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LTU932
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Question On Nose Intakes On Douglas DC-8

Wed Mar 28, 2007 12:30 pm

I searched for a thread on this, but couldn't find it. My question is about these intakes, that are right by the nose of Douglas DC-8s.

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Photo © Lars Söderström

What's the purpose of these intakes? I haven't seen such intakes on other contemporary aircraft such as the 727 or 707, just on the DC-8.
Sometimes the only thing more dangerous than a question is an answer. - Ferengi Rule of Acquisition 208
 
N231YE
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RE: Question On Nose Intakes On Douglas DC-8

Wed Mar 28, 2007 1:02 pm

Those are for the turbocompressors, used to pressurize the cabin. The Boeing 707/720 had them in the engine nacelles, just above the engine's air intake.
 
citationjet
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RE: Question On Nose Intakes On Douglas DC-8

Wed Mar 28, 2007 5:51 pm

Boeing Flown: 701,702,703;717;720;721,722;731,732,733,734,735,73G,738,739;741,742,743,744,747SP;752,753;762,763;772,773,788.
 
113312
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RE: Question On Nose Intakes On Douglas DC-8

Thu Mar 29, 2007 4:04 am

Each of those chin scoops have three holes. There is a large opening in the middle which takes in air for the air-to-air heat exchanger on that side. There are also two smaller holes that are the individual inlets for the turbocompressors. On the DC-8, there are a total of 4 turbocompressors. The Boeing 720 series had two turbocompressors and most 707 series aircraft had three.

It should be pointed out that the DC8/Boeing 707/720 generation of aircraft didn't bleed air directly from the engines for pressurization and air conditioning. Rather, a small amount of engine bleed air was used to spin turbo-compressors which drew in fresh air, compressed it and raised it's temperature. This air, cooled via heat exchangers and freon units, was directed to the cabin distribution system.

The Boeing 727, 737, and DC-9 generation of planes using JT8 series engines were the first to use bleed air from the engine itself, through air conditioning "packs" to directly pressurise the cabin and for temperature control.

In later years, when the JT3 engines, on DC8s, were replaced with CFM-56 powerplants, the turbocompressors and freon cooling units were removed and replaced with "packs" that allowed engine bleed air to be used for temperature control and cabin pressurization. The DC8 chin scoops were modified to close off the turbocompressor inlets.
 
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LTU932
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RE: Question On Nose Intakes On Douglas DC-8

Thu Mar 29, 2007 11:01 am

Quoting N231YE (Reply 1):
Those are for the turbocompressors, used to pressurize the cabin. The Boeing 707/720 had them in the engine nacelles, just above the engine's air intake.

You mean here:

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Photo © Carlos Borda

While I know all engines have these "extensions" on all engines except the number 1, it's curious to see that in this freighter, it's not on the number 4 as well.

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Photo © Normando Carvalho Jr.
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Photo © Normando Carvalho Jr.

I guess that since this aircraft isn't used for PAX flights anymore, it was removed from the number 4.

Quoting 113312 (Reply 3):
Each of those chin scoops have three holes. There is a large opening in the middle which takes in air for the air-to-air heat exchanger on that side. There are also two smaller holes that are the individual inlets for the turbocompressors. On the DC-8, there are a total of 4 turbocompressors. The Boeing 720 series had two turbocompressors and most 707 series aircraft had three.

It should be pointed out that the DC8/Boeing 707/720 generation of aircraft didn't bleed air directly from the engines for pressurization and air conditioning. Rather, a small amount of engine bleed air was used to spin turbo-compressors which drew in fresh air, compressed it and raised it's temperature. This air, cooled via heat exchangers and freon units, was directed to the cabin distribution system.

The Boeing 727, 737, and DC-9 generation of planes using JT8 series engines were the first to use bleed air from the engine itself, through air conditioning "packs" to directly pressurise the cabin and for temperature control.

