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wjcandee
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787 Pressurization And The NY Times

Mon May 08, 2006 7:47 am

A question about a passage in today's Times article about Boeing and the 787. There are as usual a number of overdramatizations and misstatements about the airliner market and the 787, but the reporter gets a fair amount of it right.

http://www.nytimes.com/2006/05/07/bu...ey/07boeing.html?pagewanted=1&_r=1

What's confusing to me is the following passage:

"Unlike the 767 and other planes designed and built in the 1980's and even the 90's, which were made to be pressurized to simulate the thin air found at 8,000 feet, the 787 will allow pressurization to be set for conditions at 6,000 feet. After putting some frequent-flier volunteers into a pressure chamber in Oklahoma to simulate flights of 9 to 15 hours, Boeing concluded that 6,000 feet was well below the level where most people experience the headaches and other symptoms of altitude sickness."

Let's leave aside, for a second, the fact that the reporter was probably born in the '80s and therefore doesn't have enough perspective to properly state the premise as "designed and built in the 1950s through the 1990s". My understanding is that pressurization at cruise -- and the cabin altitude generally -- is based upon a differential with the outside air pressure and is designed to prevent overstressing the pressure vessel in which we are all riding. Absent the danger of overstressing the vessel, the aircraft would presumably be designed to maintain a low cabin altitude for most of the flight. However, a regulator works to ensure that the pressure isn't so high inside the vessel as compared to outside the vessel that the thing goes "Pop". So the cabin alititude might be 8000 feet, or it might be proportionally lower if we're cruising lower. Conversely, if the pressurization system isn't functioning properly or there's excessive outflow from somewhere in the a/c (e.g. door seal), the cabin altitude will climb and emergency systems will kick in. So, in the 757/767 at least, you're going to get a warning at 10,000 feet cabin altitude and masks at 14,000 feet cabin altitude. It's certainly LESS desireable for the passengers to ride at a higher cabin altitude, because the people with less able cardiovascular systems are going to be less comfortable at the higher altitudes.

So here's my question: Why would Boeing need to do tests with passengers to discover that they're happier at a LOWER cabin altitude of 6,000 feet (i.e. a higher cabin pressure)? Wouldn't a cabin altitude of sea level, if attainable, be optimal? If 8,000 feet is an acceptable cabin altitude for people, why would it take tests to determine that 6,000 feet is "well below the level" at which people get sick from lack of oxygen?

Is this a "DOH!" situation -- a case of the reporter not bothering to understand what she's talking about -- or am I missing something?

[Edited 2006-05-08 00:49:52]

[Edited 2006-05-08 00:52:12]
 
kaitak744
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 7:55 am

In the real world, everything gets tested. No matter how obvious it may be.
 
11Bravo
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 8:07 am

Quoting Wjcandee (Thread starter):
Wouldn't a cabin altitude of sea level, if attainable, be optimal? If 8,000 feet is an acceptable cabin altitude for people, why would it take tests to determine that 6,000 feet is "well below the level" at which people get sick from lack of oxygen?

If you don't do tests, how do you propose to quantify what altitude is "well below the level". I'm guessing sea-level isn't practical or maybe even possible. I'm also assuming that the 6k v 8k difference isn't free.

If the engineers comes to you and says "You know Boss for only 38 bazillion more dollars we can build a structure that will handle 6,000 feet of pressure" I don't know about you, but my response would be "Okay,... show me some data that quantifies the beneficial effect of that investment, and we'll think about it". How do you provide that answer without tests?
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MD-90
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 8:10 am

Quoting Wjcandee (Thread starter):
Why would Boeing need to do tests with passengers to discover that they're happier at a LOWER cabin altitude of 6,000 feet (i.e. a higher cabin pressure)?

To discover if it's worth the extra weight to be able to increase pressurization.

Quoting Wjcandee (Thread starter):
Wouldn't a cabin altitude of sea level, if attainable, be optimal?

Yup. There are bizjets that can maintain a sea level cabin for a pretty good height. Gulfstreams, GLEXs, Lears, and other aircraft that can cruise at FL510 of course have higher pressure differentials.

Quoting Wjcandee (Thread starter):
If 8,000 feet is an acceptable cabin altitude for people, why would it take tests to determine that 6,000 feet is "well below the level" at which people get sick from lack of oxygen?

If the NYT reporter wrote that, than that's just ignorance. It's about passenger comfort, not purely safety. 12,000 feet is the highest altitude that most private pilots feel comfortable flying without supplemental oxygen, because skills and reaction times start degrading above that.
 
roseflyer
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 8:18 am

I don't really understand what your problem with the article is. From the excerpt you included, it just talked about how there 787 will have a higher cabin pressure.

