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frugalqxnwa
Topic Author
Posts: 550
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Early DC-8 Engine Nacelle Question

Mon Jul 03, 2006 1:53 am

What is the purpose of the ring in back of the exhaust nozzles? They aren't thrust reversers since this picture was taken on takeoff with the rings extended:


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Photo © Art Brett - Photovation Images



About the only time I have seen the rings retracted is at the gate.


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Photo © John F. Ciesla

 
bohica
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RE: Early DC-8 Engine Nacelle Question

Mon Jul 03, 2006 2:18 am

I'm curious about that myself. However you might want to ask this question in the Tech/Ops forum. Someone there will be able to explain its purpose.  Smile
 
rabenschlag
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RE: Early DC-8 Engine Nacelle Question

Mon Jul 03, 2006 2:35 am

If I remember correctly, they were noise reduction devices which were extended during takeoff.
 
SPREE34
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RE: Early DC-8 Engine Nacelle Question

Mon Jul 03, 2006 2:38 am

Thrust reversers. Clam Shell type that were somewhat separated from the main nacelle. The "organ pipe" type exhaust outlets were supposed to reduce noise somewhat. Not much by todays standards.
I don't understand everything I don't know about this.
 
n8076u
Posts: 419
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RE: Early DC-8 Engine Nacelle Question

Mon Jul 03, 2006 3:06 am

Those rings are called ejectors. They were extended before takeoff and before landing. They had two purposes. One was to cover/uncover the reverser buckets (before the reversers could be used, these had to be deployed), and the other was for reducing noise during takeoff and landing. In simple terms, the exhaust coming out of the back of the engine would get mixed and enveloped by fresh outside air, then go through the ejectors (which had some noise muffling materials as well) which would reduce the exhaust noise. Something like that anyways.  Smile

Chris
Don't blame me, I don't work here...
 
Jetfixr757
Posts: 138
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RE: Early DC-8 Engine Nacelle Question

Mon Jul 03, 2006 5:03 am

Well said 8076U, and you are right on target with that. DC-8 got me my current airline job, it also got me a 3 year world tour/company paid vacation.
Jet
 
timz
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RE: Early DC-8 Engine Nacelle Question

Mon Jul 03, 2006 6:25 am

Quoting SPREE34 (Reply 3):
Thrust reversers.

So the UA DC-8 in the pic is taking off with reverse thrust?
 
n8076u
Posts: 419
Joined: Mon Jun 19, 2006 10:52 am

RE: Early DC-8 Engine Nacelle Question

Mon Jul 03, 2006 6:51 am

Quoting Timz (Reply 6):
So the UA DC-8 in the pic is taking off with reverse thrust?

No, the reverser buckets themselves are not deployed (they are controlled by the reverser levers on the throttles). They were merely "exposed" when the ejectors moved to the rear (controlled by two switches on the overhead for inboard and outboard pairs) so the reversers "could" be used during landing or a rejected take-off.

The ejectors, when stowed in the forward position, would be physically in the way of the reverser buckets (also called clamshells) movement to the reverse thrust position, and would also be in the way of the path the exhaust takes (out the sides) during reverse thrust.

Hope that made sense.

Chris
Don't blame me, I don't work here...
 
crownvic
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RE: Early DC-8 Engine Nacelle Question

Mon Jul 03, 2006 1:27 pm

Wow! And I only thought there were 15-19 years olds on this site! Chris has it all correct. The only thing that I can add is that the ejector ring when in the extended position (this ring traveled on a track on the lower pylon) for noise reduction, actually pulled the sound into a narrow channel through the ring, thus reducing the noise outflow footprint. Using today's standards, it probably took it from Stage I noise levels to Stage I 1/4!
 
n8076u
Posts: 419
Joined: Mon Jun 19, 2006 10:52 am

RE: Early DC-8 Engine Nacelle Question

Mon Jul 03, 2006 1:39 pm

Quoting Crownvic (Reply 8):
it probably took it from Stage I noise levels to Stage I 1/4!

That's probably being generous as well...

I forgot to add that the DC-8s with this ejector arrangement were turbojet powered, so that must have been one hell of a "noise footprint".  Smile

Chris
Don't blame me, I don't work here...
 
Flyinround731
Posts: 181
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RE: Early DC-8 Engine Nacelle Question

Mon Jul 03, 2006 4:02 pm

I'd been led to believe that the DC-8-20 and DC-8-30 models were not equipped with thrust reversers, after reading that one use of ring deployment was to allow use of them during landing and possible aborted takeoff, I guess I was wrong. What about the DC-8-10s, did they have thrust reversers?
 
n8076u
Posts: 419
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RE: Early DC-8 Engine Nacelle Question

Mon Jul 03, 2006 4:23 pm

Quoting Flyinround731 (Reply 10):
What about the DC-8-10s, did they have thrust reversers?

There's another thread around here somewhere where I explained some of this, but I'll give a brief summary: The first DC-8 that ever flew did not originally have any thrust reversers, but just this one single aircraft was like that. At some point during the flight testing program, they did modify that aircraft and installed thrust reversers on it. All other production aircraft after that came off the line with thrust reversers.

