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Guest

DC-8s With "blowtorches"

Thu Jan 03, 2002 7:37 am

Hi everybody!

I am probably not the first to ask this but I'd love to know the exact function of all of that daunting equipment on the rear part of the DC-8-40's RR Conways,did Douglas have the intention to create something of a "Harley-touch" on it's jetliners in order to boost sales..... (just kidding).I assume it truly makes the dash 40 the hottest-looking of the Baby-eights.

Click for large version
Click here for full size photo!

Photo © Giorgio Adamo



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Photo © John F. Ciesla


And why did the Conway that powered the 707-420 have a less bulky exhaust-nozzle(the more classic"daisy-petals")

Thankz and Regards,
Ganymed
 
FBU 4EVER!
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RE: DC-8s With "blowtorches"

Thu Jan 03, 2002 7:55 am

These are part of the sound silencers.The early turbojets were very noisy,and because of the DC-8's design it was somewhat more prone to become a victim of what became known as "sonic fatigue".This was,partly,solved by increasing the fuselage skin thickness aft of the wing by almost 50% and using a different silencer than the 707.
"Luck and superstition wins all the time"!
 
411A
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RE: DC-8s With "blowtorches"

Thu Jan 03, 2002 9:56 am

That "daunting equipment" was......the thrust reverser.
 
flight152
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RE: DC-8s With "blowtorches"

Thu Jan 03, 2002 10:21 am

No 411A, they are noise reducing equimpment.
 
411A
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Joined: Mon Nov 12, 2001 10:34 am

RE: DC-8s With "blowtorches"

Thu Jan 03, 2002 11:04 am

You're wrong, Flight152, the picture of the Alitalia DC8 clearly showes the thrust reverser. Have a close look, you can see the tracks on which it moves aft.
How do I know you ask.....the chief production test pilot at Douglas was a good friend of mine.
When the engine is removed, the thrust reverser stays behind. Clever design, but rather high drag.
As you can see, I have been around a long time.
Any more questions son?
 
777236ER
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RE: DC-8s With "blowtorches"

Thu Jan 03, 2002 11:33 am

The "daisy petal" is a sound silencer! It mixes the hot exhaust with cold ambiant air. Actually, it still exists to day in RB211s and i THINK, Trents.

The first pic looks like a thrust reverser. I don't know, and i don't want to get pulled into the little war i know y'all are gonna have  Big grin
Your bone's got a little machine
 
411A
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RE: DC-8s With "blowtorches"

Thu Jan 03, 2002 11:54 am

Actually 777236ER, the "daisy petal", altho very effective on the JT3 and Conway at reducing noise, is NOT used on the RB.211, and never has been on production aircraft. "Tis not needed, the RB.211 is a VERY quiet engine.
 
Minuteman
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RE: DC-8s With "blowtorches"

Thu Jan 03, 2002 1:31 pm

Well pops, "always" and "never" are never absolute ...  Big grin

Here's one application that uses a "daisy-petal" type mixer. Granted, its not with the free-stream, but with bypass air. Perhaps this mixer is why its a quiet engine?

Rolls-Royce RB211-524D4D, as can be found on some 747s.

 
KCLE
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RE: DC-8s With "blowtorches"

Thu Jan 03, 2002 1:32 pm

How did the "diasy petals" work, and how did the thrust reversers work on any DC-8. I've never seen a DC-8 fly, and the only one I've ever seen is the DC-8 at CLE, that never moves, at least not when I'm there. Here are some pics I found in the 1980 Childcraft book I got from the library awhile ago when they had a book sale, I believe it was 1995. What version of DC-8 is that United, and are the mechanics working on the thrust reverser for that DC-8?

I had to piece the pic together because I couldn't fit the whole book on the scanner, hence the color differences and jagged edges.

 
411A
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RE: DC-8s With "blowtorches"

Thu Jan 03, 2002 4:33 pm

I think you will find Minuteman that the "mixer" is for the hot stream and that these -524D4D engines have been modified to eliminate the mixer from the series, by service bulletin.
 
Guest

RE: DC-8s With "blowtorches"

Thu Jan 03, 2002 5:38 pm

OK guys ,now I know some about the purpose of all of this equipment (noise reduction + thrust reversing).
But didn´t those silencers have the disadvantage to increase fuel consumption + weight significantly ?
 
411A
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RE: DC-8s With "blowtorches"

Fri Jan 04, 2002 1:06 am

Ganymed---
On the early DC8's, yes they did increase fuel consumption, and their was a Douglas service bulletin to remove the reversers from some models of the DC8, because as you know, thrust reverse is not used in calculating field length requirements for FAA certification, unlike the UKCAA which DO include reverse in their calculations.
Having flown several models of the B707 with non-fan engines, I can tell you that reverse on these engines made mostly....noise, with very little deceleration.
 
YWG
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RE: DC-8s With "blowtorches"

Fri Jan 04, 2002 5:46 am

When a DC-8 "wet starts" it might let out an 8ft flame from the engine. A good example of this today is when say an A330 starts up, a puff of smoke will come out the end. It happens when the engine gets to much fuel.
Contact Winnipeg center now on 134.4, good day.
 