Why wouldn't they increase the pressure if they could without weight problems? Sure it has worked in the past, but a higher pressure could be good. They tested to see if it is beneficial and it looks like Boeing showed that it would be preferable.

As others have said EVERYTHING is tested. I did a bit of work on the testing of the 787 environmental control system, and know how complicated of a project that was. The 787 is trying to be innovative. If they can realistically change the pressure, then it is good. From the little I know, I would assume that the higher cabin pressure is the balance of added complications and passenger comfort. Yes they could pressurize to 101.5 KPa, but that is a tradeoff that does not make sense. 6,000ft might be a better compromise with today's technology.

Quoting Wjcandee (Thread starter):
Let's leave aside, for a second, the fact that the reporter was probably born in the '80s and therefore doesn't have enough perspective to properly state the premise as "designed and built in the 1950s through the 1990s".

I have no idea what you are trying to point out. Leslie Wayne is a well respected aviation reporter with the New York Times. As far as being born in the 1980s, I myself was born then and although I am in my 20s, I still think that I can understand the pressurization system of the 787, especially because I have worked with people responsible for the testing of many different systems on the 787 and myself have worked on it.

I think the article title is a little overdramatic, because Boeing has bet the house before on project. Boeing risked far more during the 707 and subsequent 747 developments than on the 787. Boeing is a more diversified company now.

[Edited 2006-05-08 01:27:06]
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zvezda
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 8:28 am

Boeing did tests to determine at what cabin pressure passengers didn't experience adverse affects. They were looking for the knee of the curve.

Quoting 11Bravo (Reply 2):
I'm also assuming that the 6k v 8k difference isn't free.

It's not free. It requires a stronger (i.e. heavier) fuselage -- in tension. CFRP is very strong in tension and -- because the B787 is would around a mandrel -- there is ample tensile strength in the needed direction as a side benefit of having enough strength and stiffness in other directions to have 6000' cabin pressure at very little, if any, additional cost.
 
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TVNWZ
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 8:40 am

[quote=Wjcandee,reply=0]Let's leave aside, for a second, the fact that the reporter was probably born in the '80s and therefore doesn't have enough perspective to properly state the premise as "designed and built in the 1950s through the 1990s". [/quote/]

This is the proper perspective. We are not flying planes built in the 50's nor 60's. Well almost not in the 60's if you include the NW DC 9's.  Smile
 
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centrair
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 9:14 am

Quoting MD-90 (Reply 3):
Yup. There are bizjets that can maintain a sea level cabin for a pretty good height. Gulfstreams, GLEXs, Lears, and other aircraft that can cruise at FL510 of course have higher pressure differentia

I am not an engineer (math is not my friend), so I must ask questions to benefit my own curiousity.

If you have an aircraft with a smaller frame, wouldn't it be easier to keep a sea level pressure? You have less volume to deal with.

How hard would it be to maintain sea level pressure on a wide body aircraft? That is a lot of volume.

How would the frame handle sea level pressure internally when the pressure outside is less? If you had sea level pressure and were flying at 35,000 ft, then lost pressure what would the body do? What would happen to the frame?
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Jet-lagged
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 9:15 am

Quoting Zvezda (Reply 5):
Boeing did tests to determine at what cabin pressure passengers didn't experience adverse affects. They were looking for the knee of the curve.

Yes, presumably they also testing lower than 6000 feet, but concluded that the additional comfort was small compared to the additional strength and cost that would require.

At least, from an engineering design perspective, that would have been the preferred approach - testing all the way from sea level to 8,000 feet.
 
JayinKitsap
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 9:58 am

People with various medical conditions have a difficult time with high altitude. For example, my Grandmother lived in Denver 5,300 ft high. Her doctor advised her to not go over 8,000 for breathing and stroke reasons. Others with more serious conditions couldn't even do the 8,000 of the current planes. Lowering the cabin altitude by even 1,000 feet is signficant, 2,000 is substantial. Yet as an earlier post says, the knee is about 6,000' there is little difference on people's health or comfort below 6,000 that the return isn't there.

It is the difference in pressure between the inside and outside that is the controlling design factor. Hull stress is proportional with radius so a widebody needs a thicker skin, frame spacing also has an effect.

