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787 Cabin Pressure  
User currently offlineChamonix From France, joined Mar 2011, 327 posts, RR: 1
Posted (2 years 11 months 2 weeks 1 day 19 hours ago) and read 9440 times:
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Does the 787 have the highest of all cabin pressures ever due to its bleedless air system?
The 787 will operate with a higher cabin pressure giving passengers the feeling of being at 6,000 feet above sea level rather than the 8,000 feet for other airliners.

21 replies: All unread, jump to last
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17014 posts, RR: 67
Reply 1, posted (2 years 11 months 2 weeks 1 day 18 hours ago) and read 9422 times:

I don't think the bleedless system per se enables higher pressure. The hurdles were likely more in fatigue life and such. Also the increased humidity means you have to watch more for corrosion.


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineKELPkid From United States of America, joined Nov 2005, 6369 posts, RR: 3
Reply 2, posted (2 years 11 months 2 weeks 1 day 18 hours ago) and read 9401 times:

Quoting Starlionblue (Reply 1):
Also the increased humidity means you have to watch more for corrosion.

Only if you're using a material subject to corrosion to construct the fuselage   IIRC, there is no metal structure at all in the fuselage...100% composite. There is structural metal on the 787, just not in the fuselage.



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineStitch From United States of America, joined Jul 2005, 30852 posts, RR: 86
Reply 3, posted (2 years 11 months 2 weeks 1 day 16 hours ago) and read 9352 times:
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Quoting Chamonix (Thread starter):
Does the 787 have the highest of all cabin pressures ever...

The A380-800 also offers a higher cabin pressure than commercial airlines currently in service, but I am not sure if it's ASL equivalent is equal to the 787's.


User currently offlineWingscrubber From UK - England, joined Sep 2001, 848 posts, RR: 0
Reply 4, posted (2 years 11 months 2 weeks 1 day 15 hours ago) and read 9318 times:

All of the newer generation airliners will start to be designed to cope with the higher pressure differential caused by higher cabin pressure. On the 787 the electric air compressors of course have to be capable of matching what the bleed air system used to do, but as Starlionblue points out, it is the improvements in stress and fatigue life of the fuselage which make it possible.

Quick example

Sea level air pressure = 15 psi (ref)
40,000ft standard air pressure = 2.72 psi

8000ft = 10.64 psi
6000ft = 11.7 psi

So differential presure at 8000ft is 7.92psi, and at 6000ft it's 8.98psi. So only 1psi difference, but remember that's pounds per square inch - think about the internal surface area of an airliner, an extra pound of force for every sqaure inch of of the skin, that adds up to a lot of additional stress overall.



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User currently offlinerwessel From United States of America, joined Jan 2007, 2338 posts, RR: 2
Reply 5, posted (2 years 11 months 2 weeks 1 day 14 hours ago) and read 9292 times:
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Quoting Chamonix (Thread starter):
Does the 787 have the highest of all cabin pressures ever due to its bleedless air system?
The 787 will operate with a higher cabin pressure giving passengers the feeling of being at 6,000 feet above sea level rather than the 8,000 feet for other airliners.

Several biz-jets allow higher cabin pressures. The G550, for example, has a hard limit of 10.48PSI, although is normally operated so that you get 6000ft in the cabin when at 51,000ft (which is 10.17PSI). It can normally maintain sea level cabin pressure up to 31,000ft (10.28PSI - which is the normal operating limit – and if you run that at 51,000ft, you get a 5500ft cabin altitude). The G550 should be able to maintain a lower cabin altitude than the 787 at any altitude the 787 can operate at (although neither aircraft will usually go below sea level).


User currently offlinebikerthai From United States of America, joined Apr 2010, 2093 posts, RR: 4
Reply 6, posted (2 years 11 months 2 weeks 1 day 14 hours ago) and read 9274 times:

Quoting Starlionblue (Reply 1):
there is no metal structure at all in the fuselage...100% composite.

Technically the fasteners are metal.   

