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 Circular Cross Sections?
 A380900 From France, joined Dec 2003, 1120 posts, RR: 1Posted Wed Jun 26 2013 18:04:21 UTC (1 year 9 months 1 week 1 day 21 hours ago) and read 3483 times:

 I read in another thread that the 777 was the first Boeing with a "circular cross section". I don't know whether that's true but I wonder which airliners have a perfectly circular cross section. It is quite obvious that the A380 does not. But what about the A300/10/20/30/40/50? The 787? And how far are 757, 737 and 767 from a circle? And I guess if these are perfect circles, it must have a technical explanation. Is it more about aerodynamics or more about pressurization issues? And how bad is it for the A380 and the others not to not have this shape? How about concorde by the way? Was the fuselage cross section close to a circle?[Edited 2013-06-26 18:07:33]
 Starlionblue From Greenland, joined Feb 2004, 17288 posts, RR: 67 Reply 1, posted Wed Jun 26 2013 18:23:21 UTC (1 year 9 months 1 week 1 day 21 hours ago) and read 3470 times:

 Quoting A380900 (Thread starter):And how far are 757, 737 and 767 from a circle?

The 707/720, 737, 727 and 757 all share the same cross section, but have different lower lobes. The upper lobes are quite circular, with the different models having different degrees of bulge on the bottom. The 727 and 737 are closest to circular.

707

727

737

757

The 767 is rather oval.

AFAIK the 330/340 is circular.
 Quoting A380900 (Thread starter):And I guess if these are perfect circles, it must have a technical explanation. Is it more about aerodynamics or more about pressurization issues?

Pressurization and structure in general. A cylinder is inherently stronger, and thus can be made lighter. However in other respects a cylinder could be heavier. It really depends on the capacity of the airliner you're designing whether a cylinder is a good idea or not.

 Quoting A380900 (Thread starter):And how bad is it for the A380 and the others not to not have this shape?

It's all about design compromises. Methinks a cylinder on the scale of the 380 would be too wide for the middle decks to be practical I think. Imagine three aisles with 3-4-4-3 seating. Also it would mean shorter wings, and the 380 is already hard up against the ICAO size limit of 80 meters wingspan.

One of the 380 proposals was basically two 340 fuselages side by side but structurally it would have been quite heavy.

 "There are no stupid questions, but there are a lot of inquisitive idiots."
 BMI727 From United States of America, joined Feb 2009, 16066 posts, RR: 27 Reply 2, posted Wed Jun 26 2013 19:21:30 UTC (1 year 9 months 1 week 1 day 20 hours ago) and read 3420 times:

 Quoting A380900 (Thread starter):And I guess if these are perfect circles, it must have a technical explanation.

It's structurally the most efficient: pressurization is trying to push a vessel into a circle anyway. A circle also has the most area per perimeter of any shape, but that brings you to the reason why you might not want a circle: drag.

A circle gives the most interior space, but not all of it is useful, especially as you get bigger. A large circular cross section will have space in the crown and keel that is not revenue producing. Now a lot of it can be used for EE bays, ductwork, hydraulics, etc. but eventually some of it is not needed. So a circle is the best area per perimeter (which translates pretty directly to surface area, which translates to drag), but maybe not the least perimeter for a given useful area. A non-circular shape can cut drag for a given cabin/cargo bay size, but at the cost of structural efficiency. It turns into a big tradeoff, but for all the circular planes out there, Gulfstream switched from circular to a flat bottom cross section on the G650.

 Why do Aerospace Engineering students have to turn things in on time?
 LH707330 From United States of America, joined Jun 2012, 1017 posts, RR: 1 Reply 3, posted Thu Jun 27 2013 00:27:13 UTC (1 year 9 months 1 week 1 day 15 hours ago) and read 3335 times:

 One reason for the non-circular cross-sections is the floor. If you don't have anything inside, a cylinder makes the most sense, but if you have floor beams in tension, then it makes sense to have two partial cylinders in a double-bubble (or figure-8) arrangement, because it cuts down on drag.
 Viscount724 From Switzerland, joined Oct 2006, 26562 posts, RR: 22 Reply 4, posted Thu Jun 27 2013 16:14:35 UTC (1 year 9 months 1 week 23 hours ago) and read 3195 times:

 Didn't the KC-135 have a circular fuselage section (unlike the wider 707 fuselage)?
 Starlionblue From Greenland, joined Feb 2004, 17288 posts, RR: 67 Reply 5, posted Thu Jun 27 2013 16:36:32 UTC (1 year 9 months 1 week 23 hours ago) and read 3184 times:

 Quoting Viscount724 (Reply 4):Didn't the KC-135 have a circular fuselage section (unlike the wider 707 fuselage)?

