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Trimeresurus
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Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Wed Sep 09, 2020 5:17 pm

If you look at a 727 from behind, you'll see that it has only one exhaust opening, located at where normally APU is in most other A/C. I assume that belongs to the engine #2, whose inlet is oddly located further up from where the exhaust is(so the air doesn't follow a straight path from inlet to exhaust, I am not sure if this creates extra wear and tear as well as inefficiency but Boeing saw it fit that way 60 years ago), and unlike a 707, we know that 727 also has an APU. So, where does it emit it's stuff from? Do they share that hole with the middle engine? If so, is it possible run both the APU and that engine at the same time?

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JustAnFO
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Wed Sep 09, 2020 5:26 pm

The APU exhaust is a louvered vent, located in the upper skin of the right wing near the root.


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Polot
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Wed Sep 09, 2020 5:33 pm

The APU on the 727 is between the main landing gear. Important to remember that the 727 was one of the first jets if not the first to have one; APU being in the tail section wasn’t “standardized” yet. As mentioned the outlet is on the upper part of the right wing.

The inlet for the number 2 engine is not centered with the exhaust not because of any avoidance of straight airflow reasons (remember the DC-10/MD-11 exists, and think about how number 1 and 3 engines are...) but rather because the number 2 engine is physically in the fuselage. The outlet is the back of the engine. Obviously the cabin/rest of the aircraft is blocking the immediate front of the engine, so the inlet is an S duct that brings air from above the fuselage down to the front of the engine. See the below photo, in which the number 2 engine has been removed and you can see through the s duct:



S ducts versus DC-10 style is a trade off between easier number 2 engine mounting vs having to get airflow into engine to work well.
 
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DL_Mech
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Wed Sep 09, 2020 10:25 pm

A picture of the APU exhaust on top of the right wing:

Image

From https://live.staticflickr.com/2120/5779450013_fbdb77cf9a_b.jpg

A nice picture of the APU in the wheel well:

Image

From http://i183.photobucket.com/albums/x312/CAPFlyer/727/P7120048.jpg
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fr8mech
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Wed Sep 09, 2020 10:57 pm

DL_Mech wrote:
A picture of the APU exhaust on top of the right wing:


A torching APU was always an interesting sight, and rather exciting if there were passengers onboard.

DL_Mech wrote:
A nice picture of the APU in the wheel well:


Wow, I just had a flashback to changing the generator on that APU.

All funnies aside, I read that the APU was actually an afterthought. The airlines began to ask for it, and Boeing Had to find a hole for it. The keel beam area of the wheel well provided a robust area that didn’t require a whole lot of beefing up.
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tb727
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Thu Sep 10, 2020 1:02 am

I'm thankful they added an APU too. The airplane is very self sufficient with it combined with the air stairs. You can operate all over the world with no real need for ground equipment, top off the oil and check the gas and blast off to the next middle of nowhere spot.
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Max Q
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Thu Sep 10, 2020 11:13 pm

There’s a well written story on the integration of the 727 APU out there somewhere.


Correct it wasn’t initially planned, once Boeing decided to incorporate it the issue was where


They could probably have found space for it in the tail area but the last thing they needed was more weight back there along with the three engines, additional structure to support them and the heavy T tail



So the wheel well was chosen, closer to the CG, a clever design


But man was that noisy and hot, with the two screaming pack fans nearby you wouldn’t want to spend too long in the gear well area during your walk around !
The best contribution to safety is a competent Pilot.


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SheikhDjibouti
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Fri Sep 11, 2020 10:25 pm

Trimeresurus wrote:
If you look at a 727 from behind, you'll see that it has only one exhaust opening, located at where normally APU is in most other A/C. I assume that belongs to the engine #2, whose inlet is oddly located further up from where the exhaust is(so the air doesn't follow a straight path from inlet to exhaust, I am not sure if this creates extra wear and tear as well as inefficiency but Boeing saw it fit that way 60 years ago),

The S-duct design wasn't just Boeing.
Add Hawker-Siddeley, Lockheed, Tupolev, Yakolev, Dassault.

In fact, isn't it McDonnell-Douglas who are the odd-ones out, favoring a straight -through engine fitting?






And here come the fun facts.
The earlier Trident models came with an APU in the main body of the aircraft, but most/all were modified to position the APU at the base of the tail, with an outlet just above the #2 engine exhaust


However, when it came to the Trident 3B, that extra exhaust outlet at the base of the tail isn't for the APU; it's for engine #four.
No, seriously.. It really was.


Oh, and I nearly forgot; the BA Trident 1E shown above with a backdrop of BA Tristars, originally also featured (you are gonna love this)... seven-abreast seating.
Again, I'm not making this up. Honest.
Nothing to see here; move along please.
 
