Big airplanes--where does it end?

Factual Questions
1

There is a new world’s largest passenger airplane. Is there an engineering limit to how big you can make an airplane?

My question has nothing to do with whether airports are big enough to handle it or other such practical considerations.

I am wondering about when you hit the limits of what you can do with any known materials. How big can it get before there is no way to attach the wings so they won’t fall off from their own weight? How about lift and drag?

If we have not reached those limits yet, are there economic limits, such as diminishing returns on fuel economy, that make it unlikely that bigger planes will be built?

2

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.

3

i’m not a structures specialist but the same old square-cube relationship gets in the way. The wing area, lifting capacity, increases as the square and the weight goes up as the cube. So, we need to develop materials having the same strength and lighter weight/volume, as well as all the other desirable characteristics. such as resistance to fatique, weathering, and so on.

4

The reasons why no passenger aircraft larger than the A-380 exist (and why this one is having some difficulties) are overwhelmingly practical rather than theoretical. The basic story is that you not only have to make it big, but also light enough to carry the passengers, baggage and/or frieght that will pay the hideous purchase price, along with enough fuel to get where you’re going.

This isn’t any sort of issue in current designs. It’s safe to say that wings could be much bigger without falling off. It is fair to say sufficiently strong wings and main spars are both more expensive and heavier than they would be in an ideal world.

No inherent limits there. Really large aircraft can have good L/D - both the A-380 and the 747 certainly do (indeed, they must, in order that their fuel consumption be acceptable).

There are, but they aren’t principally realted to fuel economy (which is really quite good for giant aircraft, and indeed is one of the reasons for their existence).

Probably the biggest is simply the staggering cost (and risk) of developing a new design like this. Boeing is said to have “bet the company” on the success of the 747 (they won that bet, big time) and Airbus (EADS) is probably in much the same case with the A-380.

There are some sound real-world reasons why giant aircraft are challenging. The nature of our atmosphere makes something like 35,000 to 39,000’ the right altitude to fly at - you are above most weather and where air density and temperature makes modern jet engines happy. Giant aircraft would like to cruise a bit lower than that, and so need somewhat oversized (thus heavier and more expensive) wings to fly where they must. This makes it challenging to have your giant plane light enough to carry sufficient payload.

There are plenty of other issues. Not least is getting, say, 700 people aboard in a reasonable time (time is money - an A-380 will probably cost over $4000/hour just sitting on the ground). Baggage handling could get interesting. Runways must be strong enough to take the weight.

Another consideration is wake turbulence. When a large plane takes off it creates wingtip vortices that make it unsafe for another plane to follow for a certain time. Recent tests indicate what was expected - that the A-380’s wingtip vortices will be substantially worse than for any other aircraft. It may be necessary to double the amount of time before any aircraft is allowed to take off after an A-380. In response to this, airports have said that they will look at doubling the landing fees - it might cost an extra $30k or so. This would have a nasty impact on the profitability of an A-380, and thus could seriously hurt sales.

5

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

6

Another limiting factor is the FAA’s requirement that in an emergency a plane at maximum capacity must be capable of being completely evacuated in 90 seconds. 33 people were injured when Airbus ran their test on the A380, but they made the time limit and thus the test was still a success. Story here: What makes the airplane evacuation test so dangerous?

7

I’m sure Jack Northrop feels vindicated by your post. :slight_smile:

8

The X/YB-35s and the XB-49 both showed substantial dynamic instabilities in flight, limiting manueverability and airspeed. (The also had a number of design and component problems which contributed to their numerous test failures.) Northrop was right about the benefits of flying wings, but lacked complex automatic control systems required to actively stabilize such aircraft in flight. We have this capability now, but only on expensive military aircraft; developing this for commercial use will require substantial investment and an even higher degree of confidence in the reliability of electronic systems and software.

Stranger

9

Actually, right now all commercial aircraft designs are fatigue tested on full scale aircraft configurations. Here is a description of the Boeing 777 fatigue test set-up. All other commercial aircraft undergo the same sort of testing. It’s probably a result of de Haviland Comets having broken apart in midair. I don’t believe than anyone trusts a computer simulation of fatigue and there isn’t any reason to do so.

My concern about composite structures is that they are all individually made and repeatability is suspect, at least to me. With metal sturctues, the elements are out in the open and visible. With laminates the quality of the lamination is hidden and can’t be detected without fairly sophisticated methods. I think thebreaking off of that Airbus’ tail in Jamaica, Queens is a good example of what I see as the problem. I know that Airbus and the NYSB blame improper use of the rudder control, but I don’t think a metal vertical stabilizer and rudder have ever broken off because of excessive rudder pedal movement.

10

Bah!

60+ years ago, Howard Hughes & Henry Kaiser built the HK-1 (“Spruce Goose”) out of wood(!). And it is half again bigger than the A380, and designed to carry half again as many passengers. This plane was so well-designed aerodynamically that it took off accidentally while being taxied across the harbor.

