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NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 2:32 am

The 748-I length is under debate because the longer version doesn't have enough range for EK, and the shorter version doesn't have enough capacity for BA and LH and possibly others.

Meanwhile, there is a lot of ceiling space that Boeing wants to configure as
executive suites and which airlines want to use for crew rests, galley carts
and maybe lavs. It seems that if airlines get their way with regard to crew rests and galleys etc, there will still be a lot of space left over.

So the question is: why don't they put extra fuel tanks into the ceiling, say, in a short block directly above the wing (so as not to change CG too much, if at all)? It must have a very obvious answer to an Engineer, but as a non-Engineer I would like to know what Engineers think about this. It can easily be pumped up into place through relatively thin pipes and the pilot could use up that tank first, before beginning to use the wing and belly and tail tanks.

Is it unsafe? If safety is the issue, how is the situation significantly different from sitting above the center fuel tanks if you sit above the wing?

Or just that requires too much modification to the existing design to be feasible? Ceiling floor not strong enough to bear the load?

Or the 748-I is limited by weight and not by fuel volume?

Or is leakage a concern? (So that leaking from wing and belly tanks won't affect passengers, but leaking from above would be a hazard?

To my naive mind, this would make everyone happy (for both extreme range and good capacity at a nice midpoint between the 77W and A380).

Oh yes, another one: why they give the 748I a screwdriver tail like the MD80
and the 777? It won't significantly improve the aerodynamics?
 
geniusjacky
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 2:36 am

Just my guess:
Maybe you will then need a duct to move fuel from the ceiling down to the wing. I dont think there is any room that is left for the purpose. And most likely, you probably don't want the fuel duct to run through the wall of the aircraft next the the passenger.
 
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N328KF
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 2:40 am

Aside from what I would have figured would be the obvious problem of having flammable material above passengers (imagine a crash situation, especially like that of the dual 747 Canary Island crash), putting fuel up high could possibly introduce CG issues.
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OPNLguy
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 2:47 am

I'll leave the technical stuff to the engineers, but personally, I don't like the idea of fuel tanks above passengers, if only for the reason that Murphy's Law being what it is, that one will eventually leak in normal use. IIRC, the Shorts 330 and 360 turboprops had fuel tanks inthe cabin ceilings, and I always hated flying them for that reason.
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aeroman444
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 3:08 am

It will never happen. If you spring a leak it will be in the passenger cabin.
 
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Buyantukhaa
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 3:12 am

Quoting OPNLguy (Reply 3):
I don't like the idea of fuel tanks above passengers,



Quoting N328KF (Reply 2):
the obvious problem of having flammable material above passengers

Then don't fly F50, C130, Atr42/72, BAe146, ARJ, Dash8 etc etc.

I know, it isn't the same. I think the answer is that the fuselage would have to be strengthened so much, that it would not make much economic sense. But that's merely a guess. We may want to move this thread to Tech/Ops BTW.
I scratch my head, therefore I am.
 
aeroweanie
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 3:35 am

Usually, you don't put fuel in the ceiling, due to crash survivability considerations. However, the Shorts 330 and 360 had fuel in a tank right above the cabin, in the wing centersection. Later model C-17s have such a tank too.


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eatmybologna
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 3:39 am

I'm not an aerospace engineer either, so just a guess here. I believe that the weight of fuel is beneficial when located in the wings. The airplane's wings bare their highest loads of force during take off. This is due to the dynamic pressure of wind velocity against the bottom face of the wing during the initial ascend (take off.) The weight of fuel offsets the upward force on the wings, helps them remain more rigid, and helps keep them more horizontal. If you observe the wing over stretched intervals during a long cruise, you may see an increased upwards flex over time due to fuel consumption.

E-M-B
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ThrottleHold
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 4:06 am

Quoting AeroWeanie (Reply 6):
However, the Shorts 330 and 360 had fuel in a tank right above the cabin, in the wing centersection

As does the BAe 146/Avro RJ series.
 
