707 Engine pylons

[ramsrc]

Just a quick question relating to Engine pylons on the Boeing 707.
Whilst leafing through a book of 707 photos I noticed that in many cases the number one engine (left hand outboard) has a different type of pylon.

For example (http://www.airliners.net/open.file/696227/L/)

Strangely, this does not seem to be repeated on the other side

See here (http://www.airliners.net/open.file/601998/L/)

Is there a reason for this? For instance different equipment in the number one pylon - or is it simply a case of interchangeable types?

I would be grateful of any explanations a 707 expert could offer to satisfy my curiosity!

[Groundloop]

The extension on the top of the engine pylon contains a turbogenerator. Most 707's had 3 of these so the 4th pylon did not need the extension to house one.

[SeldomFixit]

Does any other question ever get asked about a 7 'ort 7 ? :=

[411A]

generator?

Close, but no cigar.

The correct term is turbocompressor.

The JT-3 and JT-4 engines fitted to the 'ole 707 (forgetting for a moment the RR Conway, altho the same applied here as well) supplied engine bleed air to power the turbocompressors, and these, in turn, supplied compressed air to pressurize the cabin.

Most models of 707 had only three turbocompressors, fitted to engines 2,3,4.
Number one engine normally did not have one, so the pylon looks different.

Some may indeed ask...why use turbocompressors when bleed air is available from the engines?

The short answer is....

The engine bleed air tended to be a bit oiley and, under engine idle conditions, there was not enough engine bleed air to properly keep the aeroplane pressurized and allow the cabin altitude to be reduced for descent/landing.

The turbocompressors were more efficient, and therefore used to advantage.

B707...a classic machine. I flew in Command on this aircraft for eight years and truly found it most enjoyable.

[ramsrc]

411a and Groundloop

Many thanks for your replies.

Was flicking through that book again last night and noticed that the Boeing 720 only has two of these Turbocompressors (on numbers two and three) as opposed to the three on the 707. Do I presume correctly that this was to reduce weight?

Perhaps someone can answer this too... Any idea why there were only three Turbocompressors on the 707 and not four?


SeldomFixit

Does any other question ever get asked about a 7 'ort 7 ? Not round 'ere it don't... I did a search before I asked the question - so unless I missed it it isn't that common a question :p

[Notso Fantastic]

The 720 was quite considerably shorter than a 707, and designed for short range routes. I would assume that 2 turbocompressors were more suitable. As for the 707, if 3 turbocompressors were sufficient, why force airlines to go through the added cost & maintenance of 4? It saved ducting through the wing & pylon as well as a whole air unit and control system attached.

[CATIIIBnoDH]

Talking about history, The DC8 was also equipped with 4 cabin compressors. Those were the 4 significant inlets in the nose. They were switched on after TO by the Fl-Eng in pairs and it needed manual manupulation of the cabin pressurization lever to stabilize the cabin rate of climb.
Before descent the Fl-Eng had to switch the engine bleeds to high to prevent the cabin compressors from stalling (quite frightening for the pax) when the pilot pulled the throttles to idle. At around 15000 ft 2 compressors were shut down ( again manual control of the cabin press.)
Just before touchdown the last 2 were switched off to raise the pneumatic pressure for the engine reverse system. Now this is almost history (':sad:')

[woftam]

The T/C's on the B707 were also fairly unreliable and would "trip off" occasionally.

[411A]

ramsrc,

Notso is quite correct, the 720, designed for shorter routes, did not need the redundancy of three turbocompressors...nor the air volume for the shorter cabin/fuselage.
The only 707's that I recall having 4 turbocompressors were the aircraft assigned to the US presidental fleet.

As to the turbos 'tripping off'....this never occured on my flights that I recall.

The normal procedure was to start the T/C's just after engine start, set the cabin pressure controller....and well, that was it, more or less.

The 'auto' mode of the rate controller would automatically adjust the cabin air inflow to match the outflow...and cabin altitude would remain stable.

For descent, usually the inboard engines were left at a slightly higher thrust setting to allow for the cabin altitude to be lowered.

T/C's were switched off after landing.

In the event that aircraft range became a critical factor with fuel remaining, the T/C's could be switched off in flight (ACM equipped aircraft only), and engine bleeds used for pressurization, thereby saving a bit of fuel...about 2-3%.
Cabin ventilation was reduced substantially however.
In this case, T/C's were generally re-started from time to time during the flight (to avoid cold soak), to be sure they would re-start when desired for descent.

The F/E was kept rather busy...altho from the looks of it, not as busy as on the 'ole DC8.
:ooh:

[RUDAS]

Yeah,the difference in pylon shape is due to the 7 oh having three turbocompressors.Apparently a fourth was a(rather unpopular) option,but as i recall reading,only the current Airforce One of the time had four installed.

[Bigwhitelovemachine]

411A,

If I recall correctly, they used to be prone to tripping off due overspeed (106%) or low oil pressure and it did happen every so often on the ones I flew (coincidentaly I also commanded them for 8 years). Mind you with talk of 757 aircon/fume related probs, perhaps the T/C's were a good idea.

Nothing quite like them - slick,sleek and smokey. Wouldn't ever admit it to them, but FE's are nice to have around too!

[woftam]

Bigwhitelovemachine,
Thanks,
That's what I remember as well.
:ok: