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Purpose Of The Turbocompressor On P&W JT3D?

Breeze From Australia, joined Jul 2006, 8 posts, RR: 0
Posted (11 years 4 months 4 days 14 hours ago) and read 2787 times:

Does anyone know the purpose of the small turbocompressor inlet at the top of the nacelle on Pratt & Whitney JT3Ds mounted on most Boeing 707s? Also, why does this inlet usually appear only on the number 2, 3, and 4 engines and generally not on the number 1 engine?

Thanks for your replies,

Breeze

Coming to you from my loungeroom in Australia!

10 replies: All unread, jump to last

JETPILOT From United States of America, joined May 1999, 3128 posts, RR: 37
Reply 1, posted (11 years 4 months 4 days 14 hours ago) and read 2746 times:

The turbocompressor was integral to the pressurization system on the 707 and not an engine system.

Pressurization on modern airplanes comes from air bled off the compressor of the engine, which is routed through air conditioning PACK (Pneumatic Air Conditioning Kit) and then directly into the cabin.

The JT3D and other aircraft engines of the period lost a large amount of efficiency when air was bled from the engine for operation of systems.

Turbo compressors were implimented on the DC8 and the 707 to reduce the amount of bleed air required for pressurization allowing the engine to operate more efficiently.

A relatively smaller volume of air was bled off the engine to turn a turbocompressor supplying the aircrafts pressurization needs. The engine bleed air was then dumped overboard once it powered the turbocompressors turbine. There is a small opening on the side of the upper nacelle just behind the turbocompressor. That is the turbine exhaust.

The reason for only three turbocompressors is that only 3 were required for adequate pressuriztion and redundancy.

JET

VC-10 From United Kingdom, joined Oct 1999, 3656 posts, RR: 42
Reply 2, posted (11 years 4 months 4 days 13 hours ago) and read 2724 times:

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There were also concerns about engine oil contaminating the air cond system if the air was bled directly off the engine.

Prebennorholm From Denmark, joined Mar 2000, 5810 posts, RR: 56
Reply 3, posted (11 years 4 months 4 days 12 hours ago) and read 2717 times:

Notice that the smaller non-intercontinental - non ocean crossing 707 sister, the 720, had only two aircon turbo compressors. They are found on #2 and #3 engines only.
Less redundancy needed for mostly overland flight with closer diversion airports.
Best regards, Preben Norholm

Always keep your number of landings equal to your number of take-offs, Preben Norholm

Timz From United States of America, joined Sep 1999, 6232 posts, RR: 8
Reply 4, posted (11 years 4 months 4 days 10 hours ago) and read 2706 times:

Didn't some 707-320C's have them on #2 and #3 only? AA, maybe?

Timz From United States of America, joined Sep 1999, 6232 posts, RR: 8
Reply 5, posted (11 years 4 months 4 days 10 hours ago) and read 2702 times:

http://www.airliners.net/open.file?id=126610

Feret From , joined Dec 1969, posts, RR:
Reply 6, posted (11 years 4 months 4 days 10 hours ago) and read 2701 times:

Just as a matter of interest, on the 707 #4 TC is normally shut down for takeoff (giving the outboards equal EPR settings) then started during initial climb. Engine bleed air can be used to supplement a TC should it shutdown or be subject to a MEL, or in addition if extra airflow is required. I found it amusing that the airflow indicator was calibrated in "inches of water".

JETPILOT From United States of America, joined May 1999, 3128 posts, RR: 37
Reply 7, posted (11 years 4 months 4 days 10 hours ago) and read 2700 times:

One of the top ten coolest things I've ever heard is a DC8 doing a TC overspeed check before engine start.

If anyone ever heard this you know what I mean.

The TC's spool up to about 13,000 RPM in about 2 seconds and make this amazing sound. They were really loud.

I loved doing this check as a DC8 FE.

Anyone ever hear it?

JET

Crjmech From United States of America, joined Jun 2000, 260 posts, RR: 1
Reply 8, posted (11 years 4 months 4 days 8 hours ago) and read 2697 times:

JET,

Is that what that sound is? I always hear this loud "whoop" from the vicinity of Airborne Express' DC-8 here in FAT. I just figured it came from the big-ass GPU that they plug into the plane. You learn something new everyday.

Thou shalt mind thine altitude,lest the ground reach up and smite thee.

JETPILOT From United States of America, joined May 1999, 3128 posts, RR: 37
Reply 9, posted (11 years 4 months 4 days 6 hours ago) and read 2686 times:

The sound should be heard twice. The first two TC's then the second two TC's. The sound lasts about 3 seconds each.

It's the sound of the TC's being spooled for the overspeed test.

Is it a cool sound or what !!!

JET

Breeze From Australia, joined Jul 2006, 8 posts, RR: 0
Reply 10, posted (11 years 4 months 3 days 4 hours ago) and read 2653 times:

Jet, VC-10, Prebennorholm, Timz, Feret, Crjmech:

Thanks to all of you for your replies, answers, and insight on this question that has perplexed me for so long.

Breeze

Coming to you from my loungeroom in Australia!

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