Chris
Don't blame me, I don't work here...
 
cedarjet
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RE: Early DC-8 Engine Nacelle Question

Mon Jul 03, 2006 8:02 pm

So after takeoff the ejectors were retracted for better aerodynamics, and redeployed at TOD? Interesting, I've always found the DC8 too weird to take a serious interest - a lot of variants, every picture of a DC8 I ever see has a different exit configuration, different engines, etc. I've seen the (I now know) ejector rings but just dismissed them as the umpteenth DC8 eccentricity and never investigated further. Thanks for the info.
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz
 
morhas
Posts: 34
Joined: Sat Jun 17, 2006 11:30 pm

RE: Early DC-8 Engine Nacelle Question

Mon Jul 03, 2006 11:58 pm

As described by N8076U, the additional structure was used in early engine to reduce the noise. This structure also called corrugated silencer. It help to mix hot air and cold air faster. After engineer came out with the design of High Bypass Ratio engine, this structure appeared to be hidden inside the engine.

Recently, Boeing 787 and 747-8 has introduced special shape of structure at the nacelle. This structure also will do the same job.
 
n8076u
Posts: 419
Joined: Mon Jun 19, 2006 10:52 am

RE: Early DC-8 Engine Nacelle Question

Tue Jul 04, 2006 3:08 am

Quoting Cedarjet (Reply 12):
So after takeoff the ejectors were retracted for better aerodynamics, and redeployed at TOD?

Maybe not as soon as TOD, but at least in the time period around when flaps are being brought out for landing, but otherwise, you've got it.

Quoting Cedarjet (Reply 12):
a lot of variants, every picture of a DC8 I ever see has a different exit configuration, different engines, etc.

Apparently, to give them an edge in competition with the 707, Douglas allowed customers to customize their aircraft to suit their needs far more than Boeing ever did. So it is safe to say that no two DC-8s are alike, and certainly different than Boeings of the time.

Here's another oddity to add to the list, DC-8 thrust reversers could be used in flight, discussed here:
DC-8 And Reverse Thrust Use In Flight (by Ttailsteve Jul 3 2006 in Civil Aviation)


Chris
Don't blame me, I don't work here...
 
wjcandee
Posts: 10232
Joined: Mon Jun 05, 2000 12:50 am

RE: Early DC-8 Engine Nacelle Question

Tue Jul 04, 2006 3:56 am

Quoting N8076U (Reply 14):
Maybe not as soon as TOD

Or maybe yes, if they were using the reversers as speed brakes.

Quoting N8076U (Reply 14):
DC-8 thrust reversers could be used in flight

Yeah, although my recollection of that is that they could *deployed* in flight, but you wouldn't typically bring thrust above idle.

[Edited 2006-07-03 20:58:12]
 
aeroweanie
Posts: 1576
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RE: Early DC-8 Engine Nacelle Question

Tue Jul 04, 2006 10:50 am

Quoting N8076U (Reply 4):
Those rings are called ejectors. They were extended before takeoff and before landing. They had two purposes. One was to cover/uncover the reverser buckets (before the reversers could be used, these had to be deployed), and the other was for reducing noise during takeoff and landing.

Exactly! Here are further examples of the use of ejectors to reduce engine noise:

Nordam's 737-200 Phase 1 hush kit (this adds a fixed ejector in front of a stock 737-200 thrust reverser):

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Photo © Caz Caswell


QTA's GII/III hush kit (this adds a fixed ejector aft of the stock thrust reverser):

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Photo © Spencer Wilmot - Photovation Images


QTV's BAC 1-11 hush kit (this adds a fixed ejector aft of the stock thrust reverser):

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Photo © Royal S King


Really Quiet's GII hush kit (this adds a moving ejector, with an all-new thrust reverser):

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Photo © Gerhard Plomitzer


Stage III Technologies GII/III hush kit (this adds a fixed ejector aft of the stock thrust reverser):
 
ImperialEagle
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Joined: Sat Jan 14, 2006 10:53 am

RE: Early DC-8 Engine Nacelle Question

Tue Jul 04, 2006 10:57 am

Every now and then "back in the day" I would find myself on one of (DL's) ex-PA DC-8-30's. As I mentioned in another thread, I used to enjoy the window seats at either the #2 or #3 position----just in case inflight reverse thrust was used.

I used to enjoy watching the ejector rings wobble along their cable/track arrangement after takeoff. And I liked to anticipate their travel aft in preparation for a serious descent if circumstances demanded! The inflight use of reverse thrust was really effective!
"If everything seems under control, you're just not going fast enough!"
 
crownvic
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Joined: Wed Oct 20, 2004 10:16 pm

RE: Early DC-8 Engine Nacelle Question

Tue Jul 04, 2006 2:48 pm

Chris...Got to correct you on one point. The -40 model was actually RR Conway powered and featured the ejector ring. Many are not aware that the Conway was a very low bypass turbo-FAN. The -11 through -33 were turbo-Jets.
 
n8076u
Posts: 419
Joined: Mon Jun 19, 2006 10:52 am

RE: Early DC-8 Engine Nacelle Question

Tue Jul 04, 2006 3:29 pm

Quoting Crownvic (Reply 18):
The -40 model was actually RR Conway powered and featured the ejector ring.

I know, I know.  Wink

It is confusing enough without adding the -40s into the mix, but you are right.

Chris
Don't blame me, I don't work here...
 
crownvic
Posts: 2789
Joined: Wed Oct 20, 2004 10:16 pm

RE: Early DC-8 Engine Nacelle Question

Tue Jul 04, 2006 3:44 pm

I have to look like an authority once in awhile  Smile

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