I would think that the hull reliefs are set for an absolute altitude, once below that altitude they would be full open. That is a much safer and straighforward control scheme.
 
qqflyboy
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 10:30 am

I was just reading the article in the most recent Airways where the cover story is about the 787. They did note certain aspects of the pressurization system, most notably that it will be an electric system that will not run off bleed air from the engines. This allows the engines to be more efficient, burning less fuel, while also making the plane lighter since the required ducting (usually made of titanium and very heavy) won't be needed.

They also noted the new system will allow for 20% humidity, compared to 5% now. It is dehydration that usually causes the effects of jetlag (drowsiness and headaches). They did not mention that the average cabin altitude will be lower.

A related, but side note: The article talked a little about the strength of the composites being used today. Apparently some C-130s have been outfitted with composite flaps. The previous metal flaps would start to get tiny cracks after 3,000 hours... they stopped testing the composite flaps after 60,000 hours. Perhaps this added strength will indeed allow lower cabin altitudes, ie, higher cabin pressure. Just a thought.
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wjcandee
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 10:42 am

In response to a number of posters, whose comments I appreciate, here goes:

Quoting RoseFlyer (Reply 4):
I have no idea what you are trying to point out. [...] As far as being born in the 1980s, I myself was born then and although I am in my 20s, I still think that I can understand the pressurization system of the 787

I was just rolling my eyes at the notion, implied in the article, that it has only been since the 1980s that pressurized aircraft were designed to maintain 8000-foot cabin altitudes. The reporter says, "Unlike the 767 and other planes designed and built in the 1980's and even the 90's, which were made to be pressurized to simulate the thin air found at 8,000 feet..." That's a mark of a person that thinks that the 80s were a REALLY long time ago, hence my eye-rolling. It was a snitty little point, and not the focus of my post. But just for the record: The 707 was a pressurized passenger airliner, and Boeing greenlighted it in April 1952 (at a much greater corporate risk, as others have pointed out, than that involved in the 787 decision, which was actually derided at the time as being a conservative one), and PanAm ordered the first commercial versions of the 707 in October 1955. My point was that aircraft designed as long ago as the 1950s have been designed to maintain an 8000-foot cabin altitude. Indeed, the US government has required that airliners maintain a cabin altitude no greater than 8000 feet since 1957.

Quoting RoseFlyer (Reply 4):
I don't really understand what your problem with the article is. From the excerpt you included, it just talked about how there 787 will have a higher cabin pressure.

I guess I didn't express it well enough. The reporter said, after stating that modern airliners maintain a cabin altitude of 8000 feet, that Boeing put a bunch of people in a simulator to test whether it would be okay to maintain a pressure of 6000 feet, and that "Boeing concluded that 6,000 feet was well below the level where most people experience the headaches and other symptoms of altitude sickness." I read that to man (perhaps incorrectly) that the reporter believed that one would have to do some tests to make sure that people wouldn't be harmed (i.e. "experience the headaches and other symptoms of altitude sickness") by LOWERING the cruise cabin altitude. He/she seems to be saying that people are fine (i.e. don't experience the "symptoms of altitude sickness") at an 8000-foot cabin altitude, but that Boeing needed to do testing to make sure it was okay to provide a 6000-foot cabin altitude. If I'm reading the reporter correctly, then I think the reporter doesn't understand the purpose of the testing, and is confusing lower cabin ALTITUDE with lower cabin PRESSURE. I think, as others have mentioned, that the purpose of the testing was to determine a more optimal, comfortable cabin altitude, and what altitude provided the most bang for the additional pressure buck. Indeed, the Flight Attendant unions have been suggesting for years that the 8000-foot standard was based upon cost savings and not human factors, and that they'd be more healthy if they performed their duties at a lower cabin altitude.

But maybe I'm not understanding the reporter.

Anyway, thanks for all the input.
 
wjcandee
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 10:48 am

Quoting JayinKitsap (Reply 9):

I would think that the hull reliefs are set for an absolute altitude, once below that altitude they would be full open. That is a much safer and straighforward control scheme.

I enjoyed reading your post. For what it's worth, on the 757/767 (from my 757/767 Flight Handbook from a particular airline, under System Protection Functions): "The positive pressure relief valves limit maximum differential pressure to 9.1 psi. The negative pressure relief doors prevent outside pressure from exceeding cabin pressure." Actually, this makes sense from a hull-protection point of view because it is the differential pressure that would affect the hull integrity, not the absolute internal pressure.
 
zvezda
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 11:22 am

Quoting Centrair (Reply 7):
If you have an aircraft with a smaller frame, wouldn't it be easier to keep a sea level pressure?

