How big can an airplane get?
December 11, 2008 8:55 AM   Subscribe

What's the largest size that an airplane could theoretically be and still fly? Let's assume real-world materials that exist currently (ie, no carbon nanotubes or anything like that). I'm interested in the 'why' as much as I'm interested in the actual answer.
posted by empath to Travel & Transportation (19 answers total) 4 users marked this as a favorite
I'm not sure of the answer to the big part of your question. As to the "why," as the plane gets bigger, it takes more and more energy to get it past its stall speed (the speed at which the wings start/stop generating enough lift to pick the plane off of the ground). The issue becomes how efficient our engines are, pound-for-pound.
posted by craven_morhead at 9:01 AM on December 11, 2008

Are we talking carrying capacity here (people or cargo or bombs or what?) or are you just going for sheer size, with one pilot and nothing else? Cause I imagine the largest plane you could make with the latter would be out of aluminum and some big-arse engines, and would be about as big as you want, barring it being so wide that its wing tips reach out into space. Though I suppose you could curve the plane around the curvature of the earth... I guess it comes down to what craven_morhead said about weight vs energy to create enough lift...
posted by Grither at 9:05 AM on December 11, 2008 [1 favorite]

I'm going to go with the answer that there is no maximum theoretical size, just as there is no maximum for ocean-going ships (???) or land vehicles.
posted by troy at 9:06 AM on December 11, 2008

Imagine a simple flying wing design, like the Pathfinder.

As far as power goes, you could keep adding wing sections as long as you like.

But make it too big, and the plane will either fall apart under its own weight or tear itself to bits in mildly bumpy weather.

So I imagine there's a limit with modern materials, but it's probably a very large limit. Like, wingspans into the kilometers or tens of kilometers.
posted by ROU_Xenophobe at 9:24 AM on December 11, 2008

We're assuming the plane is not hollow, right? Because I think you could make a really big plane that is essentially a giant winged zeppelin, but if the inside were instead filled with decks of passengers, that same plane would be mighty heavy for the size of its wings.

I'm thinking along the lines of The Biology of B-Movie Monsters:
The conceptual foundations of scaling relationships lie in geometry. Take any object--a sphere, a cube, a humanoid shape. Such an object will have a number of geometric properties of which length, area, and volume are of the most immediate relevance. All areas (surface area, cross-sectional area, etc.) will be proportional to some measure of length squared (i.e., length times length); volumes will be proportional to length cubed (length times length times length). Equivalently, lengths are proportional to the square root of an area or the cube root of a volume.

In each example, linear dimensions double, but area increases by four times.
If you change the size of this object but keep its shape (i.e., relative linear proportions) constant, something curious happens. Let's say that you increase the length by a factor of two. Areas are proportional to length squared, but the new length is twice the old, so the new area is proportional to the square of twice the old length: i.e., the new area is not twice the old area, but four times the old area (2L x 2L).

Similarly, volumes are proportional to length cubed, so the new volume is not twice the old, but two cubed or eight times the old volume (2L x 2L x 2L). As "size" changes, volumes change faster than areas, and areas change faster than linear dimensions. [...]
So my guess is that a plane can't get infinitely larger unless the structural materials and engines become infinitely stronger and lighter.

I have no idea what the approximate maximum size is given current materials, but I'm sure that you can't just build them bigger and bigger out of these materials.
posted by pracowity at 9:29 AM on December 11, 2008

Does it have to run on jet fuel? Presumably, you could add a nuclear engine or rockets, which I believe provide more thrust in less space, but pose bigger engineering and cost issues for a plane you intend to fly more than once.
posted by mccarty.tim at 10:09 AM on December 11, 2008

No fundamental limit that I can think of that's not awfully big. Think of a huge number of airplanes flying in formation, maintaining a fixed distance of separation. Then imagine connecting them.

The main thing is that a wing is doesn't have to be one long unsupported span. The lift/gravity forces and the thrust/drag forces can all be distributed and balanced along an arbitrary length (i.e. each portion of the wing has its own lift, and you can put engines all along the wing).

So I think if there's a limit, it's very very big unless you impose some other restriction on the design.
posted by madmethods at 10:24 AM on December 11, 2008

Well thanks to yesterday's fpp I found an article in a 1972 Popular Science magazine which described the biggest plane I've ever heard of. Check it out...

What Has 56 Wheels and Flies? The World's Largest Aircraft Of course, I don't believe that the plane was ever constructed as Google turns up nothing for the "Brute Lifter."

It's 985,000 pounds empty, but capable of handling a payload of up to 2.3 million pounds, or 8100 barrels of oil. And apparently longer than a football field between the endzones, so that's longer than 300 feet.
posted by trueluk at 10:25 AM on December 11, 2008

We're assuming the plane is not hollow, right?

Why not? Airplanes are already hollow. Think of how much empty space there is inside the fuselage to carry passengers and cargo. Fuel tanks are essentially big empty gaps in the wings. Cram it full of junk and there's still a surprising amount of empty space. If you want to get really nitty-gritty, airplane skin is generally made of aluminum honeycomb (self link) - see all that air there?

Anyway, that's sort of immaterial to the question at hand. Without thinking about it too much, we're probably limited by two major things right now: thrust and logistics. Actually, I'll throw fuel efficiency in there, too. Obviously, it takes more energy to move more weight, so you need bigger engines. Bigger engines burn more fuel, so what you'll start doing is carrying exponentially more fuel until all you've got is an aircraft full of fuel and nowhere to store anything.

I'm speaking in terms of weight simply because that's really the limiting factor on aircraft. You could make a wing as long as you want, but the beams necessary to cantilever it without it snapping off in a stiff breeze would become increasingly larger and therefore heavier.

