add2718
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Why are MD-88's so loud?

Tue Mar 13, 2018 10:41 pm

Since living near an airport and becoming aware of the different models of planes that fly regularly out of the airport, I've noticed that MD-88's seem disproportionately loud for their size. Are their engines that old and outdated? They seem louder at the same distance than bigger airplanes like the 757 or even the 767.
 
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nuke
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Re: Why are MD-88's so loud?

Tue Mar 13, 2018 10:56 pm

The JT8D on the MD-80 has a lower bypass ratio than jets like the 757 and 767 so they tend to be noisier. Basically the bypass ratio is how much air enters the inlet, and of that, how much bypasses the jet section, compared to the hot, fast jet of air coming out of the engine itself. With lower bypass ratio engines, the ratio of core exhaust is higher and thus the overall jet of air is faster (in producing a given amount of thrust) and therefore noisier.
Dependable Engines.
 
Andre3K
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Re: Why are MD-88's so loud?

Wed Mar 14, 2018 12:47 am

Bypass ratio. That's pretty much it.

That aside, I LOVE THE SOUND OF MD-80's. If they could make them more fuel efficient and still be just as loud, that would be the best of both worlds.

High bypass ration=High boredom ratio
 
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MD80
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Re: Why are MD-88's so loud?

Thu Mar 15, 2018 6:51 pm

The MD-80 was the first aircraft of its size with Chapter 3-compliance. So, this aircraft is a "loud Chapter 3"-aircraft,
Dedicated for the MD-80, MD-90, MD-95 ehhh...Boeing 717, and DC-9: http://www.MD-80.com
 
StTim
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Re: Why are MD-88's so loud?

Thu Mar 15, 2018 6:57 pm

Oh I remember the BAC-111 before the hush kits were fitted. Now that was loud!
 
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Adipasquale
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Re: Why are MD-88's so loud?

Thu Mar 15, 2018 8:29 pm

Don't forget how smoky the JT8Ds are too! As others have stated, it all comes down to bypass ratio. With that said, I absolutely love how loud the MD80s are. A few months ago I was spotting with my girlfriend, and there was a 744 departing, preceded by an MD88. I told her how loud the MD88 would be, but that the 744 would be louder. However, once both planes departed, I realized that from where we stood, the MD88 was as loud as the 744! This got me thinking to how loud 707s and DC-8s must have been, but unfortunately I'm too young to have experienced the days when planespotting would have made you deaf!
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MHG
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Re: Why are MD-88's so loud?

Thu Mar 15, 2018 9:10 pm

add2718 wrote:
Since living near an airport and becoming aware of the different models of planes that fly regularly out of the airport, I've noticed that MD-88's seem disproportionately loud for their size. Are their engines that old and outdated? They seem louder at the same distance than bigger airplanes like the 757 or even the 767.

Next to the technical reasons explained already those JT8D engines were developed way earlier than the socalled turbofans.
And as evolution usually goes engineers try to improve engines continously (make them more economical and less noisy).
The MD-88 are of an older generation and you can expect them literally disappearing within the next 2-3 years.
There was/is a "modernized" version called the MD-90 (Delta still flies them) that has turbofans fitted but not too many frames were built ...
I miss the sound of rolls royce darts and speys
 
Andre3K
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Re: Why are MD-88's so loud?

Fri Mar 16, 2018 12:53 am

Adipasquale wrote:
This got me thinking to how loud 707s and DC-8s must have been, but unfortunately I'm too young to have experienced the days when planespotting would have made you deaf!


The turbojet powered 707's have long been gone, but the low bypass turbofan ones still exist. If you ever get a chance to be near an E-8 Jstars, they are pretty loud but nothing compared to a Super VC-10. And I've been next to a runway where they both took off within 3 minutes of each other. The only planes louder than the VC-10 when I was in Qatar back in 2005 were the B-1's, F-15's and maybe the Nimrod.
 
zeeth
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Re: Why are MD-88's so loud?

Fri Mar 16, 2018 2:00 pm

Living at various addresses under the same glide path on & off for 60 years, there is an enormous difference in the type of sound as well as the decibel level. I am stunned that an A380, 200ft overhead, just sounds like a large vacuum cleaner. It's a loud sound, but that's all it is. In the 1960s, with the DC9, DC8, 707, 727, Caravelle, BAC111 & VC10, it was more of a seismic event, and for longer. Windows would rattle, crockery would vibrate off the shelf & the record player would skip tracks. Not a scientific quantification, I know, but it just doesn't happen any more.
Some modern aircraft just emit a hiss, a few different extremely high-pitched tones & white noise until overhead when you hear other whistling, roar & harmonics. Old aircraft like the DC9 just seemed to rip the atmosphere to shreds from miles away.
Turboprops seem the same though, but then I've never heard a Viscount or F27 on the same day as an ATR or Dash 8.
Not having witnessed deadstick landings except at airshows, I'm inclined to think that newer airframes generate less aerodynamic sound separate from their power plants as well.
 
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aemoreira1981
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Re: Why are MD-88's so loud?

Sat Mar 17, 2018 3:10 am

When I planespot at LGA, before the MD-88s were replaced by A321s there for Delta, you could tell when an MD-88 was departing on Runway 4 even if you didn't know the model of the planes (from there, one can view taxi-arounds and departures from a nearby park with a below-grade parkway in between and if standing on a bench, the view is virtually unobstructed to the runway).
 
EIDL
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Re: Why are MD-88's so loud?

Sat Mar 17, 2018 5:01 am

MHG wrote:
Next to the technical reasons explained already those JT8D engines were developed way earlier than the socalled turbofans..