In later years, when the JT3 engines, on DC8s, were replaced with CFM-56 powerplants, the turbocompressors and freon cooling units were removed and replaced with "packs" that allowed engine bleed air to be used for temperature control and cabin pressurization. The DC8 chin scoops were modified to close off the turbocompressor inlets.

Thanks for the detailed explanation!
Sometimes the only thing more dangerous than a question is an answer. - Ferengi Rule of Acquisition 208
 
N231YE
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RE: Question On Nose Intakes On Douglas DC-8

Thu Mar 29, 2007 1:19 pm

Quoting LTU932 (Reply 4):
You mean here:

I should have said the TCs were in the engine pylons, with the opening just above the engine air intake.
 
Grunf
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RE: Question On Nose Intakes On Douglas DC-8

Thu Mar 29, 2007 8:44 pm

Early jet engines had a bad habbit of leaking some oil troughout compressor stages so an turbocharger was used to isolate PAX from bleed air which was considered "dirty". New engines didn't have the problems anymore and turbochargers were removed...
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CaptOveur
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RE: Question On Nose Intakes On Douglas DC-8

Fri Mar 30, 2007 3:15 pm

Quoting LTU932 (Reply 4):
I guess that since this aircraft isn't used for PAX flights anymore, it was removed from the number 4.

Nope.. 707s only had 3 turbocompressors, no matter what they are being used for.
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411A
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RE: Question On Nose Intakes On Douglas DC-8

Sat Mar 31, 2007 1:43 am

Well, in actual fact, some B707 freighter aircraft did indeed have the number four engine turbo-compressor removed.
Two T/C's are sufficient on B707 freighter aircraft to keep the cabin properly ventilated, as well as, of course, pressurized.

Now, someone might ask....'what happens IF one of the turbocompressors fails in flight?'

This is generally not a problem, as most B707's that did NOT have vapor (freon) cycle air conditioning, also had engine bleeds, that cound be used to pressurize the cabin.
The only proviso with using engine bleeds in lieu of T/C's was that descents needed to be started sooner, using partial thrust as opposed to idle thrust, as the engine bleed air did not have the volume capacity as a turbo-compressor delivered.

Making bold statements about the B707 without ever having actually flown one sometimes paints one into a corner, as some of the information provided about this rather remarkable airplane on some internet sites is, at best, somewhat misleading.

As for me personally, I have flown, in Command, many models of the B707, both freighter and passenger versions, straightpipe and turbofans, for over ten years in the past, prior to moving to the L1011.

Next question?
 
N231YE
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RE: Question On Nose Intakes On Douglas DC-8

Sat Mar 31, 2007 2:35 am

Quoting 411A (Reply 8):
Next question?

Can I have your job?  silly 
 
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Starlionblue
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RE: Question On Nose Intakes On Douglas DC-8

Sat Mar 31, 2007 3:02 am

Quoting CaptOveur (Reply 7):
Quoting LTU932 (Reply 4):
I guess that since this aircraft isn't used for PAX flights anymore, it was removed from the number 4.

Nope.. 707s only had 3 turbocompressors, no matter what they are being used for.

As 411A says, aircraft come in many variations. This is especially true when they start to age.

My guess is that the 707s always had 3 turbocompressors when they left the factory. But as stated above, this was removed in some cases. After market mod just like a trunk wing on a Honda.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
411A
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RE: Question On Nose Intakes On Douglas DC-8

Sat Mar 31, 2007 3:27 am

Of course, N231YE, you can have my job...so long a I get paid.

We are, however, starting to run out of L1011 First Officers, as there are not many L1011's left in service, and the younger First Officers want to move onto an airplane that has, shall we say, a longer future.

Within the next few months, those in southern Europe will see, once again. scheduled service with the L1011...look for it in Lisbon....if all goes according to plan.

The AOC is issued, the airplane will be in maintenance shortly, and the funding is in place.

Ticket sales, on a limed basis, will be next.
One step at a time.
 
2H4
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RE: Question On Nose Intakes On Douglas DC-8

Sat Mar 31, 2007 5:12 am




Quoting 411A (Reply 11):
the younger First Officers want to move onto an airplane that has, shall we say, a longer future.