So, more weight means more lift necessary, and lift always induces drag. The two major components of drag are parasite drag (basically the body itself forcing air to stop or slow down) and induced drag, which is a function of lift. Doing some further math, you basically get drag as a function of not only lift but airspeed. So you need engines big enough to keep the airplane going fast enough that the wings can maintain lift.

Logistics will be your second hurdle to developing a *usable* large aircraft. The A380 had these problems early on - it's so large that many airports cannot accept it. Either the pavement is not sturdy enough to handle the weight, or the runways/taxiways aren't wide enough. Plus, to get all those people on and off in a timely fashion, you need multiple jetways for each aircraft. But you quickly run into a logistical limit where, even ignoring those issues, it simply takes too long to load and unload the airplane to make it cost effective. You make money by flying the airplane; if it takes you three hours to load passengers, it's not going to make much money. If all you're doing is going for size, though, this matters a whole lot less. Still, where are you going to land it? Probably in the water.

So, to answer your question - I don't have an answer. I haven't had to think about this for awhile, so I'm a little rusty, but I could probably do some back-of-the-napkin stuff when I get home if you're really that interested.
posted by backseatpilot at 10:29 AM on December 11, 2008

For the record, here's the largest aircraft ever built, and the one with the longest wingspan.

Though backseatpilot makes some good points regarding the largest *usable* aircraft, it's a bit of an aside from the main question. Lots of large airplane designs could be usable, assuming we retooled our ground-based infrastructure to handle them.
posted by craven_morhead at 10:48 AM on December 11, 2008

backseatpilot mentioned the A380, and I looked it up. The wikipedia main image is pretty sweet. Certainly a large vehicle.
posted by milqman at 10:51 AM on December 11, 2008

The technical term we use in the industry is "really fucking big".
posted by backseatpilot at 10:54 AM on December 11, 2008 [1 favorite]

(i.e. each portion of the wing has its own lift, and you can put engines all along the wing)

Yeahbut, it looks like something more complicated than that is going on, since if you look at photos of large-winged planes in flight, their wings turn up towards the tips instead of remaining in their flightline posture. Make that wing 20 km long, and who knows how big those forces would get?

As well, the wing has to be strong enough to support its own span while on the ground when it's not receiving lift.
posted by ROU_Xenophobe at 11:42 AM on December 11, 2008

There are two limits to the size of an aircraft. The first is one the OP touches on in terms of material limits. We discussed the part of the aircraft where the wings join the fuselage. It is the focus of a lot of design because it is highly stressed and also an area of aerodynamic interest. It is easy to build things that are strong and it is easy to build things that are aerodynamically efficient but it is very challenging to build strong aerodynamically efficient structures subject to the other constraints of an aircraft.

The second is the limit on size is that of profitability. This has been the subject of much speculation with respect to the 787 versus A380. Each presumes a different model of air travel.

This probably isn't as quantitative as the OP wants, but this "limit on profitability" happens in sports as well. Many coaches prefer 2 stars to 1 superstar because an injured superstar does less for the team than 1 injured and 1 healthy star. Similarly, an out-of-commission super-carrier aircraft is more difficult to manage than one out-of-commission pond-hopper.

There are engineering challenges associated with large systems. I would estimate that the complexity and difficulty of a tasks scales with the size (i.e. the physical size) of the project and that the dependence is nearly cubic. For example, the toilet designers in the A380 had to make sure that the pumps and waste tank located at one end of the aircraft could generate sufficient suction to evacuate waste from the other end of the aircraft.

Things can always be made bigger, but what force drives that? If there is profitability (either because it's easy to scale the item in question up, or because the need justifies the cost) then things will get bigger.

I would say the industry is governed by the second limit. Who wants a 1km long aircraft when there are no airports around to handle a beast of that size. You also have to find people to populate a plane of that size, passengers who all want travel along the same route.
posted by KevCed at 12:27 PM on December 11, 2008

As big as you wanted. It seems like the other posters are assuming fuselage and two wings.

The limit could be on how much air there is on the earth, before it gets so large to be called a spacecraft.
posted by wongcorgi at 2:15 PM on December 11, 2008

Response by poster: So what we're saying here is that if someone wanted to build an airplane the size of a cruise ship, there would be no theoretical reason it wouldn't be possible?
posted by empath at 2:48 PM on December 11, 2008

Yeahbut, it looks like something more complicated than that is going on, since if you look at photos of large-winged planes in flight, their wings turn up towards the tips instead of remaining in their flightline posture. Make that wing 20 km long, and who knows how big those forces would get?

As well, the wing has to be strong enough to support its own span while on the ground when it's not receiving lift.

Ah, but you're assuming one monolithic pair of wings supporting one monolithic central fuselage. That's the path large aircraft have generally taken for a variety of reasons (not least of which is the need to fit the wheels on a runway that may be narrower than the wingspan), but it's certainly not an unavoidable design choice.

On the other hand, if you distributed the mass this way in a giant plane, you couldn't land on a traditional runway... perhaps on water, or on several parallel runways.
posted by musicinmybrain at 6:15 PM on December 11, 2008

Ah, but you're assuming one monolithic pair of wings supporting one monolithic central fuselage.

I'm actually thinking of Pathfinder, which is just a wing with some tiny pods on the bottom.
posted by ROU_Xenophobe at 6:27 PM on December 11, 2008

Good point... I guess under realistic atmospheric conditions the really-long-wing topology has its limitations after all.
posted by musicinmybrain at 6:33 PM on December 11, 2008

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