Large aircraft

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 remain, as of 2016, the largest aircraft ever constructed, while the Hughes H-4 "Spruce Goose" of 1947 had the largest wing span of any fixed-wing type. The Hybrid Air Vehicles Airlander 10 hybrid airship is the largest aircraft flying today.

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.

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.

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Wikipedia