I'd be surprised if the "fork" fitting that attach the wing box to the fuselage is anything but metal. It could be a titanium fitting or strap which would reduce the corrosion issue. But it would be hard to avoid some metal in the structure. Specially at junction where large load transfer occurs.

bikerthai



Intelligent seeks knowledge. Enlightened seeks wisdom.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6369 posts, RR: 3
Reply 7, posted (2 years 11 months 2 weeks 1 day 13 hours ago) and read 9244 times:

Quoting bikerthai (Reply 6):
I'd be surprised if the "fork" fitting that attach the wing box to the fuselage is anything but metal. It could be a titanium fitting or strap which would reduce the corrosion issue. But it would be hard to avoid some metal in the structure. Specially at junction where large load transfer occurs.

Yeah, but isn't that outside the pressure vessel, anyways?   Not susceptible to corrosion due to allowing humidity into the pressurization system (unless there's a poorly placed relief valve or something along those lines...).



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 8, posted (2 years 11 months 2 weeks 1 day 7 hours ago) and read 9124 times:

Quoting Chamonix (Thread starter):

Does the 787 have the highest of all cabin pressures ever due to its bleedless air system?

It's not because of the bleedless system. Any modern aircycle machine system runs at considerably higher pressure than you need to pressurize the cabin down to 6,000'. That's part of why pressure is maintained by regulating outflow, rather than inflow. All you need to do to raise the cabin altitude on a conventional bleed system is close the outflow valves. The problem is fuselage stress...CFRP is the enabler for the 787, not bleedless.

Quoting KELPkid (Reply 2):
IIRC, there is no metal structure at all in the fuselage...100% composite.

There's lots. Bulkheads, splice plates, seat tracks, and stanchions are all Ti. Still very limited corrosion issues though.

Tom.


User currently offlinepianos101 From United States of America, joined Jan 2008, 365 posts, RR: 0
Reply 9, posted (2 years 11 months 5 days 5 hours ago) and read 8469 times:

Quoting tdscanuck (Reply 8):
Still very limited corrosion issues though.

Only with the Ti/CFRP interfaces. But remember, the 787 is still 20% Al and 15% Ti. The largest share (50%) is CFRP, but there is a lot more Al than you (not you Tom, but you know what I mean) would think. Corrosion is still a serious concern, since the 787 doesn't get sprayed all over with corrosion inhibiting compound like other models. There are specific design requirements in place to ensure that customer-promised maintenance schedules are adhered to and corrosion won't occur before those times.



Since Ti is 50% heavier that Al, it actually used sparingly, and only when the physical properties of the Ti are required. We won't make a metal fitting that fastens to CFRP out of Ti just for the sake of corrosion, since a fay seal is a perfectly adequate corrosion protection method.

Quoting Starlionblue (Reply 1):
Also the increased humidity means you have to watch more for corrosion.

Corrosion not as much as other things, such as moisture control. Higher humidity means more moisture content in the air, meaning more chances for water to collect in certain areas of the fuselage.


User currently offlineBaroque From Australia, joined Apr 2006, 15380 posts, RR: 59
Reply 10, posted (2 years 11 months 4 days 23 hours ago) and read 8402 times:

And what will the relative humidity be in practice? On a long flight, finding all that water is not going to be all that easy????

I know I got bumped to a 744 on my last expected 380 flight but AFAIK the A380 is still in service so "than commercial airlines currently in service," should perhaps be "than other commercial airlines currently in service,"??     


User currently offlinenomadd22 From United States of America, joined Feb 2008, 1846 posts, RR: 0
Reply 11, posted (2 years 11 months 4 days 21 hours ago) and read 8351 times:

Quoting pianos101 (Reply 9):
Since Ti is 50% heavier that Al, it actually used sparingly, and only when the physical properties of the Ti are required.

I'm pretty sure that's not the case. Weight isn't the reason Ti is used sparingly since it's so much stronger than aluminum that you'd use a less of it volume wise. Ti is expensive and very hard to work with.