Max Q
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Fri Sep 11, 2020 11:06 pm

SheikhDjibouti wrote:
Trimeresurus wrote:
If you look at a 727 from behind, you'll see that it has only one exhaust opening, located at where normally APU is in most other A/C. I assume that belongs to the engine #2, whose inlet is oddly located further up from where the exhaust is(so the air doesn't follow a straight path from inlet to exhaust, I am not sure if this creates extra wear and tear as well as inefficiency but Boeing saw it fit that way 60 years ago),

The S-duct design wasn't just Boeing.
Add Hawker-Siddeley, Lockheed, Tupolev, Yakolev, Dassault.

In fact, isn't it McDonnell-Douglas who are the odd-ones out, favoring a straight -through engine fitting?






And here come the fun facts.
The earlier Trident models came with an APU in the main body of the aircraft, but most/all were modified to position the APU at the base of the tail, with an outlet just above the #2 engine exhaust


However, when it came to the Trident 3B, that extra exhaust outlet at the base of the tail isn't for the APU; it's for engine #four.
No, seriously.. It really was.


Oh, and I nearly forgot; the BA Trident 1E shown above with a backdrop of BA Tristars, originally also featured (you are gonna love this)... seven-abreast seating.
Again, I'm not making this up. Honest.



Interesting

I remember riding on one from London to Glasgow in a row of backward facing seats. Didn’t see that again until years later on Southwest when they had that section



Where was the APU actually located in those early Tridents ?



One other unusual Trident detail was the offset nosewheel that retracted and extended sideways
The best contribution to safety is a competent Pilot.


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GalaxyFlyer
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Sat Sep 12, 2020 12:59 am

The original 727 APU had a door before the louvered vent was installed.
 
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Starlionblue
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Sat Sep 12, 2020 1:17 am

Max Q wrote:
SheikhDjibouti wrote:
Trimeresurus wrote:
If you look at a 727 from behind, you'll see that it has only one exhaust opening, located at where normally APU is in most other A/C. I assume that belongs to the engine #2, whose inlet is oddly located further up from where the exhaust is(so the air doesn't follow a straight path from inlet to exhaust, I am not sure if this creates extra wear and tear as well as inefficiency but Boeing saw it fit that way 60 years ago),

The S-duct design wasn't just Boeing.
Add Hawker-Siddeley, Lockheed, Tupolev, Yakolev, Dassault.

In fact, isn't it McDonnell-Douglas who are the odd-ones out, favoring a straight -through engine fitting?






And here come the fun facts.
The earlier Trident models came with an APU in the main body of the aircraft, but most/all were modified to position the APU at the base of the tail, with an outlet just above the #2 engine exhaust


However, when it came to the Trident 3B, that extra exhaust outlet at the base of the tail isn't for the APU; it's for engine #four.
No, seriously.. It really was.


Oh, and I nearly forgot; the BA Trident 1E shown above with a backdrop of BA Tristars, originally also featured (you are gonna love this)... seven-abreast seating.
Again, I'm not making this up. Honest.



Interesting

I remember riding on one from London to Glasgow in a row of backward facing seats. Didn’t see that again until years later on Southwest when they had that section



Where was the APU actually located in those early Tridents ?



One other unusual Trident detail was the offset nosewheel that retracted and extended sideways


Images of a Trident 1E show a small intake above the center engine intake, in front of the fin leading edge. There also seems to be a bulge above the center engine exhaust, which at a guess could be the APU housing. I don't know for certain though.


Image
Image

The Trident 3 famously had the "thrusting APU" fourth engine in that position, for 15% additional takeoff thrust.

Image

Seven abreast seating looks... snug...

Image
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SheikhDjibouti
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Sat Sep 12, 2020 11:12 am

I'm craving the indulgence of the OP for going slightly off-topic, whilst still discussing Trijet APUs.

SheikhDjibouti wrote:
The S-duct design wasn't just Boeing. (e.g.Hawker-Siddeley, Lockheed, Tupolev, Yakolev, Dassault.)

The earlier Trident models came with an APU in the main body of the aircraft, but most/all were modified to position the APU at the base of the tail, with an outlet just above the #2 engine exhaust


Max Q wrote:
Where was the APU actually located in those early Tridents ?

I don't have a definitive answer for the original APU position, other than a vague "in the belly".
FWIW here are photos of G-ARPP before... and after the modification to a tail mounted APU.
The difference at the base of the tail is particularly noticeable.


Starlionblue wrote:
Images of a Trident 1E show a small intake above the center engine intake, in front of the fin leading edge.
Note that the "small intake above the center engine intake, in front of the fin leading edge" is already present in the earlier photo of G-ARPP. In fact it appears to have been a feature of every Trident from the very first Trident 1 to the last 3B. I wish I could tell you exactly what it was for, but I can only speculate.

