Large aircraft allow the transportation of large and/or heavy payloads over long distances. Making an aircraft design larger can also improve the overall fuel efficiency and man-hours for transporting a given load, while a greater space is available for transporting lightweight cargoes or giving passengers room to move around. However, as aircraft increase in size they pose significant design issues not present in smaller types. These include structural efficiency, flight control response and sufficient power in a reliable and cost-effective installation. Large aircraft also require specialised ground facilities, and some countries have special regulatory environments for them. The giant airships of the 1930s are still some of the largest aircraft ever constructed, while the Hughes H-4 "Spruce Goose" of 1947 had the largest wingspan of any fixed-wing type (it's still the largest seaplane ever built, and it had the largest wingspan of any aircraft ever flown until the twin-fuselaged Scaled Composites Stratolaunch first flew on April 13, 2019, with by far largest wingspan, still widest; larger than significantly larger than an American football field). The Hybrid Air Vehicles Airlander 10 hybrid airship is the largest aircraft flying today (by volume, its wingspan isn't largest; the Caspian Sea Monster is still longest, was once largest and heaviest, the even heavier Antonov An-225 Mriya was heaviest until its destruction in the 2022 Russian invasion of Ukraine).
Characteristics
Payload space The lifting capacity of an aircraft depends on the wing size and its "loading", the weight per unit area that the wing can support. Loading is more or less constant for a given level of technology. Thus, as aircraft size increases the lifting capacity increases with the surface area. For a given aerodynamic form, the area in turn increases with the square of the wing span. If structural efficiency can be maintained, the structural weight of the airframe also increases with its surface area and the square of the span. But the internal volume increases with the cube of the span. For example, if the dimensions are all doubled in size, then the area and lifting capacity increase 2 × 2 = 4 times, while the volume increases 2 × 2 × 2 = 8 times. For a passenger aircraft, this doubling in size allows up to twice the cabin space per passenger. Alternatively, for a transport it allows up to twice the space to fit in bulky but light cargo. Thus, large aircraft are both more comfortable and operationally flexible in use than smaller types.
Structure Although a larger wing carries larger forces, it is also thicker. The main spar in the wing approximates an I-beam, whose depth equals the wing thickness. For a given overall load to be carried, the forces in the beam decrease with the square of its depth. If a wing is doubled in span it is also doubled in thickness. This reduces the forces in the spar by a factor of 2 x 2 = 4, allowing a fourfold increase in the overall load. This exactly matches the increased lift available from the larger wing area. This means that the metal parts of a large aircraft need be no thicker or heavier than those of a smaller aircraft. However, because these parts must cover four times the area they make the aircraft four times heavier. This again exactly matches the increase in laden weight, so there is no structural limit to how large (or small) an aeroplane can be made. Large aircraft do still pose a design challenge. The structural members may be no thicker, but they are now twice as long, so stiffness becomes a problem, and the design approach must be adapted to ensure adequate overall stiffness. This is typically achieved by making structural members cellular. For example, the wing spar in a small aircraft may in fact be a simple I-beam with a solid cross-section, but in a larger design the upright part of the beam or "web" will be constructed as an open lattice of trusses in a triangulated structure.
Flight control The effectiveness of a flight control such as an aileron depends mainly on its area and its distance from the centre of the aircraft - its lever arm. If the wingspan is doubled, the area increases fourfold and the lever arm doubles, making the aileron 8 times more effective. With the aircraft being also four times heavier, and with the weight on average twice as far out, it requires 8 times the effort to achieve the same acceleration of the wing tip. These balance out, so on a large aircraft the equivalent aileron will accelerate the wing tip up or down at the same speed as a smaller aircraft. But on a wing twice the span, the tip must travel twice as far to achieve the same change in aircraft attitude. This takes longer, so a large aircraft manoeuvres more slowly than the equivalent smaller aircraft. On very large types such as the Airbus A380, conventional ailerons alone are not enough, and additional lift spoilers are used to reduce the lift of the downward-tipping wing and increase the roll rate to a practical and safe level. Similar issues occur with the elevator and pitch control. Without extra design measures to ensure adequate control response, any attempt to make a last-minute correction to the flight path is likely to prove too little too late, making a last-minute landing abort and fly-around difficult and dangerous.
Engines The number of engines on an aircraft affects its reliability and safety. The more engines there are, the safer it is if one engine fails. But on the other hand, the more engines there are, the more likely there is to be a failure of one or more and the greater the workload on the flight engineer. Nowadays, two engines are preferred in practice, with even quite large wide-body aircraft having only two engines. Four is generally accepted as the limit, for both safety and cost reasons. Barring a few military types, no practical large aircraft has ever had more than four engines. As aircraft get bigger, it therefore becomes necessary to design bigger engines. The airspeed of a fan blade must be kept below the speed of sound in order to avoid damaging and noisy shock waves. This maximum speed of the tip sets a limit on the rate of rotation. For a given rate of rotation, the tip of a larger fan will travel faster. So to keep down the top speed of a large engine, the fan must spin more slowly. The fan is driven by a turbine off the same shaft, so the turbine blades also spin round more slowly.
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