The JT8D *is* a turbofan.
 
jupiter2
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Re: Why are MD-88's so loud?

Sun Mar 18, 2018 9:30 pm

Noise, smoke, vibration, all part of what made aircraft more interesting "back in the day". Used to get 727s and DC9s screaming over my old house as a kid, the house would vibrate, windows rattle and the TV would get drowned out, I loved it. Aircraft had character.

Old turboprops used to be ear piercing, with their high pitched whine. I can remember an IPEC Air Freight Argosy approaching 25 in SYD at around 0200 once, back in the mid 80's, I could hear it for at least a minute before I could see it, it was deafening. Had to laugh though, being a turboprop it could operate during curfew hours, but the jets at the time which were no louder couldn't.
 
trnswrld
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Re: Why are MD-88's so loud?

Mon Mar 19, 2018 4:59 pm

Before the re-engine of the C5, those were very loud and unusual sounding engines. That’s another example of a plane that you hear well in advanced and after the fact.
A more modern commercial aircraft that always catches my attention for how loud it is when flying over at about 9-11 thousand feet is the B738. I don’t know why, but that airplane is easily the loudest when flying over. I live under an ORD approach path where they are at 9-11k over the Joliet/Naperville area and those things are just noisy. Not sure if it’s mainly engine or aerodynamic type noise or a combination of both, but I can always pick out an MD80 or B738/9 variant out from the crowd.
 
speedbird52
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Re: Why are MD-88's so loud?

Tue Mar 20, 2018 3:52 am

Adipasquale wrote:
Don't forget how smoky the JT8Ds are too! As others have stated, it all comes down to bypass ratio. With that said, I absolutely love how loud the MD80s are. A few months ago I was spotting with my girlfriend, and there was a 744 departing, preceded by an MD88. I told her how loud the MD88 would be, but that the 744 would be louder. However, once both planes departed, I realized that from where we stood, the MD88 was as loud as the 744! This got me thinking to how loud 707s and DC-8s must have been, but unfortunately I'm too young to have experienced the days when planespotting would have made you deaf!

Be grateful for having a girlfriend who will go plane spotting with you. It's always wonderful to have someone who supports your passions even if they don't necessarily share them
"I have control" Three Words That Could Have Saved Lives.
 
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Adipasquale
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Re: Why are MD-88's so loud?

Tue Mar 20, 2018 1:33 pm

speedbird52 wrote:
Be grateful for having a girlfriend who will go plane spotting with you. It's always wonderful to have someone who supports your passions even if they don't necessarily share them

At risk of getting OT, its definitely not lost on me how lucky I am to have found someone who is so supportive of my interests!
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Anthonyg96
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Re: Why are MD-88's so loud?

Tue Mar 20, 2018 9:25 pm

The loudness of the ac depends on how mutch thrust is on the engines
Anthony Geinopolos
 
Andre3K
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Re: Why are MD-88's so loud?

Tue Mar 20, 2018 10:19 pm

Anthonyg96 wrote:
The loudness of the ac depends on how mutch thrust is on the engines



You've got to be trolling. This is simply not true at all.

An F-16 is MUCH louder than a 757 but has much less thrust.

A 767 is louder than a 777 but has much less thrust.

This list could go on for quite a while but I digress.

The easy answer is the bypass ratio. But the core of the noise difference is in how much energy remains from the core engine exhaust and how fast it is leaving the nozzle vs the bypass exhaust.

Large bypass turbofans have a 3 main ways to reduce noise (with more gimmicks based on acoustical dampening and aerodynamics). The first is from how much energy is absorbed by the turbines to drive the main fan. The second is from the decreased velocity of the exhaust leaving the core because of a larger nozzle than would be required without the fan(LP turbines need more pressure than HP turbines). The third is from the bypass air mixing with the core air and shielding it from shearing so violently with the relatively still air flowing around the engine.

Nothing is free though because noise you lose from core exhaust shearing reduction core is added in the fan/turbomachinery noise, the difference being the frequency and by extension how far that noise will travel (the exhaust noise from a jet will travel MUCH further than the turbomachinery noise).

Take it with a grain of salt, as I'm sure someone with more experience will correct any errors.
 
ImperialEagle
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Re: Why are MD-88's so loud?

Wed Mar 21, 2018 12:41 am

Some of us recall what it was like when the old pure-jets launched. If you knew someone close to a major airport that did not have the luxury of air conditioning,back in the day, then you knew some insomniacs.
The Mad Dogs are nearly silent compared to an -880. Just 'sayin.
"If everything seems under control, you're just not going fast enough!"
 
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TheRedBaron
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Re: Why are MD-88's so loud?

Wed Mar 28, 2018 3:18 pm

Now get into an Md83 and sit in the last 3 rows and:

Enjoy the view of and engine from the window...and not much else.
Enjoy the deafening sound of th turbofan....and hear not much else.
Enjoy the vibration of the spool up and down through the flight......

and then the reversers deployed..... simply AWESOME.

Best regards
TRB
The best seat in a Plane is the Jumpseat.
 
T prop
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Re: Why are MD-88's so loud?

Fri Mar 30, 2018 8:57 am

Loudest I heard were the old KC-135's and B-52's with water injection, Concorde and F-4's. B-1, F-15 and F-22 earn honorable mention in the dB category. Old U2 was loud considering they were fairly high when they passed overhead on takeoff.
 
WWads
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Re: Why are MD-88's so loud?

Fri Mar 30, 2018 12:32 pm

It's always fun to sit on the Sky Deck at the ATL F SkyClub and listen for departing MD88s. One can take off on the other side of the field and the windows still vibrate. If you get one on the nearby runway, wine glasses can shake too.

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