Yes, in general it is easier to keep small structures strong. The reason is that mass increases with the cube of the size, but the strength only increases with the square. Thus ants that can carry 10 times their weight and Gulfstreams that maintain 6000' cabin pressure at 51000' altitude.
 
2H4
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 11:36 am




Quoting Zvezda (Reply 13):
and Gulfstreams that maintain 6000' cabin pressure at 51000' altitude.

Wow....that's really impressive.




2H4


Intentionally Left Blank
 
zvezda
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 11:55 am

Quoting 2H4 (Reply 14):
Wow....that's really impressive.

There are some serious advantages in structural efficiency by staying small. That's one of the reasons I have doubts about whether or not Boeing will build a Y3.
 
a380900
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 12:04 pm

The New York Times published Judith Miller's crap for years and years, why should people even bother? I will never read this thing again.
 
SpinalTap
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 12:19 pm

Quoting Centrair (Reply 7):
I am not an engineer (math is not my friend), so I must ask questions to benefit my own curiousity.

If you have an aircraft with a smaller frame, wouldn't it be easier to keep a sea level pressure? You have less volume to deal with.

How hard would it be to maintain sea level pressure on a wide body aircraft? That is a lot of volume.

How would the frame handle sea level pressure internally when the pressure outside is less? If you had sea level pressure and were flying at 35,000 ft, then lost pressure what would the body do? What would happen to the frame?

The volume really doesn't matter so much.

For thin walled cylinders the following equations apply

Longitudinal stresses (stresses along the length of the cylinder)
Sigma(L) = p*r/(2*t)

p = Internal pressure - External pressure (pressure difference)
r = radius of cylinder
t = thickness of walls

Hoop stresses (stresses around the circumference of the cylinder)
Sigma(H) = p*r/t

Ref: http://www.efunda.com/formulae/solid.../mat_mechanics/pressure_vessel.cfm

For a fixed radius and pressure difference you can calculate the thickness of the walls to match the material strength (there are always large safety factors involved) using the hoop stress equation because the hoop stresses are twice the longitudinal stress (for normal materials such as metals).

So to answer your question the smaller the diameter (at constant wall thickness) the higher the pressure difference can be without failure.

To calculate the extra thickness the walls need to handle sea level pressure:
Condition A: Pressure at 6000 ft P = 0.0812 MPa
Condition B: Pressure at sea level P = 0.1013 MPa

Rearranging hoop stress equation: t = p*r/[Sigma(H)]

Comparing at the two pressures for the same hoop stress and radius the thickness you get
tB/tA = pB/pA = 0.1013/0.0812 = 1.25
You need roughly 25 % thicker walls.

In the event of a decompression: decompression would be more violent in higher pressure cabins. The higher the pressure difference the higher the flow rate.
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2H4
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 12:48 pm




Quoting SpinalTap (Reply 17):
You need roughly 25 % thicker walls.

...But is the strength of every different type of airframe material (such as carbon fiber composites) as linear as the pressure increase?

In other words, how can one say that a given composite material will require X% thicker walls to maintain the same strength, when the strength and overall physical properties of such materials vary as greatly as they do?




2H4


Intentionally Left Blank
 
GoAllegheny
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 1:00 pm

I agree with your comment on the 80s and 90s - this reporter should have mentioned that airliners since the 50s have been designed to 8000 feet pressure. The real problem is that the writer is missing the fact, which is only implied in the article, which is that some people do get altitude sickness at 8,000 feet in planes. Not many, but apparently enough for Boeing to try to increase the pressurization level. Including a sentence on that point would have clarified the reason why Boeing did the testing with the volunteers.
 
speedracer1407
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 1:07 pm

Quoting Wjcandee (Reply 11):
I guess I didn't express it well enough. The reporter said, after stating that modern airliners maintain a cabin altitude of 8000 feet, that Boeing put a bunch of people in a simulator to test whether it would be okay to maintain a pressure of 6000 feet, and that "Boeing concluded that 6,000 feet was well below the level where most people experience the headaches and other symptoms of altitude sickness." I read that to man (perhaps incorrectly) that the reporter believed that one would have to do some tests to make sure that people wouldn't be harmed (i.e. "experience the headaches and other symptoms of altitude sickness") by LOWERING the cruise cabin altitude. He/she seems to be saying that people are fine (i.e. don't experience the "symptoms of altitude sickness") at an 8000-foot cabin altitude, but that Boeing needed to do testing to make sure it was okay to provide a 6000-foot cabin altitude. If I'm reading the reporter correctly, then I think the reporter doesn't understand the purpose of the testing, and is confusing lower cabin ALTITUDE with lower cabin PRESSURE. I think, as others have mentioned, that the purpose of the testing was to determine a more optimal, comfortable cabin altitude, and what altitude provided the most bang for the additional pressure buck. Indeed, the Flight Attendant unions have been suggesting for years that the 8000-foot standard was based upon cost savings and not human factors, and that they'd be more healthy if they performed their duties at a lower cabin altitude.

