brighter side!
User currently offlinePyrex From Portugal, joined Aug 2005, 4025 posts, RR: 28
Reply 20, posted (8 years 11 months 1 week 5 days 23 hours ago) and read 17223 times:

Quoting BR715-A1-30 (Reply 17):
I guess that is to accelerate more air.

Not exactly. The lower the by-pass ratio the higher the optimum fan pressure ratio, so on the earlier ones a 2-stage fan was necessary. With the increase in by-pass ratio the fan doesn't need such a high pressure-ratio, so you can do it with just one stage.

Plus, I imagine newer technology on the -200 version allows for a higher fan pressure ratio per stage.



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User currently offlineOryx From Germany, joined Nov 2005, 126 posts, RR: 0
Reply 21, posted (8 years 11 months 1 week 5 days 21 hours ago) and read 17215 times:

Speaking of rockets....

The Aerion super sonic business jet will (if ever build) be powered by Pratt & Whitney JT8D-219 (Flight International, 19 Oct 2004).

Does anybody have an idea why there is no more modern design for a Mach 1.4-1.6 civil turbofan?


User currently offlineKALB From United States of America, joined Mar 2000, 573 posts, RR: 0
Reply 22, posted (8 years 11 months 1 week 5 days 18 hours ago) and read 17186 times:

Ya want noise? Listen to non-hush kitted RR Speys on BAC 111! Now, those babies can scream.

User currently offlineLAPA_SAAB340 From Spain, joined Aug 2001, 390 posts, RR: 4
Reply 23, posted (8 years 11 months 1 week 5 days 18 hours ago) and read 17183 times:

Ya want noise? Listen to non-hush kitted RR Speys on BAC 111! Now, those babies can scream

Indeed. I remember when the MD-80s came out, I thought they were incredibly quiet in comparison to the likes of the 1-11. The crackling sound of the exhaust felt like combustion in that engine was really a string of explosions. An early morning 1-11 flight that would climb out over my house every day was one of the most effective alarm clocks I ever had... Big grin

I only wish I could have heard a Trident...


User currently offlineTheSonntag From Germany, joined Jun 2005, 3632 posts, RR: 29
Reply 24, posted (8 years 11 months 1 week 5 days 17 hours ago) and read 17173 times:

Quoting Oryx (Reply 21):

I guess the answer is that supersonic airplanes must be relatively low-bypass turbofans, as the core thrust is important at supersonic speeds. Thats the reason why military airplanes and Concorde were so loud, they cannot have a high bypass ratio as this would make it impossible to go up to high supersonic speeds (don't ask me why exactly).

As conventional engines are designed for subsonic speeds (so it is tried to get higher bypass ratios), they are not suited for supersonic speeds.

The JT8D, on the other hand, meets the existing noise criterias (it isn't great, but it is legal), and its bypass ratio is low enough for supersonic speeds, I guess...


25 Pyrex : I did my final thesis at University on the design of an engine for a supersonic business jet and in the introduction I mention all the factors behind
26 Stirling : Could not said it better myself. It makes me sad they closed the staging area at SFO, at the end of runway 1R...great place to get behind a departing
27 717-200 : I love seeing and hearing especially the 737-200 Combis of AS taking off out of SEA when I am out on the South Satellite ramp. Which subseries of the
28 HAWK21M : Whats the Restrictions on JT8Ds Worldwide Hushkitted or Not.Which cities have banned the B732s. regds MEL
29 TheSonntag : It depends on the actual noise levels. In Europe, Stage 2 airplanes are banned completely, except for humanitarian or military flights. That's why th
30 Post contains images HAWK21M : Its true On Grd.The Hushkitted & non Hushkitted Engines sound the same regds MEL
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