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"Double Bubble" Vs. Circular Cross Section  
User currently offlineAmericanB763ER From Luxembourg, joined Sep 2005, 166 posts, RR: 0
Posted (8 years 10 months 2 weeks 1 day ago) and read 11330 times:

I have done a search on this subject but it didn't return anything to my satisfaction so here goes (quite a simple question):

What's the benefit of a circular fuselage cross section vs. a "double-bubble"-layout. It almost seems to me the circular version is gaining ground as most of the newer aircraft types (all the airbus types + the 777) have that one (OK Emb170/190 and the A380 are exceptions here) - as opposed to the "double-bubble" found on the 767, the older Boeings + Douglas types which have a more or less visible "kink" between the cabin and the cargo hold? Sure there must be aerodynamic reasons behind this.

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Thanks
Marco

19 replies: All unread, jump to last
 
User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 1, posted (8 years 10 months 2 weeks 23 hours ago) and read 11296 times:

No, not aerodynamics, just the desire to utilize older tooling left over from previous designs.

Boeing...for example.

The first truly sucessful design for very long range flights (Stratocruiser) used the B29 design plus an extra fuselage lobe, to enhance passenger appeal/space.
This was then ingrained in Boeing thinking, as it is slightly less expensive to design a double lobe fuselage, for the required space desired, yet keep the structure weight down.
This was carried over to the B707 design...and beyond.

Circular is all well and good, but it would appear that some may disagree.
Douglas, for example, when they developed the DC6.
The fuselage might look circular, but it is not (slightly taller than wide) and this was specifically done so that passengers would have more head/shoulder room.
How do I know?
My dear old Dad was the engineering project manager...DC6/DC7.
And, DC8.


User currently offlineDougloid From , joined Dec 1969, posts, RR:
Reply 2, posted (8 years 10 months 2 weeks 21 hours ago) and read 11258 times:

Quoting 411A (Reply 1):
The first truly sucessful design for very long range flights (Stratocruiser) used the B29 design plus an extra fuselage lobe, to enhance passenger appeal/space.

The Curtiss C46 had a double bubble fuselage but I do not know what the range was....I remember putting my old man on a Stratocruiser for a flight to the UK back in 1956....


User currently offlineMrocktor From United States of America, joined Jan 2005, 1668 posts, RR: 50
Reply 3, posted (8 years 10 months 2 weeks 18 hours ago) and read 11211 times:

Quoting 411A (Reply 1):
No, not aerodynamics

This is not correct. The E170 cross section is a double bubble, and there was no use of old tooling whatsoever. The explanation lies in balancing the desired internal dimensions (pax cabin and cargo compartment) against the drag generated by the fuselage.

To fit the desired dimensions of the E170 into a circular cross-section would mean a significantly larger frontal area and commesurate increase in drag. The same goes for the A380, if the fuselage were circular it would be huge with a lot more volume than necessary, with the extra volume in places that are not practical to use.

The tradeoff is that a non-circular cross section will increase the weight of the fuselage slightly, since a circular section is the ideal shape to resist the pressurization loads.

Several other factors affect the cross section, such as the inclination of the "walls" in the passenger cabin (you want them to be as close as possible to vertical in the area adjoining the seats) and the required volumes for systems.

The crew rest areas in the 777 are a good example of using otherwise unproductive space in a circular cross section.

mrocktor


User currently offlinePilotpip From United States of America, joined Sep 2003, 3148 posts, RR: 11
Reply 4, posted (8 years 10 months 2 weeks 17 hours ago) and read 11184 times:

The "double-bubble" is also a good marketing tool. Embraer has used this heavily to show the 170/190 family have a larger passenger cabin which would be appealing to Joe Public.


DMI
User currently offlineAmericanB763ER From Luxembourg, joined Sep 2005, 166 posts, RR: 0
Reply 5, posted (8 years 10 months 2 weeks 2 hours ago) and read 11073 times:

so in the end whatever cross section a manufacturer goes for- each one has its advantages + shortcomings.



Thanks to you all for your replies.


Btw the Vanguard/Merchantman is another great example for a double-bubble - just look how high above the ground the pax door is despite the low sitting fuselage.


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User currently offlineUAL747 From , joined Dec 1969, posts, RR:
Reply 6, posted (8 years 10 months 1 week 6 days 21 hours ago) and read 10992 times:

So am I to assume, aerodynamics and structural maters aside, that in fact in a perfect world for cargo and passengers, the best cross section design would be a square?