I understand the general (non-medical) idea of altitude sickness to be any discomfort and/or adverse reactions to altitude. You seem to assume that the author defines altitude sickness as the debilitating effects of extreme altitude. But some passengers (not me, but whatever) experience headaches, dizziness, etc from 8000 ft pressurization. I think the author indended to convey the fact that boeing's tests show that at 6000 ft equivalent altitude. passengers were significantly better off, showing little or no signs of altitude-related sickness or discomfort.

O
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Bohlman
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 1:08 pm

Quoting Zvezda (Reply 5):
It's not free. It requires a stronger (i.e. heavier) fuselage -- in tension. CFRP is very strong in tension and -- because the B787 is would around a mandrel -- there is ample tensile strength in the needed direction as a side benefit of having enough strength and stiffness in other directions to have 6000' cabin pressure at very little, if any, additional cost.

It doesn't really have anything to do with whether or not the airplane can handle it at this point, but rather whether or not it can handle it over time. The biggest benefit of CFRP is that it doesn't fatigue at nearly the rate that aluminum does. It's the same reason why the BBJ is in the works for being certified to have a higher differential, to allow a more comfortable 6000 ft CA. The reason that airlines can't pressurize, say, a 737 is that they want to be able to get a good 20000 hours out of the airplane before they have to consider major structural rebuilding, whereas a Gulfstream can allow a higher differential because weight is less of an issue, and you run a lot fewer cycles in it. So of course Boeing is doing tests to get the BBJ CA certified at lower, because after all, the average business jet only flies around 500 hours a year, well below the standard of the airlines. What does it matter to a business client that he'll only get 15000 hours, 10000 cycles out of the airplane?

Boeing initially was looking at a window between 4k and 8k feet, to try and find the greatest benefit in passenger comfort and well-being, and decided that 6k was where you saw the biggest improvement taper off. The decrease to 4k was decided against not due to a great deal of extra cost of the fuselage, but due to the implications of either decreasing the window size or increasing the window strength (can you say Concorde?). So the decision, as far as I know, was made due to procedures and difficulty should the cabin begin a rapid depressurization, meaning the vastly increased differential would cause an unacceptable amount of structural damage, which, especially considering the problems associated with repairing large sections of CFRP, would be very undesirable.

So basically, they could have a much lower cabin altitude with the same depressurization implications and, most importantly, no cost to CYCLES, not weight.
I'm not pro-Boeing or pro-Airbus, I'm pro-crew all the way.
 
SpinalTap
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 1:46 pm

Quoting 2H4 (Reply 18):
Quoting SpinalTap (Reply 17):
You need roughly 25 % thicker walls.

...But is the strength of every different type of airframe material (such as carbon fiber composites) as linear as the pressure increase?

In other words, how can one say that a given composite material will require X% thicker walls to maintain the same strength, when the strength and overall physical properties of such materials vary as greatly as they do?

2H4

I believe so (to the extent that the airframe is thin walled). A mechanical, materials or aeronautical engineer could probably tell you better (I'm a chemical engineer).

To my understanding (and memory) there are special considerations when using materials like Carbon fiber and polymers. I think in some carbon fibers composites the fiber orientation is random but in others it isn't. In materials where the fibers are aligned the strength is higher in the direction aligned to the fibers than the direction at right angles to fibre orientation. IIRC there is some angle that the fibres should be wrapped around the cylinder to minimise the thickness/maximise strength (so that the material is half is strong in the longitudinal direction as around the hoop)
"I get what they call a stipend, a stipend is like money but its such as small amount they don't really call it money"
 
2H4
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 1:56 pm




Quoting SpinalTap (Reply 22):
To my understanding (and memory) there are special considerations when using materials like Carbon fiber and polymers.

Yes, that's what I was getting at. I wonder if the whole problem of increasing mass and weight relative to pressure differential could be avoided (or at least minimized) through the use of specifically-tuned carbon fiber composite layups...

Quoting SpinalTap (Reply 22):
In materials where the fibers are aligned the strength is higher in the direction aligned to the fibers than the direction at right angles to fibre orientation.