Fools.  yes 


2H4


Intentionally Left Blank
 
113312
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RE: Question On Nose Intakes On Douglas DC-8

Sat Mar 31, 2007 6:24 am

American Airlines ordered most of their B707 aircraft with only two turbocompressors. Most of the 707-320 freighters you see today that have turbocompressor inlets only on the inboard engines will likely turn out to be -323B/C models originally from American Airlines. It isn't that the third T/C was removed, it was never installed in the first place.

I would like to note that American was an early customer of the turbojet 707-123 before the days of the JT3D "fan" engine. On these organ pipe turbojets, there is a hole above all four of the main engine inlets. On the AA aircraft, the outboard engines didn't have T/Cs so this was a dummy hole. When these ships were converted to the turbofan engines, the pylon was modified so that the inboard ones had the holes for the T/C and the outboard ones didn't just like the B720B models.
 
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LTU932
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RE: Question On Nose Intakes On Douglas DC-8

Sat Mar 31, 2007 9:28 am

Quoting 113312 (Reply 13):
Most of the 707-320 freighters you see today that have turbocompressor inlets only on the inboard engines will likely turn out to be -323B/C models

Indeed. I didn't even notice until now that this picture I linked was from a former AA bird.

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Sometimes the only thing more dangerous than a question is an answer. - Ferengi Rule of Acquisition 208
 
N231YE
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RE: Question On Nose Intakes On Douglas DC-8

Sat Mar 31, 2007 10:14 am

Quoting 411A (Reply 11):
We are, however, starting to run out of L1011 First Officers, as there are not many L1011's left in service, and the younger First Officers want to move onto an airplane that has, shall we say, a longer future.



Quoting 2H4 (Reply 12):
Fools.   

Glass cockpits make nice eye candy...but real pilots fly steam   

Quoting 411A (Reply 11):
so long a I get paid.

Well, um...you see, being a [broke] college student, um, I have holes in my pockets.

[Edited 2007-03-31 03:27:34]
 
ReidYYZ
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RE: Question On Nose Intakes On Douglas DC-8

Sat Mar 31, 2007 10:22 am

Quoting Starlionblue (Reply 10):
My guess is that the 707s always had 3 turbocompressors when they left the factory. But as stated above, this was removed in some cases. After market mod just like a trunk wing on a Honda.

Yeah, but the modified seven-oh didnt make the pilot look like a mouth-breather flying it.
 
slz396
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RE: Question On Nose Intakes On Douglas DC-8

Sun Apr 01, 2007 4:05 am

Quoting 113312 (Reply 3):
It should be pointed out that the DC8/Boeing 707/720 generation of aircraft didn't bleed air directly from the engines for pressurization and air conditioning. Rather, a small amount of engine bleed air was used to spin turbo-compressors which drew in fresh air, compressed it and raised it's temperature. This air, cooled via heat exchangers and freon units, was directed to the cabin distribution system.

I thought bleedless engines were something new, but it seems the 787 is simply going back to a design from the past?
 
redflyer
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RE: Question On Nose Intakes On Douglas DC-8

Fri Apr 06, 2007 7:41 am

Quoting Slz396 (Reply 17):
I thought bleedless engines were something new, but it seems the 787 is simply going back to a design from the past?

If you really think about it, they are using a form of "glue" -- the resin in the CFRP -- to build the 787. So I guess that is quite the throw back to the early age of aviation when glue was used to hold together components of an airplane.  Wink
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Jetlagged
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RE: Question On Nose Intakes On Douglas DC-8

Fri Apr 06, 2007 8:32 am

Quoting RedFlyer (Reply 18):
So I guess that is quite the throw back to the early age of aviation when glue was used to hold together components of an airplane.

Glue never went away. De Havilland (then Hawker Siddeley, then BAe) used glue (called redux) for aircraft like the Comet, Trident and BAe146. It was a metal to metal bond used to replace rivets, which can cause fatigue problems.
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