[QUOTE=St_Ides]
From what I’ve read, the A380 is pretty much the biggest we can go with current materials.
That’s not to say we can’t go bigger in the future, but not with the materials we have right now. 20 years ago, nobody would have thought we could ever build an airliner out of mainly carbon fibre, but Boeing’s doing that with their Dreamliner.
Lift and drag’s not that much of a concern, as long as the materials to build it are there. And the logistics to support the aircraft.
[/QUOTE]
Lift and drag are major considerations; as David Simmons already noted, the mass of making wings longer increases faster than your ability to generate lift. It’s actually even worse than that; lift can be more or less reduced to a matter of wing length (one dimension…making them broader doesn’t really increase the lift) while giving the wings sufficient stiffness requires more volume (thicker, broader), so the longer you make them the worse the relationship gets, and also increases induced drag by a square factor (more important at low airspeeds than high ones) and power as a cube. I suspect the limiting factor would be on takeoff; you’d end up having an unreasonably high takeoff speed and enormous power requirements, which would demand an incredibly long runway and gigantic engines. Even if you can make the wings out of some superlight, superstiff material you’re still going to have poor low speed lift/drag ratio, making the plane difficult and unsafe to fly at takeoff and landing speeds, which was an issue with the Tupelov Tu-144 and (to a lesser extent) the Concorde SST. Then there are the manufacturing difficulties of making and handling really long wings as a separate structure, but that’s a manufacturing, rather than aerodynamic, issue.
It should be noted that this refers to tube-and-swept wing configuation aircraft, which quite frankly should be considered obsolete. A blended body wing would be better, and a flying wing type aircraft better yet at generating lift and internal volume while minimizing high stress cantilevered structure, also permitting a low-to-no stall speed, higher efficiency at cruise speed, and better aerodynamics in the near-transonic range. The problems with blended bodies and flying wings are control (they tend to be dynamically unstable, requiring active control and thrust-vectoring, especially at high speeds), lack of cabin window area, more off-axis area (contributing to disorientation and air sickness in passengers), and the general lack of knowledge about these type of aircraft in commercial aircraft design. Because of how radical they are (and thus, requiring both a lot of research and develpment, and good salesmanship to the typically conservative business of commercial air travel and transport) no manufacturer wants to take on the cost and risk of trying to develop this without assurances of profit, but in practical terms it would be a vastly better design than the conventional toothpaste-tubes-with-wings we currently use.
BTW, carbon fiber structures are very, very tricky. CFCs using Kevlar or some other high tensile strength fiber are very strong in tension, and very stiff–sometimes too stiff for their own good–but it is difficult to make a transverse joint that is of equivilent strength, and under load cycling (particularly tensile-to-compressive) the material is under progressive non-linear failure, and it is offen difficult to discern the critical failure state and lifetime of a composite structure without a body of empircal information about that specific design. (Aluminum also undergoes progressive failure, but failure modes and indications of imminent failure states are better known and easier to see by inspection in situ.) Airbus has had some signficant problems with composite vertical stabilizers due to improper joint design and a lack of adequate inspection and repair criteria. As a result, I’m not super-excited about the extensive use of composive fiber material in aircraft structures.
Stranger