Right, but remember, fibers can be aligned at angles other than right angles. Perhaps somedday, with proper stress analysis, and more precise fiber alignment capability, layers could be aligned to perfectly match local stress vectors.

Metal is just so 1992...  biggrin 




2H4


Intentionally Left Blank
 
wjcandee
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 2:14 pm

Quoting Bohlman (Reply 21):
Boeing initially was looking at a window between 4k and 8k feet, to try and find the greatest benefit in passenger comfort and well-being, and decided that 6k was where you saw the biggest improvement taper off. The decrease to 4k was decided against not due to a great deal of extra cost of the fuselage, but due to the implications of either decreasing the window size or increasing the window strength (can you say Concorde?).

AND


This is fascinating. Had the Times put it this way, I (and I think many people) would have understood the point ever so much better.

Quoting 2H4 (Reply 23):
Metal is just so 1992...

 rotfl  rotfl  rotfl  rotfl  Smile

Thanks again for all these great responses!
 
SFORunner
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 2:40 pm

I'm surprised nobody's taken the time to post and comment on this Aboulafia gem from the article:

Airlines placed orders for 154 of the 777's last year, a record, while Airbus booked only 15 orders for the A340. The 777 has captured about 70 percent of all orders in its market since the A340 made its debut in 1997.

"The A340 is dying slowly and horribly," Mr. Aboulafia said. "It is one of the more colossal failures in aviation. With higher fuel prices, no one wants four engines."
 
NoUFO
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 2:59 pm

Quoting Wjcandee (Thread starter):
Let's leave aside, for a second, the fact that the reporter was probably born in the '80s ...

Then he would be 25 or 26. Not very realistic against the background that those people spend some years at the University. Anyway, his age is not really relevant, since some much younger people know that pressurized aircraft have been around much longer.

Quoting Wjcandee (Thread starter):
... and therefore doesn't have enough perspective to properly state the premise as "designed and built in the 1950s through the 1990s".

I think he merely wanted to refer to aeroplanes you frequently fly on nowadays when booking a long haul flight - such as the 767. It would not make any sense to mention the 707: no longer around, the 707 (or the pressurazion system on board) will not be the reason for headaches anymore.
The only reason would be to stress that it actually took decades to achieve a major technological progress in this field.

Quoting Wjcandee (Reply 11):
If I'm reading the reporter correctly, then I think the reporter doesn't understand the purpose of the testing, and is confusing lower cabin ALTITUDE with lower cabin PRESSURE.

No, sorry, I think you are on a wrong track here. Could it be that you confuse his "well below the level" with lower pressure when in fact he meant to say "the 787 does not reach the level after which people often suffer from headaches."

Quoting Speedracer1407 (Reply 20):
I think the author indended to convey the fact that boeing's tests show that at 6000 ft equivalent altitude. passengers were significantly better off, showing little or no signs of altitude-related sickness or discomfort.

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AirWillie6475
Posts: 2372
Joined: Thu Jan 27, 2005 1:45 pm

RE: 787 Pressurization And The NY Times

Mon May 08, 2006 4:55 pm

This is way overblown. First of all NOT MANY planes flying right now actually go up to an altitude where the cabin altitude would be 8000, well expcept for Southwest since they always try to fly at the highest altitude possible. If you look at flights right now the planes actually fly from 30-37k feet. I think cabin altitude is also considered when planing cruising altitudes at some airlines. At 30-37K feet, the A320/330/340/B737ng/757/767/747/777, cabin would be around 6000k feet. Infact the 747 cabin cruising altitude is around 5000 at 35K feet which is what altitude most 747s fly that is why 747s are favorites for long routes.

I don't think pax headaches are cause by altitudes but by changing cabin pressures anyways. Sometimes I fly up to 8500 in the cessna 172 and I've gone up to 12k with no oxygen it's not a big deal for regular healthy people. For those of you who don't know cessna 172s are not pressuriesed. There is a big difference between pressurised 8000 and unpressurised 8000. Inside a pressurised cabin the oxygen level will be constant or even higher than a unpressurised cabin.

[Edited 2006-05-08 10:01:32]
 
zvezda
Posts: 8886
Joined: Sat Aug 28, 2004 8:48 pm

RE: 787 Pressurization And The NY Times

Mon May 08, 2006 5:56 pm

Quoting 2H4 (Reply 18):
...But is the strength of every different type of airframe material (such as carbon fiber composites) as linear as the pressure increase?

