I was watching youtube videos of truly massive aircraft. I was wondering, based on ‘back of the envelope’ calculations, how big can aircraft get?
I understand that presently, their size is a matter of economics- whats the point of making a huge flying monstrosity if you don’t have any need to carry something inside it? But still, we built planes that could piggyback the space shuttle, how big can these things get?!
The Boeing 747 is big, but there are bigger planes out there.
The Shuttle Carrier Aircraft are actually just specially modified, but otherwise fairly ordinary, Boeing 747s.
What about that massive aircraft the soviets used to test the Buran? Isn’t that the largest airplane currently?
The Antonov An-225 is currently the world’s largest aircraft. (Though the Hughes H-4 Hercules has a wider wingspan and is taller, but it only flew the one time.) There is only one An-225.
EDIT: The ‘Spruce Goose’ is also shorter and lighter than the Antonov.
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Specs of An-225:
General characteristics
Crew: 6
Payload: 250,000 kg (550,000 lb)
Door dimensions: 440 x 640 cm (14.4 x 21 ft)
Length: 84 m (275.6 ft)
Wingspan: 88.4 m (290 ft 2 in)
Height: 18.1 m (59.3 ft)
Wing area: 905 m2 (9,743.7 ft2))
Aspect ratio: 8.6
Cargo Volume: 1,300 m3 (46,000 cu ft)
Empty weight: 285,000 kg (628,315 lb)
Ok, the length of the An-225 is 275 feet (84m). Now suppose somebody tried to increase that to 150m (almost 500 feet). I would assume that it would become more and more difficult for the craft to structurally support itself while still remaining light enough to fly. Perhaps an engineer can explain the principles behind these diseconomies of scale. Better switch to dirigibles.
If your goal is simply extreme size, yes. But there ain’t never gonna be no dirigible that will carry even a decent fraction of the An-225’s payload.
A case can be made that the size limit is much larger than anything flown to date.
Large aircraft have been built that are conceptually a number of small aircraft joined together. Examples would include the Helios solar-powered aircraft and their predecessors. Following such a scheme, I’d guess that a span of 500’ is possible - but this would be so expensive and impractical that we’re unlikely ever to see it.
Why? The soon to be completed JHL-40, the world’s largest VTOL aircraft, can lift 40 tons or 80,000 lbs. I would think that dirigibles would scale better than aircraft, since volume grows faster than surface area and the skin doesn’t have to support itself. The JHL-40 will be some 300 feet long: double the length and cross section and the capacity should go up by ~8, exceeding the aircraft’s specs. I don’t have an engineering background though: where am I amiss?
Are we counting ekranoplans?
I love them ekranoplans! I heard somewhere that the space shuttle is not particularly heavy compared to other payloads and in fact is situated so that it provides some of its own lift once airborn. It is a glider, after all. I’m sure the rocket scientists here can flesh out the details, but it’s too late for me to bother with teh google.
Yes, I was thinking along those lines. You could probably bolt a couple of Airbus A-380s together wing-to-wing, and expect it to fly if you linked up the control systems. It would be impractical, and very, very dangerous, but could probably be made to work. IIRC, the US air force experimented with the concept of fighters that would attach to the wing-tips of bombers mid-flight, to save fuel. It proved to be impractically dangerous due to wing-tip turbulence.
If you’ve ever looked at the wing of a 747 flying through any kind of turbulence (like this mild turbulence during final), you’d suspect the entire airframe was made of low-durometer silicone rubber; the wings are so flimsy at that scale that harmonic vibrations are visible in addition to the fundamental tip-up/tip-down motion. I suspect that if the scale of an aircraft gets much larger than this, it will be difficult to build an airframe that exhibits satisfactory stiffness. Flexing may be difficult to damp out, and the kinetic energies stored in the flexing may result in overstressing much more easily.
The Helios solar powered aircraft cited upthread has a wingspan considerably longer than that of a 747, and is a good example of how the construction might be approached, i.e. by distributing the payload over the length of the wing instead of in one central fuselage. This avoids having to build a wing that can bear huge bending moments at its center, but that in turn means that the wing will be very elastic (watch the Helios video @1:46), and light turbulence could tear the thing apart (and in fact did, in the case of the Helios). Moreover, a flimsy flippy-floopy aircraft will result in a nauseating ride for its passengers.
Just a word for what was probably the largest military aircraft, although it never dropped a bomb. The B-36. Smaller than the biggest cargo planes, it was pretty impressive. And made an impressive sound, with its six piston engines & four jets…
IANA aero engineer, but I know a bit about it.
Completely ignoring economics and infrastructure compatibility, and just considering the air machine in isolation …
The thing which tore the big dirigibles apart, and the reason we don’t have any now, is that the atmosphere is turbulent, at least in spots. Once the craft gets big enough, you get to the point where the bumps at the front are going one way & the bumps at the rear are going another way. The vehicle needs to be strong enough to withstand that.