777236ER
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RE: DC-8s With "blowtorches"

Fri Jan 04, 2002 7:53 am

Thank you Minuteman.

is NOT used on the RB.211, and never has been on production aircraft.

Read what Minuteman said, 411A

Your bone's got a little machine
 
Minuteman
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RE: DC-8s With "blowtorches"

Fri Jan 04, 2002 12:07 pm

Easy there 777236ER ... its a discussion, not an argument.

I'm certainly not an expert in nozzle design, but I just seemed to remember a picture from one of my old propulsion books that showed an RB211 with this kind of mixer and it struck me as odd that they're still using something like this.

Apparently a variant of the blowtorch is still in use, or at least the picture on Rolls-Royce's website is outdated.
http://www.rolls-royce.com/civil/products/turbofans/rb211-524/Default.htm

411A, I'm confused as to making an exception for the mixer being used for the hot stream. I was of the impression that the purpose of the mixer was to mix the turbine exhaust flow with the cooler free-stream or bypass air for two main reasons.

First, to mix the cooler, denser bypass air with the core flow to quiet noise from the combustor. Also, to energize the column of air surrounding the core exhaust. There is a significant difference in the gas temperatures and velocities and a lot of shearing going on. It seemes like you don't want to do this (wanting high exhaust velocity in high-speed cruise) and would be good for only low speed, quieter flight.

Another point in the case against 'em; these mixers live in a very high-energy part of an aircraft and, I believe, have been known to crack and deform.
 
EssentialPowr
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RE: DC-8s With "blowtorches"

Fri Jan 04, 2002 12:15 pm

The AE3007 has a daisy petal nozzle; the daisy petal nozzle of the RB211-535s is readily apparent from a rear view.

A nozzle is as basic a component on a jet as a compressor; the daisy petal design accomplishes exactly what Minuteman describes above...
 
411A
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RE: DC-8s With "blowtorches"

Fri Jan 04, 2002 12:19 pm

Well Minuteman, notice in your diagram that the mixer is "fitted" to the hot stream only, so that it mixes the hot stream with the bypass air for quiet(er) operation.
And yes they are subject to continued stress, in fact I had one depart the number two engine on a B707-331 some years ago, while landing at Luanda.
Then it was REALLY loud, but I don't think the locals there noticed.
 
exPratt
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RE: DC-8s With "blowtorches"

Fri Jan 04, 2002 10:57 pm

On the early DC-8s that were equipped with JT3Cs and JT4Cs, the collar around the exhaust mixer was known as the ejector. It was operated by switches in the overhead panel between the pilots and had to be extended for landing and takeoff. When the ejectors were extended, a light (I think it was blue) would illuminate over the engine gages in the center panel. The primary purpose was to duct air around the mixer to break up and mix with the core flow to reduce the noise. Because the ejector covered the thrust reversers, the ejector had to be extended for landing and also for takeoff in the event of a rejected takeoff. With the development of the JT3D fan engine that was much quieter than the 3Cs and 4Cs (hard to believe there were engines louder than 3Ds), the mixers and ejectors were no longer required.

The picture of the UAL DC-8 taking off with the ejectors stowed and mixers showing while it is a great shot suggests the picture was staged and not just a photo of a revenue service takeoff. Stowing the ejectors were on the after takeoff checklist that would be accomplished well after the landing gear had been retracted and not immediately after rotation.
 
rmm
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Joined: Sun Feb 04, 2001 10:34 am

RE: DC-8s With "blowtorches"

Fri Jan 04, 2002 11:24 pm

Just out of curosity, here's a pic of a CFM56-5C out of
an A340 with the mixer.

http://www-psao.grc.nasa.gov/Images/engines/cfm56-5c.gif

Rmm
 
access-air
Posts: 1577
Joined: Thu Sep 21, 2000 5:30 pm

RE: DC-8s With "blowtorches"

Sat Jan 05, 2002 4:17 am

Quite actually,

The early design for noise suppression and thrust reverse on the older DC8-10, 20, 30, 40 series...was more or less duplicated but to a different specification on the later stretched DC8-62 and 63..the whole assembly that slides rearwards ont he 62 and 63 only slides back when the thrust reversers are being apllied and then moved back forward and tucked flush with the rest of the nacelle when roll out is complete. The DC8 seris 50 and 61 had completely different design all to gether that had cascades on the ducted fan portion on their JT3Ds.....as for the Boeing 707's witht he JT3Cs and JT4s and the Conways..they had cascade thrust reversers just ahead of the exhaust mixer nozzle....im imagining that reverse thrust setting was always an earth shattering rumble on these planes as the noise suppression is well behind that type of set up....The cascade system was also used for reversers on the BAC-111, VC-10, Trident, Caravelle with Avons and P&WJT8s, and later versions of the 727 reversers...early 727 reversers were target type clam shells..okay ...ive said waaaaay too much.....

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