Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
MD80 & 727 Engine Sounds Questions  
User currently offlineBruce From United States of America, joined May 1999, 5056 posts, RR: 15
Posted (11 years 9 months 3 days 6 hours ago) and read 3489 times:

Couple things I'm curious about after this week's trip.

I was sitting near the back of an MD88 (Delta) and i could see the fan inside the engine. When the pilot started up the engine, the fan didn't turn!!!! The engine also made a kind of high pitched loud whine until we started our takeoff roll then it got quieter (when you would think it would be louder at full takeoff power!). While inflight i noticed that the fan was spinning. Why doesn't the fan turn all the time that the engine is "on"??

I also flew on a 727. Let me tell you those hushkits really make it quiet inside!!!! But while we were at the gate, I could hear from outside a loud, deep roar (I sat in row 28). it could not be the engine running because they were still loading bags into the aft compartment. What causes that noise??

bruce


Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
9 replies: All unread, jump to last
 
User currently offlineMirrodie From United States of America, joined Apr 2000, 7443 posts, RR: 62
Reply 1, posted (11 years 9 months 3 days 5 hours ago) and read 3439 times:
Support Airliners.net - become a First Class Member!

Well, I have been trying to learn on this forum, so I will take a stab at the 727 portion of the question. I think the sounds you heard prior to engine start was the APU (or the start cart, but I doubt this). (Guys, did I learn anything?)

We'll both learn about the fan turning question.

What 727 did you fly? NWA? I just flew their 727s and didn't feel it was that quiet. But I have noticed that yes, upon spooling up the engines of most aircraft, they get louder, then start to go into a lower tone of whine. Interesting. I hope to hear on this from you users.


regards, mirrodie



Forum moderator 2001-2010; He's a pedantic, pontificating, pretentious bastard, a belligerent old fart, a worthless st
User currently offlineBruce From United States of America, joined May 1999, 5056 posts, RR: 15
Reply 2, posted (11 years 9 months 3 days 3 hours ago) and read 3427 times:

It was a Delta 727, N518DA. I know that all planes have APU's but are the 727's in a different location or somehow different that would cause the noise to be heard? I know it sounds different than other planes.


Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
User currently offlineAA737-823 From United States of America, joined Mar 2000, 5820 posts, RR: 11
Reply 3, posted (11 years 9 months 3 days 3 hours ago) and read 3426 times:

Okay, an answer to both questions. First, the 727.

The 727 APU is in a very poorly chosen place- right between the wings, under the pax cabin floor. This makes it a poor place because it is oh-so-loud all of the time. You can definitely hear it. Also, it exhaust out the top of the wing, near where the wing joins the fuselage, if I am not mistaken. In some pics, you can see soot on the port side of the fuselage from the APU exhaust. This is rare (do a search of the photo database) because the outlet was designed to prevent this. Of course, later planes- starting for Boeing with the 737- put the APU in the very end of the tail empennage. This let it exhaust out the back end, and brought the noise down to almost imperceptable levels.

The MD-88 on the other hand, is a different explanation, in that your observations are partially incorrect.

That engine- a Pratt & Whitney JT8D-200 series, has a vane sort of thing on the front. It never rotates. Many people look into it an think the engine is not spinning when in fact it is. The first compressor blades are actually BEHIND that non-moving vane. The vane is there to direct the air going into the inlet. The point at which your observation is incorrect is when you say that later, it was turning. In fact, it was not. You may not have been able to see it at that time, due to changing daylight conditions or something I don't know. BUT- point is, it never turns.

Merry Christmas!!!!

Randy


User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1341 posts, RR: 26
Reply 4, posted (11 years 9 months 3 days 2 hours ago) and read 3415 times:

I was sitting near the back of an MD88 (Delta) and i could see the fan inside the engine. When the pilot started up the engine, the fan didn't turn!!!!

I'll list a couple of different possibilities:
- The engine was turning, but you were looking at the inlet guide vanes, which direct airflow to the compressor. These vanes are stationary
- As mentioned, on the Md80, the APU was probably starting and you mistook that for the engine starting. This is a possibility since you said you later saw the engine spinning.
- The flightcrew didn't start the engine until nearthe runway. Airplanes are often taxied using one engine. Because it qwas so loud, after the first engine was started, u probably didn't even notice when the 2nd was started even though it was right next to u.


User currently offlineBruce From United States of America, joined May 1999, 5056 posts, RR: 15
Reply 5, posted (11 years 9 months 3 days ago) and read 3404 times:

i know what you're talking about with the vanes in front and then the fan. I saw the stationary vane. And the fan behind it. It was the FAN behind the vane that was not turning. In fact, i believe I have a photo of it. I was trying to get a good shot (I was 1 row in front of it) but of course the shadow of the fuselage made only part of the diameter light. I wonder if it would be "uploadable". perhaps I'll sharpen it up and put a link to it on my site so you can see it.

