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Joined: Wed Apr 04, 2001 7:57 am

B707 Engine Pylons

Wed May 02, 2001 10:35 am

This might seem like a strange question, but I've always wondered: Why are the engine pylons different on the #2 and #3 engines on 707's? The pylon comes all the way to the front of the engine nacelle, as in this picture.

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Photo © Kjell nilsson

I haven't seen any pylons like this on other jetliners, nor have I seen jetliners where the outboard engines have standard pylons, and the inboard ones are different. Is this simply more favorably aerodynamically, or does the longer pylon house some sort of equipment. Any insights would be appreciated, since when I know the answer to this question, I'll no longer have to lie awake at night thinking about it! Big grin


RE: B707 Engine Pylons

Wed May 02, 2001 11:02 am

The thing on the inboard pylons is a turbocompressor so the cabin can maintain pressurization...there used to be one on all four when the 707 had turbojets, but with the turbofans, for some reason two were enough, so they omitted the outer ones.

RE: B707 Engine Pylons

Wed May 02, 2001 11:06 am

Wait...I know now...the pic you posted is that of a 720...the shorter version of the 707. The reason why this one has only two turbocompressors is the shorter cabin...doesn't need the extra oomph of four turbocompressors.
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RE: B707 Engine Pylons

Wed May 02, 2001 11:16 am

Yup, if you look at other 707 pics you'll see the turbocompressor (or at least the housing for it) on #4 as well-- usually. Some freighters lacked the #4.
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RE: B707 Engine Pylons

Wed May 02, 2001 8:59 pm

The 720, which is pictured, had the turbocompressors on engines 2 and 3. The 707 had the turbocompressors on engines 2, 3, and 4.
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RE: B707 Engine Pylons

Thu May 03, 2001 4:24 am

Expratt is correct.
The reason for putting three turbocompressors on the 707 (while only two on the 720) was not just the longer cabin. Greater capacity could have been had much easier and cheaper by just scaling up the two a little.

The main reason for having three is greater redundancy needed on an intercontinental airliner.

Imagine a 720 having a problem with #2 engine and a problem with the torbocompressor on #3 engine in the middle of an ocean. It would have a real problem.

But since the 720 was made for transcontinental flights, then it would always have a runway within reach even if it had to cruise at low altitude.

Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs
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RE: B707 Engine Pylons

Tue May 08, 2001 5:21 pm

Hey Prebennorholm, what you said about the 720 is true but don't forget Aer Lingus operated them transatlantically from Dublin to NY so this coud have happened
Once you have tasted flight, you will walk the earth with your eyes turned skyward; for there you long to return

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