jeffry747
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Curious Engine Noise On DC-8s

Thu Jun 30, 2005 6:47 pm

I've often noticed on Douglas DC-8s at the UPS hub in Louisville, KY,that the CFM56 engines emit a very loud and deep bass noise on start up. Anyone know what causes this? Iv'e only heard the noise come from CFM56 engines.
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lamedianaranja
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RE: Curious Engine Noise On DC-8s

Thu Jun 30, 2005 7:01 pm

And what are the holes for under the nose? Might they cause the noise?


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jeffry747
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RE: Curious Engine Noise On DC-8s

Thu Jun 30, 2005 7:13 pm

The foward facing inlets are for the air conditioning ram air system, and the "slots" on the side are the air system heat exhaust
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broke
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RE: Curious Engine Noise On DC-8s

Thu Jun 30, 2005 9:48 pm

What I think you are hearing is the combustion section of the engine during the start sequence. I have heard similar sounds, that you have described, on the DC-8-70's and on L-1011's.
 
air2gxs
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RE: Curious Engine Noise On DC-8s

Thu Jun 30, 2005 10:02 pm

Quoting Jeffry747 (Thread starter):
CFM56 engines emit a very loud and deep bass noise on start up

The engine has either fuel nozzles or fuel injectors. If fuel nozzles are installed, you will hear (and feel) that deep rumble at start-up. If the engines have injectors installed, the engine will not produce that rumble. You know, I may have it backwards, but either way, it's the type of fuel delivery into the combustor that makes the noise.
 
hirisk
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RE: Curious Engine Noise On DC-8s

Thu Jun 30, 2005 10:58 pm

i love that rumble.like it even more then the boom from the JT8D's on mad dogs when they start.
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N79969
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RE: Curious Engine Noise On DC-8s

Thu Jun 30, 2005 11:01 pm

Interesting question. I have wondered about the same sound from those exact airplanes. It is very distinct. Interestingly, I have never heard that from any other CFM56 powered airplane-- neither the 737 or A320.
 
pilotpip
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RE: Curious Engine Noise On DC-8s

Fri Jul 01, 2005 1:34 am

Every now and then you'll hear it out of a 737. However, the DC-8 has a great sound when it starts. That's the highlight of my night on the ramp.
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flysc
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RE: Curious Engine Noise On DC-8s

Fri Jul 01, 2005 8:32 am

I work on the UPS ramp at CAE and I have to say it is amazing to stand about 20-30 ft away from that engine when it is starting up. It just kind of shakes your body. Starting up the DC-8 is really noisy because of the engines and the airstart equipment that we use.
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jetjack74
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RE: Curious Engine Noise On DC-8s

Thu Jul 07, 2005 2:08 am

Quoting Lamedianaranja (Reply 1):
And what are the holes for under the nose? Might they cause the noise?

The 707 has the same ram-air inlet under the fuselage, on the belly, same function
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avioniker
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RE: Curious Engine Noise On DC-8s

Thu Jul 07, 2005 2:15 am

You'll hear the same noise from the CF-6 as well. It's caused by the turbulence created when the fuel is first introduced and the flow isn't yet smooth through the cans and turbine section.
Common in GE designs.
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Jetlagged
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RE: Curious Engine Noise On DC-8s

Thu Jul 07, 2005 7:17 pm

Quoting Jetjack74 (Reply 9):
The 707 has the same ram-air inlet under the fuselage, on the belly, same function

The air conditioning inlets on the 707 are located in the engine pylons (well 3 out of the 4). The much smaller ducts you see in the centre-wing section of many airliners are for the ram air cooling of the pack heat exchangers. This air doesn't enter the cabin.
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jetjack74
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RE: Curious Engine Noise On DC-8s

Sat Jul 09, 2005 12:41 am

Quoting Jetlagged (Reply 11):
The air conditioning inlets on the 707 are located in the engine pylons (well 3 out of the 4)

On the early JT3/4 engines straight-pipes, they're on all four. Interestingly, the 720/B's and AA's 323B/C's have only 2 on both inboard engines. Sorry, my explanation was mistaken concerning the underside inlets. I was equating the the 2 nozzles under the nose of the DC8 with the nozzles under the centre wing section of the 707. But the DC8-50/61 have the same turbocompressor inlets on the underside of the nacelles.
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JETPILOT
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RE: Curious Engine Noise On DC-8s

Fri Jul 15, 2005 2:03 pm

The inlets on the nose of the DC8 are for the turbo compressors to pressurize the cabin. There is also an inlets to cool the heat exhangers on the air aonditioning pacs.

On the 70 series the pressurization comes from air bled off the engines, and the turbocompressors are removed and the openings plugged.

On the 50/62/63 series the turbocompressors are still installed and are spun up for an overspeed test just before engine start, and they make an awesome sound.

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JET

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