AirWillie6475 From United States of America, joined Jan 2005, 2448 posts, RR: 1 Posted (7 years 6 months 1 week 1 day 4 hours ago) and read 7711 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?
SlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 71 Reply 1, posted (7 years 6 months 1 week 1 day 3 hours ago) and read 7702 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.
AC21365 From United States of America, joined Jul 2005, 402 posts, RR: 0 Reply 2, posted (7 years 6 months 1 week 1 day 2 hours ago) and read 7653 times:
Access-Air From United States of America, joined Sep 2000, 1939 posts, RR: 15 Reply 3, posted (7 years 6 months 1 week 1 day 1 hour ago) and read 7637 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....
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....
113312 From United States of America, joined Apr 2005, 536 posts, RR: 1 Reply 4, posted (7 years 6 months 1 week 1 day 1 hour ago) and read 7624 times:
It was a lot quieter than the engines that came before like the JT3, JT4, CJ805 or the Rolls Royce Conway.
AeroWeanie From United States of America, joined Dec 2004, 1600 posts, RR: 52 Reply 6, posted (7 years 6 months 1 week 23 hours ago) and read 7579 times:
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.
CRJonBeez From United States of America, joined Nov 2004, 317 posts, RR: 4 Reply 8, posted (7 years 6 months 1 week 19 hours ago) and read 7467 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!
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
Okie From United States of America, joined Jul 2003, 2348 posts, RR: 3 Reply 13, posted (7 years 6 months 1 week 8 hours ago) and read 7341 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.
Lightsaber From United States of America, joined Jan 2005, 10651 posts, RR: 100 Reply 15, posted (7 years 6 months 6 days 19 hours ago) and read 7194 times:
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.
HAWK21M From India, joined Jan 2001, 31201 posts, RR: 58 Reply 16, posted (7 years 6 months 6 days 14 hours ago) and read 7153 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
BR715-A1-30 From , joined Dec 1969, posts, RR: Reply 17, posted (7 years 6 months 5 days 5 hours ago) and read 7028 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.
NKP S2 From United States of America, joined Dec 1999, 1714 posts, RR: 6 Reply 18, posted (7 years 6 months 5 days 5 hours ago) and read 7019 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.
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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LAPA_SAAB340 From Spain, joined Aug 2001, 389 posts, RR: 5 Reply 23, posted (7 years 6 months 4 days 5 hours ago) and read 6839 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...
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 HAWK21M: Its true On Grd.The Hushkitted & non Hushkitted Engines sound the same regds MEL
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