UAL


User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 7, posted (8 years 10 months 1 week 6 days 20 hours ago) and read 10983 times:
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DATABASE EDITOR




Quoting UAL747 (Reply 6):
So am I to assume, aerodynamics and structural maters aside, that in fact in a perfect world for cargo and passengers, the best cross section design would be a square?



Shorts certainly thought so:




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2H4





Intentionally Left Blank
User currently offlineZSOFN From United Kingdom, joined Jun 2005, 1413 posts, RR: 6
Reply 8, posted (8 years 10 months 1 week 6 days 19 hours ago) and read 10977 times:

One issue not raised is that of pressure. Obviously the most resistant cross sectional shape to the pressures of the cabin vs external pressures in high altitude flight is the circle. So with a "double bubble" are there weaknesses/pressure points to overcome? Are they serious considerations?

User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2543 posts, RR: 24
Reply 9, posted (8 years 10 months 1 week 6 days 16 hours ago) and read 10943 times:

Quoting ZSOFN (Reply 8):
One issue not raised is that of pressure. Obviously the most resistant cross sectional shape to the pressures of the cabin vs external pressures in high altitude flight is the circle. So with a "double bubble" are there weaknesses/pressure points to overcome? Are they serious considerations?

Mroctor touch upon this in reply 3. It is a factor in the design decision, which will be a trade off with other demands. Also there can be in service issues, such as the 747 fatigue problems in the area where the forward fuelage is not circular (resulting in section 41 mods).

Where the cabin shape is not circular the skin will not be in pure tension, but will have bending loads as well trying to force the skin circular. Either the cabin structure will have to be heavier, or the differential pressure limit will be less. It's possible that the shape can lead to local stress concentrations which will lead to premature fatigue failure.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineAviation From Australia, joined Dec 2004, 1143 posts, RR: 21
Reply 10, posted (8 years 10 months 1 week 6 days 15 hours ago) and read 10927 times:

Here is the ultimate Double-Bubble design:


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StratoFreighter is the ultimate in the double bubble design

Thanks,
Aaron J Nicoli



Signed, Aaron Nicoli - Trans World Airlines Collector
User currently offlineHiFi From Brazil, joined Apr 2005, 192 posts, RR: 0
Reply 11, posted (8 years 10 months 1 week 6 days 6 hours ago) and read 10877 times:

Quoting ZSOFN (Reply 8):
One issue not raised is that of pressure. Obviously the most resistant cross sectional shape to the pressures of the cabin vs external pressures in high altitude flight is the circle. So with a "double bubble" are there weaknesses/pressure points to overcome? Are they serious considerations?

With a double-bubble, your fuselage is usually heavier, since it needs to be thicker.. Internal volume is reduced (but of course you have the second bubble to add even more volume than with a simple circular cross-section).



no commercial potential
User currently offlineMrocktor From United States of America, joined Jan 2005, 1668 posts, RR: 50
Reply 12, posted (8 years 10 months 1 week 6 days 5 hours ago) and read 10861 times:

Quoting Jetlagged (Reply 9):
Either the cabin structure will have to be heavier, or the differential pressure limit will be less. It's possible that the shape can lead to local stress concentrations which will lead to premature fatigue failure.

Exactly. Double bubble designs mitigate this issue by placing the "floor" at the level where the bubbles connect. The floor acts like a cross beam and helps to bear the pressure load. In a circular cross section the floor structure is practically unloaded.

Still, the double bubble design will be heavier than a circular design of the same internal volume. The thing is, sometimes the shape of the volume is very important. In airliner design you will be most likely comparing a larger circular cross section versus a smaller double bubble cross section - for the same useful volume.

It boils down to a drag vs weight vs manufacturing cost analysis.

mrocktor


User currently offlineZvezda From Lithuania, joined Aug 2004, 10511 posts, RR: 64
Reply 13, posted (8 years 10 months 1 week 4 days 22 hours ago) and read 10754 times:

In both circular and double-bubble (assuming the floor attaches exactly where the two bubbles meet) designs, the skin is simply in tension where metals are strongest. In other shapes (including the WhaleJet's ovoid), the pressure loads change the shape (not just the size) of the fuselage. Resisting such deformations requires extra strength which adds extra weight.

Note that the surface area/volume ratio is higher for a double-bubble than a circle. Increasing the surface area increases the weight. A circle would definitely be the best design if all the volume inside could be used effectively, but that is not always the case.

Also note that all the nearly square fuselage cross sections pictured above are unpressurized. A pressurized nearly square cross section would be excessively heavy.