{With some trepidation, I will offer a link to an image claiming to be a cut-away for the RB.162 boost engine, showing it located in exactly this position. The problem I have is that it doesn't quite add up, and I am more inclined to think this is an image pertaining to a re-positioned APU because of the RB.162 fitting. i.e. the drawing is mis-labelled. But this still doesn't explain why the intake was there on earlier models. We probably need a new thread for that discussion....}
https://www.casimages.com/i/191102031441443449.jpg.html
And this a.net image of 'ZG best illustrates some of inlet/outlet vents that accord with the cut-away drawing. i.e. vents not present on Trident 1 or 2.
https://www.airliners.net/photo/British-Airways/Hawker-Siddeley-HS-121-Trident-3B/659499/L

Yet another curiosity with these earlier Trident photos, is the use of both forward exit doors.
Indeed, the third photo I have selected shows at least one Trident hooked up to a LHR Terminal 1 airbridge, together with steps up to the second door! :o


Starlionblue wrote:
Seven abreast seating looks... snug...
Brilliant image. Thx for that. :D

Image
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Tristarsteve
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Sat Sep 12, 2020 4:35 pm

That casimage of the Trident 3B shows the APU installation above the S duct in front of the fin..
All the BEA Trident 1 and 2 had the APU at the base of the fin by 1970 when I started working on them.

The RB162 was an odd engine. The compressor blades were glass fibre, and the oil system was total loss, no return system was fitted.
It ran for about 10 minutes when needed and was very noisy.
 
Max Q
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Mon Sep 14, 2020 4:56 am

GalaxyFlyer wrote:
The original 727 APU had a door before the louvered vent was installed.



I remember seeing that on some older NW aircraft
The best contribution to safety is a competent Pilot.


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GZM1
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Mon Sep 14, 2020 4:35 pm

What an interesting thread! I have two questions: What was the type (and the thrust) of the fourth engine on the Trident 3?
And second, don’t you think that the APU in reply #4 is shown upside down or am I the only one who thinks so?
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DL_Mech
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Mon Sep 14, 2020 5:29 pm

GZM1 wrote:
And second, don’t you think that the APU in reply #4 is shown upside down or am I the only one who thinks so?


The wheel well picture is looking forward into the left wheel well standing next to the landing gear. It shows the air intake of the APU and the bottom of the picture is the inside surface of the left main gear door. And, this wheel well is unusually clean.



Image

From http://i183.photobucket.com/albums/x312/CAPFlyer/727/P7120048.jpg
This plane is built to withstand anything... except a bad pilot.
 
extender
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Mon Sep 14, 2020 5:47 pm

Panel was always disbonding. Someone came up with an STC for a metal panel with extrusion for stiffening rather than a honeycomb core.
 
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SheikhDjibouti
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Re: Where is the APU of the 727 located? Does it use the same exhaust opening as the middle engine(#2)?

Mon Sep 14, 2020 5:57 pm

GZM1 wrote:
What an interesting thread! I have two questions: What was the type (and the thrust) of the fourth engine on the Trident 3?

How much "history" do you want? :lol:
The original Trident was supposed to be a direct equivalent to the 727, but BEA insisted it was downsized to meet their requirements.
So that's what de Havilland (later Hawker-Siddeley) did, allowing them to make do with the RR Spey engine.

Fast forward a few years (not that many really), and BEA needed a slightly larger aircraft, roundabout 727-200 sized.... :roll:
Unfortunately the RR Spey engine had reached the practical limit in terms of increased thrust, so the only solution was a "quick-fix", strapping on a small (and very noisy) boost engine, to be used for maybe 10 mins only i.e. at take-off.

This engine was the RB162, which for the Trident 3B offered 5,250 lb thrust
https://en.wikipedia.org/wiki/Rolls-Royce_RB162


The RB162 engine was originally designed as a vertical lift engine and thus featured an outstanding thrust-to-weight ratio.
The main applications for the RB162 were the following, (all three flew, but none progressed beyond the prototype stage).
The Mirage IIIV featured EIGHT RB162 lift engines, in addition to a regular afterburning turbofan.
The Do31 also featured EIGHT RB162, this time paired with a couple of Bristol-Siddeley Pegasus vectored thrust engines, as used in the Harrier.
I consider the Do31 as a 1970's forerunner of the CV-22 Osprey.
And the VAK-191 was pretty much a Harrier clone, with one main vectored thrust engine, and TWO RB162 lift engines.



Fun fact
The Rolls-Royce Spey engine bore the company designation RB163 (not to be confused with the un-named RB162!)
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