In other words, how can one say that a given composite material will require X% thicker walls to maintain the same strength, when the strength and overall physical properties of such materials vary as greatly as they do?

Since we're fighting hoop stresses here, we care about tensile strength. Yes, the tensile strength of CFRP increases linearly with thickness (keeping a fixed layup strategy).

Quoting SpinalTap (Reply 22):
IIRC there is some angle that the fibres should be wrapped around the cylinder to minimise the thickness/maximise strength (so that the material is half is strong in the longitudinal direction as around the hoop)

Different layers will be laid down at different angles. Also, the ratio given (half as strong in the longitudinal direction as around the hoop) will be true for one length/diameter ratio. Lengthening the tube requires increasing the strength in the longitudinal direction, but doesn't require increasing hoop strength.

Quoting 2H4 (Reply 23):
fibers can be aligned at angles other than right angles. Perhaps somedday, with proper stress analysis, and more precise fiber alignment capability, layers could be aligned to perfectly match local stress vectors.

That is exactly what Boeing are trying to do with the B787. A lot of calculations are being done to optimize the angles at which each tape layer will be laid.
 
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zeke
Posts: 14913
Joined: Thu Dec 14, 2006 1:42 pm

RE: 787 Pressurization And The NY Times

Mon May 08, 2006 6:08 pm

Quoting Zvezda (Reply 28):
Since we're fighting hoop stresses here, we care about tensile strength. Yes, the tensile strength of CFRP increases linearly with thickness (keeping a fixed layup strategy).

I don't agree with that statement, its also has time-temperature dependence.

Quoting Zvezda (Reply 28):
Different layers will be laid down at different angles. Also, the ratio given (half as strong in the longitudinal direction as around the hoop) will be true for one length/diameter ratio. Lengthening the tube requires increasing the strength in the longitudinal direction, but doesn't require increasing hoop strength.

Incorrect.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
onetogo
Posts: 286
Joined: Thu Feb 02, 2006 1:40 pm

RE: 787 Pressurization And The NY Times

Mon May 08, 2006 6:19 pm

Quoting AirWillie6475 (Reply 27):
I've gone up to 12k with no oxygen it's not a big deal for regular healthy people.

Oh my..
 
zvezda
Posts: 8886
Joined: Sat Aug 28, 2004 8:48 pm

RE: 787 Pressurization And The NY Times

Mon May 08, 2006 7:16 pm

Quoting Zeke (Reply 29):
I don't agree with that statement, its also has time-temperature dependence.

Now you're talking about fatigue. Anyway, for the small differences in thickness that are relevant here, the difference in fatigue is probably not significant for a CFRP fuselage. If it lasts 100,000 cycles, it doesn't really matter whether or not it could last 120,000 cycles.

Quoting Zeke (Reply 29):
Incorrect.

You need to be more specific if you're willing to provide an opportunity for rebuttal.
 
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zeke
Posts: 14913
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 8:56 pm

Quoting Zvezda (Reply 31):
You need to be more specific if you're willing to provide an opportunity for rebuttal.

I dont, your statement is incorrect.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
aerosol
Posts: 500
Joined: Mon Oct 23, 2000 10:31 pm

RE: 787 Pressurization And The NY Times

Mon May 08, 2006 9:03 pm

Just one question, what about radiation in high altitudes?

I always thought metal was more of a protection than plastic.
Wha is the difference there? Will it be more dangerous for crews and frequent flyers?
 
2H4
Posts: 7960
Joined: Tue Oct 19, 2004 11:11 pm

RE: 787 Pressurization And The NY Times

Mon May 08, 2006 9:05 pm




Quoting Zeke (Reply 32):

I dont, your statement is incorrect.

Without some kind of explanation, Zeke, you're contributing absolutely nothing to this discussion.




2H4


Intentionally Left Blank
 
zvezda
Posts: 8886
Joined: Sat Aug 28, 2004 8:48 pm

RE: 787 Pressurization And The NY Times

Mon May 08, 2006 9:13 pm

Quoting 2H4 (Reply 34):
Without some kind of explanation, Zeke, you're contributing absolutely nothing to this discussion.

That's not typical of Zeke. While I don't always agree with him and his biases are occasionally annoying (though some others here are far worse), he nearly always contributes something valuable to the discussion. Anyone can have a bad day, so let's cut Zeke some slack.
 
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zeke
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RE: 787 Pressurization And The NY Times

Mon May 08, 2006 9:20 pm

Quoting 2H4 (Reply 34):
Without some kind of explanation, Zeke, you're contributing absolutely nothing to this discussion.