Consider a hypothetical 500’ vehicle (airplane or dirigible). Any force appled at nose or tail has a 250 foot lever arm to work with when trying to break the vehicle in half at the midpoint. That’s a lot of leverage.
So it has to be strong. Real strong.
The other challenge is stiffness. An airplane (though not a dirigible), flies because of its shape. If you distort the shape much, it quits flying. So the wings may be able to flex up & down some, but they can’t flex fore and aft much at all, nor twist when they flex. Any twisting is likely to lead to flutter, which is disasterous. Likewise, a fuselage which twisted or flexed much would be altering how the tail plane was oriented to the airstream, leading to control problems. That would be bad.
So it needs to be stiff. Real stiff.
But it also need to be light. Wings still rely on air to hold them up. As long as the atmosphere is the same (i.e. we’re not on Venus where the “air” is much thicker), there’s a hard physics limit to how much lift a square foot of wing can produce.
So overall, the machine needs to be light, stiff, and strong. Those are opposing goals and so a compromise must be made. This is where the engineers earn their pizza.
The evolution of aircraft follows two upward curves over time. The curve of better materials and the curve of better aerodynamics.
The biggest practical wood aircraft was limited by the charateristics of wood. Arguably, the Hughes H4 was beyond that point of practicality. It never got out of ground effect and therefore never really flew by an honest definition of the word. Even if the H4 could have flown, the limitations of wood wouldn’t have permitted something much bigger than it was.
The biggest practical aluminum airplane is limited by the characteristics of aluminum. We have not built the largest possible aluminum airplane, but we’re not that far away. 50% bigger than the AN-225 is plausible. 300% that size is not.
The industry is just now exploring all- or nearly all-composite aircraft. Those materials offer better strength / stiffness / weight values than aluminum. Something 300% the size of an AN-225 sounds plausible, although again I’m not actually an expert nor am I doing the calcs just now to prove it’s possible.
All the above assumes a more or less typical tube-and-wing design. The other way to grow aircraft is with novel designs with better aerodynamics or better structural efficiency. Or both.
The “span loader” is a design whch is essentially all wing with no fuselage. You can readily see that design eliminates the huge stress concentration where the wings join the fuselage. In a typical tube-and-wing design, right there the two long outstretched wings are pulling up & the two long outstretched fuselage ends are pulling down. A lot of stress happens right there. Eliminating that concentration allows for a bigger lighter design with the same materials.
As others have said, with fancy computer-controlled flight controls it becomes at least theoretically possible to design a very wide segmented aircraft where each segment is flying more or less independently and they’re all trying to stay in (very!) close formation. Gust alleviation is still a big problem with these designs They whole thing can only flex so far before it breaks. And the bigger it is, the greater the odds of encountering really different conditions at one end than the other.
Up at very high altitude (75K - 100K ft) there is essentially no wind and therefore no turbulence. *If *you can get from the ground up to there safely, *then *you could safely operate a vehicle with very poor turbulence-tolerance. Things like the Aerovironment machines mentioned above fall into that category. If you only land & take off once every couple months or so, you can afford to wait a day or so if today is windy or stormy. For a machine which lands every few hours, that’s not practical.
So over the next 5-15 years we’ll see some very long wingspan but very light & flimsy aircraft for long duration missions. They will be much bigger than current aircraft in wingspan, but smaller in almost every other measure. And almost certainly much slower as well.
I know wings that twist as they flex. 
Oh, and they have a lead-lag hinge for fore-and-aft movement, too. 
(Completely irrelevant to the discussion at hand, of course.)
I thought of helos & then of you when I wrote that. Always the wise guy 
I can’t help it. It’s in my nature.
The wings of a modern jet liner are specifically designed to flex. This technology was first used in the B-47, and the first passenger plane to use the idea was the 707. These wings are not rigid, and control of the wing does not derive from super stiffness. However these wings do exhibit some potentially nasty issues. The main failure is “Dutch Roll” which is capable of flipping a plane, and has been the cause of a significant number of crashes. Learning to control a flexible wing, especially a swept wing was key to the development of the modern jet liner. One key to the success is mass damping of the wing - which is one reason you see the engines mounted on pylons. The mass of the engine on the end of the pylon is tuned to the wing flex. Next time you are on a 747 watch the manner in which the engines gently nod. That is the wing flexing. Another component is a dynamic yaw damper, which actively stops the plane from getting into an oscillatory yaw motion, which is the prelude to a full Dutch Roll. A huge tail (well vertical stabiliser) is the other big ticket item. That is why the tail fins of modern jets are so huge.
As the jet liners got bigger the issues with managing a flexible wing seem to have become easier. Possibly the scale issues run in the designer’s favour. The relative balance of stiffness, mass, area, and volume don’t scale linearly.
It’s not just the flimsiness. If you build a plane with the passengers distributed across a long wingspan, the people at the ends will experience very sudden and severe up-and-down motions when the plane banks to enter a turn.