Andy Goetsch
User currently offlinedynamicsguy From Australia, joined Jul 2008, 869 posts, RR: 9
Reply 12, posted (2 years 11 months 4 days 20 hours ago) and read 8328 times:

Quoting nomadd22 (Reply 11):
Weight isn't the reason Ti is used sparingly since it's so much stronger than aluminum that you'd use a less of it volume wise.

That assumes that you design a part to be the same size regardless of the material. If you are not volume-limited then aluminium structure should be more weight efficient than Ti.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 13, posted (2 years 11 months 4 days 19 hours ago) and read 8294 times:

Quoting Baroque (Reply 10):
And what will the relative humidity be in practice?

Depends on the operation...apples-to-apples (i.e. same flight duration and profile) it would be higher than a conventional aluminum airliner but the amount higher would depend on a lot of things. I don't know what the A380 humidity control system is like so I can't say anything intelligent there.

Tom.


User currently offlinebikerthai From United States of America, joined Apr 2010, 2093 posts, RR: 4
Reply 14, posted (2 years 11 months 4 days 17 hours ago) and read 8255 times:

Isn't the relative humidity in the aircraft a red herring in the discussion of corrosion?

Even with the current humidity setting in the aircraft, you still get rain in the plane. Humidity, from the passenger breath will still freeze on the fuselage at altitude and melt when the airplanes lands, causing water drain down to the bilge. The problems still exist either way. So you have to deal with it.

Now with the graphite skin, you'll not have any skin corrosion. But any aluminum in the vicinity will need to be protected. They have been doing that on the Boeing and Airbus panes for a long time now (using serious methods) with varying results.

bikerthai



Intelligent seeks knowledge. Enlightened seeks wisdom.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6369 posts, RR: 3
Reply 15, posted (2 years 11 months 4 days 17 hours ago) and read 8242 times:

Quoting tdscanuck (Reply 13):
Depends on the operation...apples-to-apples (i.e. same flight duration and profile) it would be higher than a conventional aluminum airliner but the amount higher would depend on a lot of things. I don't know what the A380 humidity control system is like so I can't say anything intelligent there.

Tom.

Doesn't the A380 use standard air packs (like just about every other jet transport)? I thought the air machine cycle naturally eliminates humidity through the course of its normal operation...



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlinetod From Denmark, joined Aug 2004, 1724 posts, RR: 3
Reply 16, posted (2 years 11 months 4 days 14 hours ago) and read 8173 times:

Quoting Chamonix (Thread starter):
The 787 will operate with a higher cabin pressure giving passengers the feeling of being at 6,000 feet above sea level rather than the 8,000 feet for other airliners.

I usually carry an altimeter with me as a pax.

Typical 744 and 777 longhaul flights seem to range 5600 to 6500
Domestic 737, MD80, etc seem to typically run about 7000 to 7500
I have only once seen one get all the way up to 8000


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 17, posted (2 years 11 months 4 days 6 hours ago) and read 8072 times:

Quoting bikerthai (Reply 14):
Isn't the relative humidity in the aircraft a red herring in the discussion of corrosion?

No.

Quoting bikerthai (Reply 14):
Even with the current humidity setting in the aircraft, you still get rain in the plane. Humidity, from the passenger breath will still freeze on the fuselage at altitude and melt when the airplanes lands, causing water drain down to the bilge. The problems still exist either way.

The problem magnitude is different. With low relative humidity the rain-in-the-plane problem is reduced because the air contains less water...assuming you're holding the same temperature, lower relative humidity means you've got less water available to condense on the inside of the skin and more water is harmlessly going out the outflow valve.

Quoting KELPkid (Reply 15):
Doesn't the A380 use standard air packs (like just about every other jet transport)?

Yes.

Quoting KELPkid (Reply 15):
I thought the air machine cycle naturally eliminates humidity through the course of its normal operation...

Somewhat, but the ACM has very dry air coming in to start with so that's not really a big factor. The absolute humidity at cruise altitude is functionally nil. Virtually all humidity present comes from the passengers (or humidifiers if so equipped)...the easiest way to regulate humidity is to control the air re-circulation rate.