I see your post in the same context. The statement that made was incorrect, columns cannot be increase infinitely.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
2H4
Posts: 7960
Joined: Tue Oct 19, 2004 11:11 pm

RE: 787 Pressurization And The NY Times

Mon May 08, 2006 9:29 pm




Quoting Zeke (Reply 36):
I see your post in the same context. The statement that made was incorrect, columns cannot be increase infinitely.

Oh, come on, Zeke. You know perfectly well that saying "No, it isn't....end of story" is pretty arrogant and doesn't help others understand your point of view.

I'm not sure if you're having a bad day, or of it's simply some kind of ego thing, but just about everyone here really loves hearing explanations from those as knowledgeable as yourself. Discussions can be constructive and educational, but only if everyone puts a little effort into it.




2H4


Intentionally Left Blank
 
zvezda
Posts: 8886
Joined: Sat Aug 28, 2004 8:48 pm

RE: 787 Pressurization And The NY Times

Mon May 08, 2006 9:45 pm

Quoting Zeke (Reply 36):
The statement that made was incorrect, columns cannot be increase infinitely.

I neither wrote nor implied that a column can be increased infinitely. I have no idea where you go that idea from.
 
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zeke
Posts: 14913
Joined: Thu Dec 14, 2006 1:42 pm

RE: 787 Pressurization And The NY Times

Mon May 08, 2006 10:12 pm

Quoting Zvezda (Reply 28):
Lengthening the tube requires increasing the strength in the longitudinal direction, but doesn't require increasing hoop strength.



Quoting Zvezda (Reply 38):
I neither wrote nor implied that a column can be increased infinitely.

The fuselage is a hollow column, subject to dynamic and static buckling, torsion, and compression loads. It is not just an idealised beam.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
comorin
Posts: 3858
Joined: Sun May 29, 2005 5:52 am

RE: 787 Pressurization And The NY Times

Mon May 08, 2006 10:30 pm

Quoting Zeke (Reply 39):
The fuselage is a hollow column, subject to dynamic and static buckling, torsion, and compression loads. It is not just an idealised beam.

This discussion is about the fuselage as a pressure vessel, which is the right way to look at it when discussing cabin pressure differentials. If we were talking about flight loads and related stresses, then we would consider the column model for buckling, torsion loads and so on.
 
2H4
Posts: 7960
Joined: Tue Oct 19, 2004 11:11 pm

RE: 787 Pressurization And The NY Times

Mon May 08, 2006 10:35 pm




Quoting Zeke (Reply 39):
The fuselage is a hollow column

Do the floors not provide structrual stiffness above and beyond that of a hollow column?





Certainly not idealized, but surely far more effective at resisting buckling, torsion, and compression loads than a hollow column...right?




2H4


Intentionally Left Blank
 
zvezda
Posts: 8886
Joined: Sat Aug 28, 2004 8:48 pm

RE: 787 Pressurization And The NY Times

Mon May 08, 2006 10:41 pm

Quoting Zeke (Reply 39):
The fuselage is a hollow column, subject to dynamic and static buckling, torsion, and compression loads.

Yes, though the compressive loads in the longitudinal direction are negligible compared to the other loads. Anyway, I never wrote that it could be lengthened infinitely.

Quoting Zvezda (Reply 28):
Lengthening the tube requires increasing the strength in the longitudinal direction, but doesn't require increasing hoop strength.

Any additional tape laid down along the longitudinal axis to cope with the major increase in longitudinal stresses would provide some measure of additional strength in every other direction. This would be more than enough to carry any increases in hoop stresses. So, getting back to my main point:

Quoting Zvezda (Reply 28):
the ratio given (half as strong in the longitudinal direction as around the hoop) will be true for one length/diameter ratio.

... is true. Changing the length/diameter ratio changes the ratio of longitudinal strength required to hoop strength required. The average angle of tape would also change with the length/diameter ratio. A greater length/diameter ratio would require relatively more tape to be laid down longitudinally or more nearly longitudinally.
 
texfly101
Posts: 343
Joined: Fri Jan 06, 2006 6:42 am

RE: 787 Pressurization And The NY Times

Tue May 09, 2006 6:37 am

Quoting Qqflyboy (Reply 10):
They also noted the new system will allow for 20% humidity, compared to 5% now. It is dehydration that usually causes the effects of jetlag (drowsiness and headaches).

This is the correct reason. Its easier to maintain the relative humdity at that pressure. Hopefully I won't have the cotton mouth, dry nose, etc after a cross country flight.

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