SlamClick
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 4:25 am

One issue is compliance with FAR 25.963

Fuel tanks within the fuselage contour must be able to resist rupture and to retain fuel, under the inertia forces prescribed for the emergency landing conditions in § 25.561. In addition, these tanks must be in a protected position so that exposure of the tanks to scraping action with the ground is unlikely.


25.561 is pretty technical and too lengthy to quote here but it describes upward G-forces for passenger seats of 3.0G That indicates to me that they envision the affected fuselage section to be landing upside down.

US Special Forces rejected the Shorts Skyvan in about 1968 in part because it had fuel cells above the cargo/passenger compartment. This was seen as undesirable for a combat aircraft. I think paying passengers may feel that they are entitled to at least as much safety as that given to green berets.
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KELPkid
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 4:37 am

Quoting SlamClick (Reply 9):
US Special Forces rejected the Shorts Skyvan in about 1968 in part because it had fuel cells above the cargo/passenger compartment. This was seen as undesirable for a combat aircraft. I think paying passengers may feel that they are entitled to at least as much safety as that given to green berets.

Interesting...there is at least one Shorts 330/360/Skyvan variant in active use with the Army Reserve...I've seen them down in SLE flying the things in "weekend warrior" duty. Maybe it was just the Special Forces that rejected them.

Also, the US Forest Service uses them as a smoke jumper aircraft...I asked the pilots when I saw them on the ground at S07 (Bend, Oregon) one time, and they said that all of their birds are ex-US Army aircraft.

When I worked at LRU, in the early 1990's a bunch of them in UA colors found there way into desert storage down at our field (not sure exactly who they belonged to...the running rumor at the airport was that YV took over UA express flying for someone and immediately parked all the Shorts). Not sure what became of them.
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Dalmd88
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 5:37 am

I think the biggest problem with a fuel tank above the passenger cabin is the structral mod work. An easier solution would be the tail like the MD-11. It would have an effect on CG, but I bet be workable.
 
FredT
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 6:34 am

I'd say structure is the main reason. You would have to strengthen the fuselage to support the weight of the fuel, meaning you'd get a huge zero fuel weight increase per kilo of fuel compared to what you get with fuel tanks in wings, keel or other parts of the aircraft which are already carrying loads.

Regards,
Fred
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Starlionblue
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 7:29 am

Quoting Eatmybologna (Reply 7):
I'm not an aerospace engineer either, so just a guess here. I believe that the weight of fuel is beneficial when located in the wings. The airplane's wings bare their highest loads of force during take off. This is due to the dynamic pressure of wind velocity against the bottom face of the wing during the initial ascend (take off.) The weight of fuel offsets the upward force on the wings, helps them remain more rigid, and helps keep them more horizontal. If you observe the wing over stretched intervals during a long cruise, you may see an increased upwards flex over time due to fuel consumption.

It is true that wing fuel offsets wing bending. That's why center tanks are drained first in flight. However the wings are already full today.
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mohavewolfpup
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 7:55 am

I thought 737's had the fuel in the wings, and in the middle top cabin area? or is it underneath the floor area?
 
SlamClick
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 8:14 am

Quoting Mohavewolfpup (Reply 14):
or is it underneath the floor area?

Underneath, where the wing structure carries through the fuselage. The forward bulkhead of the main landing gear well is actually the rear bulkhead of the center tank and a wing center section spar.
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SlamClick
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 8:21 am

Quoting KELPkid (Reply 10):
Interesting...there is at least one Shorts 330/360/Skyvan variant in active use with the Army Reserve...

Eventually the Army did buy the C-23 Sherpa:

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But I am talking about the original Skyvan

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...which would likely have had a non-military paint scheme with no markings other than the tail number. SF had a good deal of latitude back then and evaluated aircraft not available to the regular Army.
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darkblue
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RE: NonEngineer's Question About Fuel In Ceiling

Fri Nov 03, 2006 9:13 am

A Payload vs Range chart is one good way of displaying how fuel capacity, payload, structural limit and range are all related. Below I've sketched up an example diagram.

The topmost line represents the payload limit of the aircraft. The aircraft just can't hold any more passengers or freight.