We can certainly make planes quite a bit bigger than the A-380, or bigger than the HK-1. The limiting factors on size are economic – can the airlines make a profit flying a plane that big?

11

“That’s no moon…it’s a Space Station.”
:smiley:

12

If you consider Ekranoplans (ground effect craft), the answer may be “very big indeed”.

In general, it’s a matter of energy density and economies of scale. Sufficient mass lets you use unconventional power plants (such as nuclear reactors). If you have sufficient thrust, you will have flight. A better question might be, “How big of an aircraft will current economic conditions support?” That question may ultimately lie in just how manufacturing will get distributed around the globe, and how dependent on JIT inventory it will be. If lots of mass must be flung around the world FAST in order for the economy to grow, we may see aircraft (well, ekranoplans probably) that rival superfreighters in mass.

13

The Spruce Goose wasn’t a viable aircraft. It took off once, and stayed in ground effect. And that’s when it was empty. There’s no way it could’ve operated at the capacity it was intended to.

The H-4 was basically just a WIG like Cresend just mentioned… Which is a whole different can of fish than what an airliner should be.

When I say “biggest we can go” I mean have a aircraft with a decent range, payload and ride (and to have a ride that customers will tolerate, you’re looking at 25,000+ feet)

14

Well, this paper is about a nuclear ramjet-powered flyer that weighs 100kg, and has a power density of 2 Mw/L. The engine itself is described here. I’d say that given a multi-regime engine that can use stored reaction mass for slow flight, and ramjet mode for fast flight, you could scale these up considerably. Don’t think ‘wing-and-tube’ construction, think ‘battleship’ construction.

After all, even a brick will fly given enough thrust.

15

Exactly, but that’s all just theoretical at this point. When they have a working prototype of that powerplant, I’ll believe it.

We can still improve the turbine engine. New metals, different designs. We can always improve.
I say “above 25,000’” because of weather and turbulence. A WIG is down on the surface, where wind and temperature can cause turbulence. When you’re in the area most aircraft cruise at, those aren’t as much of as concern.

16

A blended wing structure seems to be a viable direction.

Essentially, a cross between the flying wing and lifting body concepts.

http://www.google.com/search?hl=en&q=blended+wing
As we all know, flying wings are notoriously unstable: indeed, the B-2 stealth bomber would be utterly unflyable without computerized flight controls.

And the lifting body becomes very unstable at landing speeds (opening credits of The Six Million Dollar Man, anyone?)

As each concept adresses the shortcomings of the other, the Blended Body certainly seems the way to go.

17

FWIW I have the same concerns. I also do not like the large windows on the 787 for a related reason.

18

Correction: a blended (wing) body is more of a combination of a traditional wing-and-fuselage design and a flying wing; the idea being to maximize lift while still keeping a centered fuselage and some tail control surfaces. The B1B and the Tu-160 are examples of blended wing body design. Blended wings and flying wings are statically stable in level flight up to the near-transonic region, but don’t transition well into supersonic flight and even if you could would have a large amount of parasitic drag (primarily from wave drag) which would increase power requirements dramatically.

A lifting body is designed to fly only from the lift generated by the body, without anything like what would normally be considered an airfoil. This can be highly efficient in the supersonic/hypersonic range because it reduces the overall size of the shockwave–a rounded brick would be pretty reasonable for this–but fails to generate a sufficient L/D in the subsonic region, meaning that it flies like, well, a brick. The STS Space Shuttle is an example of a lifting body modified with a delta wing to allow it to glide to a landing with a reasonable approach speed and altitude rate; as a compromise, it’s highly problematic–the wings add significant weight and thermal protection requirements while actually limiting the amount of cross range the vehicle has–but certainly flyable even at subsonic speeds, albeit without the kind of agility of a real aircraft. (Early plans had it including a complement of air-breathing jet turbines that would pop out of the fuselage just prior to landing to give it a landing abort mode, but were deleted due to weight and cost concerns.)

Making a plane that acts both like a lifing body and a flying wing is the Holy Grail of winged spacecraft designers; a rocketplane that could boost up to 50k or 60k feet on jet power would be significantly more efficient than one that has to fly on rocket power alone, due to the order of magnitude superiority in specific impulse, but is unachievable without being able to dramatically modify the shape and flying characteristics of the vehicle at will.

As a practical matter–maximizing useable cabin space and passenger comfort and safety–a BWB would probably be the best compromise, and almost as efficient as a flying wing. But it’s a massive change for an industry that is adverse to risk.

Stranger

19

It’s not the size of the airplane that’s the limiting factor – it’s the size of the treadmill!

20

Well, we know for a fact that heavier than an A-380 plane can work: the An-225

Though that is cargo plane (wonder how many people could you cram into one?)

There are also large volume aircraft like the Boeing Dreamlifter - Wikipedia
and the
Airbus Beluga - Wikipedia

But again those are cargo planes, not sure if they could handle all the people you could cram in them.

Brian