DTW_Steve From United States of America, joined Apr 2001, 93 posts, RR: 1 Posted (12 years 7 months 2 weeks 1 day 1 hour ago) and read 2168 times:
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.
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!
KonaB777 From , joined Dec 1969, posts, RR: Reply 1, posted (12 years 7 months 2 weeks 1 day 1 hour ago) and read 2137 times:
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.
KonaB777 From , joined Dec 1969, posts, RR: Reply 2, posted (12 years 7 months 2 weeks 1 day 1 hour ago) and read 2115 times:
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.
Prebennorholm From Denmark, joined Mar 2000, 6129 posts, RR: 55 Reply 5, posted (12 years 7 months 2 weeks 7 hours ago) and read 2075 times:
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, Preben Norholm