As you add fuel, range increases until you reach the structural limit of the aircraft. Even though the fuel tanks are not full, the aircraft cannot carry any more weight. So what do you do? Trade passengers for fuel to increase range.

However, this only works up to a point, because eventually you will reach the fuel capacity limit. At this point no more fuel can be added to the aircraft. The only thing left to do to increase range is to remove more payload until you have an empty aircraft.

Big version: Width: 701 Height: 425 File size: 32kb


Now back to your question, what happens if you add more fuel tanks to the aircraft? The figure below shows that if you add fuel capacity, you can increase range. However, due to the slope of the structural limit, you are still limited to how much fuel you can add. Because of this diminishing return on payload vs. range, it is not always economically feasible to add fuel capacity just to increase range.

Big version: Width: 702 Height: 426 File size: 35kb



To increase range, the focus needs to be on 2 things:
1.) decrease aircraft weight (to allow for more fuel)
2.) increase the structural limit (without increasing weight)


Hope this helps.

DarkBlue
 
Rj111
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RE: NonEngineer's Question About Fuel In Ceiling

Tue Nov 07, 2006 9:49 pm

Dark blue is correct and the 748's tanks are indeed weight limited. According to Boeing's generic figures*, a 748 fully loaded with fuel will only allow a 31t payload which is approximately 310 pax. So firstly the MTOW will have to be increased to improve range at a typical configuration. Then a mixture of MTOW and Fuel capacity will be needed to increase it further.

Assuming a 440 pax capacity you could only fill the tanks up to about 58,500 gallons - about 90%. So you'd need an MTOW increase of about 18t to get the rest in, and that's assuming no OEW increase as a result. I don't know exaclty how much further that would take it but i'd guess about 400nm.

Quoting DALMD88 (Reply 11):
I think the biggest problem with a fuel tank above the passenger cabin is the structral mod work. An easier solution would be the tail like the MD-11. It would have an effect on CG, but I bet be workable.

The 744 already has a 3,300 USG tail tank capacity and i assume the 748 will too.

*Source: http://www.boeing.com/commercial/airports/acaps/7478brochure.pdf

Rj111

[Edited 2006-11-07 13:54:46]
 
darkblue
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RE: NonEngineer's Question About Fuel In Ceiling

Tue Nov 07, 2006 11:06 pm

Quoting RJ111 (Reply 18):
The 744 already has a 3,300 USG tail tank capacity and i assume the 748 will too.

Yep, the 744 has fuel tanks in the tail, but only the horizontal stabilizer is wet. No fuel in the vertical stabilizer. The 748 will certainly have the horizontal stabilizer tanks may even add tanks to the veritical tail.

Emirates really wants a longer range 747-8 but Boeing seems to be heading in the other direction to satisfy other customers, adding capacity at the expense of range. Adding fuel to the vertical tail may be a good solution for Emirates since although the structural limit will lower the maximum payload (as i explained above), they have already told Boeing they want the slightly smaller 747-8 with longer range.
 
BAE146QT
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RE: NonEngineer's Question About Fuel In Ceiling

Wed Nov 08, 2006 12:05 am

Quoting DarkBlue:
they have already told Boeing they want the slightly smaller 747-8 with longer range.

They want a 747-800SP. Yes. That's what they want.
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Starlionblue
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RE: NonEngineer's Question About Fuel In Ceiling

Wed Nov 08, 2006 12:55 am

Quoting BAe146QT (Reply 20):

They want a 747-800SP. Yes. That's what they want.

When you say it like that I want one too. Love the SPBig grin
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Rj111
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RE: NonEngineer's Question About Fuel In Ceiling

Wed Nov 08, 2006 2:24 am

I suspect a 744 sized aircraft with the -8 enhancements would have a range of about 9,300nm give or take. If you subtract the weight disparity between the 748 and the 744ER and the weight for 40 pax on the 748I range payload chart you'll get this figure. This requires no MTOW increase on the 744ER either.

I can see why EK would be interested.

[Edited 2006-11-07 18:28:46]

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