Quoting tod (Reply 16):
Quoting Chamonix (Thread starter):
The 787 will operate with a higher cabin pressure giving passengers the feeling of being at 6,000 feet above sea level rather than the 8,000 feet for other airliners.

I usually carry an altimeter with me as a pax.

Typical 744 and 777 longhaul flights seem to range 5600 to 6500
Domestic 737, MD80, etc seem to typically run about 7000 to 7500
I have only once seen one get all the way up to 8000

When they say "Aircraft X has cabin altitude of Y" they're really saying "Aircraft X has a *maximum* cabin altitude of Y". Most pressure controllers will use a linear cabin altitude schedule until they hit maximum deltaP, then hold that deltaP up to maximum cruise altitude.

As a result, most of the time, the cabin altitude on any aircraft will be lower than the spec number. However, since they're almost all using linear scaling, a lower max means a lower cabin altitude at any cruise altitude. So a 777 on a longhaul might have 6000' cabin altitude (indicating it's not up at max cruise altitude) but a 787 at the same cruise altitude would have a lower cabin.

Tom.


User currently offlineyeelep From United States of America, joined Apr 2011, 653 posts, RR: 0
Reply 18, posted (2 years 11 months 2 days 16 hours ago) and read 7857 times:

Quoting KELPkid (Reply 15):
I thought the air machine cycle naturally eliminates humidity through the course of its normal operation...

The ACM's sole purpose is to reduce air temp. On a 737 the air exits the ACM then travels through a water separator, where the water is removed and the dry air continues through the distribution system.


User currently offlinecmf From , joined Dec 1969, posts, RR:
Reply 19, posted (2 years 11 months 2 days 11 hours ago) and read 7780 times:

Quoting KELPkid (Reply 2):
Only if you're using a material subject to corrosion to construct the fuselage   IIRC, there is no metal structure at all in the fuselage...100% composite. There is structural metal on the 787, just not in the fuselage.

It is not just fuselage being affected. Electrical connections are known to create gremlins after interacting with humidity.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 20, posted (2 years 11 months 1 day 19 hours ago) and read 7668 times:

Quoting yeelep (Reply 18):
On a 737 the air exits the ACM then travels through a water separator, where the water is removed and the dry air continues through the distribution system.

This is pretty common in all ACM designs, since liquid water blobs can wreak havoc on downstream components (especially other ACM wheels). The water load still isn't that high in cruise...fully saturated air at cruise altitude (which would be very unusual) is still less than 5% relative humidity once you bring it up to cabin temperature.

The major water removal happens when the ACM is running on the ground in hot/humid climates.

Tom.


User currently offlineBaroque From Australia, joined Apr 2006, 15380 posts, RR: 59
Reply 21, posted (2 years 10 months 4 weeks 1 day 17 hours ago) and read 7387 times:

Quoting tdscanuck (Reply 17):
Quoting bikerthai (Reply 14):
Even with the current humidity setting in the aircraft, you still get rain in the plane. Humidity, from the passenger breath will still freeze on the fuselage at altitude and melt when the airplanes lands, causing water drain down to the bilge. The problems still exist either way.

The problem magnitude is different. With low relative humidity the rain-in-the-plane problem is reduced because the air contains less water...assuming you're holding the same temperature, lower relative humidity means you've got less water available to condense on the inside of the skin and more water is harmlessly going out the outflow valve.

No argument there, or

Quoting tdscanuck (Reply 17):
Somewhat, but the ACM has very dry air coming in to start with so that's not really a big factor. The absolute humidity at cruise altitude is functionally nil. Virtually all humidity present comes from the passengers (or humidifiers if so equipped)...the easiest way to regulate humidity is to control the air re-circulation rate.

there. Nor with repisly 20.

Folk do seem a bit odd about RH (not you tds!!!!). In Indonesia, I constantly explain to folk about the need to dehumidify to avoid fungi growing on expensive optics. The common reply is that the A/C takes the water out and they take me outside to the drain to show the water outflow. And then do not listen as I explain that the off-coil air under those circs is guaranteed 100% RH which is just what fungi love!!!!

So good luck with the logic tds!!!


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