Nope. Incidentally, this was Boeing model 717.

 "There are no stupid questions, but there are a lot of inquisitive idiots."
 LH707330 From United States of America, joined Jun 2012, 1017 posts, RR: 1 Reply 6, posted Thu Jun 27 2013 17:45:44 UTC (1 year 9 months 1 week 22 hours ago) and read 3166 times:

 Quoting Viscount724 (Reply 4):Didn't the KC-135 have a circular fuselage section (unlike the wider 707 fuselage)?

Same lower lobe, slightly narrower upper lobe. It's oval, not figure-8.

 vikkyvik From United States of America, joined Jul 2003, 10790 posts, RR: 26 Reply 7, posted Thu Jun 27 2013 19:06:02 UTC (1 year 9 months 1 week 20 hours ago) and read 3138 times:

 Quoting LH707330 (Reply 3):One reason for the non-circular cross-sections is the floor. If you don't have anything inside, a cylinder makes the most sense, but if you have floor beams in tension, then it makes sense to have two partial cylinders in a double-bubble (or figure-8) arrangement, because it cuts down on drag.

Maybe I'm misunderstanding, but how does drag, whether for circular or double-bubble cross section, relate to floor load?

 Do all philosophers have an "s" in them?
 Starlionblue From Greenland, joined Feb 2004, 17288 posts, RR: 67 Reply 8, posted Thu Jun 27 2013 19:45:01 UTC (1 year 9 months 1 week 20 hours ago) and read 3125 times:

 Quoting vikkyvik (Reply 7):Quoting LH707330 (Reply 3): One reason for the non-circular cross-sections is the floor. If you don't have anything inside, a cylinder makes the most sense, but if you have floor beams in tension, then it makes sense to have two partial cylinders in a double-bubble (or figure-8) arrangement, because it cuts down on drag. Maybe I'm misunderstanding, but how does drag, whether for circular or double-bubble cross section, relate to floor load?

I believe LH707330 is not speaking about floor load from cargo, but rather about the fact that the deck beams are part of the structure and thus partially relieving the load on the fuselage itself. Thus the fuselage can be made as strong as if it were circular even if it is not.

 "There are no stupid questions, but there are a lot of inquisitive idiots."
 rwessel From United States of America, joined Jan 2007, 2549 posts, RR: 2 Reply 9, posted Thu Jun 27 2013 22:16:30 UTC (1 year 9 months 1 week 17 hours ago) and read 3092 times:

 Quoting vikkyvik (Reply 7):Maybe I'm misunderstanding, but how does drag, whether for circular or double-bubble cross section, relate to floor load?

While a circular fuselage will contain the maximum volume for a given surface area (wetted area = parasitic drag), much of that space is not usable (see, for example, the very large crown area on the 777). For typical loads, a double (or triple) bubble can enclose more *usable* space for a given wetted (surface) area, hence less drag. Structurally, the circular fuselage is simpler and lighter for the enclosed volume (hence less induced drag), and a double bubble requires heavy reinforcement along the plane joining the bubbles (although it does save on skin area, and hence skin weight). For airliners in particular, the main deck floor needs to be there anyway, and slightly strengthening that to handle the chore is usually not a big increase over the other design loads for the floor.

From a passenger and cargo perspective, you'd ideally like a rectangular cross section (think railroad box car), but that's problematic from a pressurization perspective (and the angular corners are not particularly aerodynamic).

 LH707330 From United States of America, joined Jun 2012, 1017 posts, RR: 1 Reply 10, posted Fri Jun 28 2013 20:49:08 UTC (1 year 9 months 6 days 19 hours ago) and read 2914 times:

 Quoting vikkyvik (Reply 7):Maybe I'm misunderstanding, but how does drag, whether for circular or double-bubble cross section, relate to floor load?

Apologies for the confusion, I wasn't being very clear.

These two did a good job explaining what I was trying to say: a cylinder is best from a structural standpoint, and an oval is best for drag. A double-bubble consisting of two partial circles with a tensioned floor beam is a good compromise.

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