User currently offlineLeanOfPeak From United States of America, joined Oct 2004, 509 posts, RR: 1
Reply 14, posted (8 years 10 months 1 week 4 days 21 hours ago) and read 10737 times:

Quoting Zvezda (Reply 13):
Also note that all the nearly square fuselage cross sections pictured above are unpressurized. A pressurized nearly square cross section would be excessively heavy.

Witness the 3.3 psi differential pressure limit on the Cessna P337 and P210, 3.9 psi on the Beech 58P Baron, and 5.1 psi on the Piper Malibu/Mirage series, which rose to 5.5 psi for the Meridian.

Such low differential pressure limits are not practical for aircraft cruising at jet altitudes. Therefore, the shape of the cross-section becomes more important.


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6388 posts, RR: 54
Reply 15, posted (8 years 10 months 6 days ago) and read 10536 times:

Quoting Zvezda (Reply 13):
In both circular and double-bubble (assuming the floor attaches exactly where the two bubbles meet) designs, the skin is simply in tension where metals are strongest. In other shapes (including the WhaleJet's ovoid), the pressure loads change the shape (not just the size) of the fuselage. Resisting such deformations requires extra strength which adds extra weight.

The A380 is a triple bubble design. It uses both floors as cross beams - one floor exactly at each junction between the three bubbles.

Neither double bubble nor triple bubble designs require extra strength, except that the floors must be designed to take the tension at the bubble junctions.

The A380 triple bubble design makes it a lausy outsize cargo plane. It needs the two floors to keep the three bubbles in perfect bubble shape. If anyone of the floors were removed the fuselage would break apart when pressurized.

The mostly circular fuselage on the 747 (especially with front loading door) is in a totally different league concerning outsize cargo. Even if the floor on the upper deck also is a cross beam on the forward double bubble part. The "problems" on the 747 is where the double bubble part blends gradually into the circular (single bubble) part. That is one very complicated piece of structure.

The British military Nimrod is also a very obvious double bubble, but different. The upper bubble is an ordinary Comet circular single bubble, and the lower bubble is not pressurized. No cross beam needed.

And the A300 Beluga: Not pressurized at all. Only the flight deck is pressurized.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2543 posts, RR: 24
Reply 16, posted (8 years 10 months 3 days 15 hours ago) and read 10396 times:

Quoting Prebennorholm (Reply 15):
If anyone of the floors were removed the fuselage would break apart when pressurized.

I doubt that very much. The floor contributes to the strength of the structure, but is not a critical part of the design, or shouldn't be.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineMrocktor From United States of America, joined Jan 2005, 1668 posts, RR: 50
Reply 17, posted (8 years 10 months 2 days 23 hours ago) and read 10353 times:

Quoting Jetlagged (Reply 16):
I doubt that very much. The floor contributes to the strength of the structure, but is not a critical part of the design, or shouldn't be.

All certification tests are performed with the floor in place, given the geometry, the floor is loaded - thus the floor's contribution is used for certification purposes.

Maybe the fuselage would not burst with the floor removed, since all structures are overdesigned by a specific safety margin, however the structure would certainly not meet certification specs without the floor.

mrocktor



[Edited 2005-09-30 22:30:45]

User currently offlineMD-90 From United States of America, joined Jan 2000, 8502 posts, RR: 12
Reply 18, posted (8 years 10 months 2 days 20 hours ago) and read 10329 times:

Quoting 411A (Reply 1):
The first truly sucessful design for very long range flights (Stratocruiser) used the B29 design plus an extra fuselage lobe, to enhance passenger appeal/space.

It wasn't much of a moneymaker, though. The DC-7 seemed to be better at that, although they all had the engine reliability issues (-7C, 1649A, Stratocruiser).


And I'm not sure if describing the A380 as a triple bubble design is quite accurate. I've lost the picture, but I saw a graphic (from Airbus) showing how they came up with the A380's cross-section. It was two large circles, with two smaller internally tangent circles. It basically is a double-bubble design.


User currently offlineStirling From Italy, joined Jun 2004, 3943 posts, RR: 21
Reply 19, posted (8 years 10 months 1 day 4 hours ago) and read 10246 times:

Quoting Mrocktor (Reply 12):
The floor acts like a cross beam and helps to bear the pressure load. In a circular cross section the floor structure is practically unloaded.

Then that explains the differences I've felt underfoot walking in a B737 and A320....no one has been able to explain the "flimsier" feeling of the A320 when compared to the B737....until now. Now, it makes perfect sense.

I love how answers to questions get answered in such roundabout ways! Wink



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