I guess it's possible that only the left engine was "on" during taxi. The Galley was on the left side, and i could hear that annoying noise thru the Galley Exit door, so maybe that engine was on.

Thanks for the 727 APU explanation. That really makes sense considering what I heard. I didn't realize that APU's were so loud.

bruce



Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
User currently offlineTrnsWrld From United States of America, joined May 1999, 931 posts, RR: 0
Reply 6, posted (11 years 9 months 2 days 22 hours ago) and read 3393 times:

To try and answer your other question regarding why the aircraft was noisy then seemed to queit down after full throttle was applied. My thoughts are probably that since takeoff thrust is applied basically from a stop it would be louder, then, as the aircraft accelerates the noise for the most part is "behind" the aircraft. Ever hear a jet at cruising altitude only to see the aircraft and the sound in two completely different parts of the sky? same concept. Hope this helps.

User currently offlineBruce From United States of America, joined May 1999, 5056 posts, RR: 15
Reply 7, posted (11 years 9 months 2 days 22 hours ago) and read 3393 times:

I never thought of it like that. On the jets I was on, the Md80 and 727, the engines are behind me...so maybe the sound could trail back. But for example on a 757, with the engine in front of you (if you're behind the wing) then once they throttle up it's all just a roar.
bruce



Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
User currently offlineWilcharl From United States of America, joined Jun 2000, 1166 posts, RR: 3
Reply 8, posted (11 years 9 months 2 days 21 hours ago) and read 3385 times:

the engine wasnt spinnig because they were saving fuel (tell a pax in the last row to look at the engine and that the airline is saving fuel by not running the engines, the pax will turn white) in all seriousness you saw the IGVs the aft of the 9 and the 80 seem to get quiter in the back after the cabin starts to pressurize(aft door back there) once the engine sync is on, the engines also loose that nasty ressinence that gives pax in the back a headache

User currently offlineAA737-823 From United States of America, joined Mar 2000, 5820 posts, RR: 11
Reply 9, posted (11 years 9 months 1 day 22 hours ago) and read 3370 times:

Okay, to follow up to my post from last night.

I had not even considered that they were possibly taxying (taxiing?) on one engine. I had forgotten this. This is the most likely explanation. TWA taxied their -9s and -80s- and later, even the 717s as I found out- on only one engine. Once they reached a predetermined distance form the runway, the second engine was spooled up.

Also, a quality of the JT-8D is that, when it spools up to full throttle, it is indeed quieter than at idle. You can notice this on any MD-80 flight.

About loud APU's- well, the new ones are far quieter, but keep in mind that the 727 was originally designed in the early 60s, and APUs at that time were brand new. In fact, the 727 was the first American built aircraft to even HAVE an APU on board- 707s and DC-8s had to have a "start cart" wheeled up to the plane, an air hose plugged in, and air pumped to each jet engines starter in series. Even the later 727s (the -200advanced model) had loud apu's.

The sound trailing behind you theory is partially true, but not so much the main cause of your observation. You have to remember that the engine is bolted (we hope!) directly to the aircraft, and that the sound waves (transmitted through vibrations, as you know) are entering the aircraft that way. Certainly- a large portion of the noise is due to the exhaust of the engine, but I wouldn't think you would get very far from in front of it while on the runway... you'd have to be approaching supersonic for it to be truly quiet... interesting theory to think about... whoa, deep...

Anyhow, Merry Christmas!
Randy


Top Of Page
Forum Index

Reply To This Topic MD80 & 727 Engine Sounds Questions
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
P&W Waterfall Engine Noise On 757's posted Fri Jun 30 2006 21:42:08 by Rick
Change In Engine Sounds At Max Power posted Thu Jan 26 2006 06:35:53 by Dr.DTW
Aircraft Engine Tests - Questions posted Sat Oct 30 2004 01:36:24 by StudentFlyer
Engine Sounds-what Is It? posted Thu Jul 15 2004 03:36:40 by Ba97
Question About Jet Engine Sounds posted Sun Jun 20 2004 22:12:17 by Ngr
Runup Of P&W4360 Engine posted Tue Dec 2 2003 23:58:39 by Reedyreed
A330 & B767 Engine Component Question? posted Wed Oct 1 2003 01:34:29 by Mr Spaceman
737 Engine Sounds posted Mon Jul 28 2003 09:52:56 by TWAMD-80
Object In 737 & 747 Engine's Tailpipe - Question posted Thu Jun 5 2003 22:36:54 by Mr Spaceman
DC8 & DC10 Engine Strakes? posted Thu Jan 30 2003 01:29:33 by Boxsmasher

Sponsor Message:
Printer friendly format