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Inflatable structures in boats have proved extremely successful and versatile since they began to find widespread use in the 1930s.

I'm aware that experiments have been made with inflatable wings - very light, delicate structures mainly on human-powered and unmanned aerial vehicles.

On boats, they're used differently: in some cases they form the main structure of the craft itself, in some, the craft has a rigid hull and the rest of the structure is inflatable, but either way, the inflatable sections are very far from delicate.

Experience gained with multiple air cells in inflatable engineering means that it's now possible to build effectively rigid structures, that are extremely tough, reliable and resilient. Their behaviour in failure modes is well-known.

Many large aircraft have built-in air compressors - the engines - that could keep the inflatable structures correctly pressured at all stages of a flight.

Materials science has produced enormous improvements in the longevity and resistance to chemicals/sunlight/temperature ranges of the rubber skins of inflatable structures.

What opportunities are there for designing parts of an aircraft as inflatable structures (perhaps winglets, or undercarriage doors)?

Is there any research or experimentation into more heavy-duty use of inflatable engineering in aircraft?

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    $\begingroup$ If you're inflating the structure with air what would be gained from doing so? Inflatables in water craft are desirable due to buoyancy. That would not apply in aircraft. $\endgroup$ – TomMcW Jun 19 '16 at 0:44
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    $\begingroup$ @TomMcW Compact storage? $\endgroup$ – Steve Jun 19 '16 at 7:46
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    $\begingroup$ @TomMcW Light weight would be the main one. Inflatable structures also have different modes of extension and collapse, compared with rigid ones, that might be potentially useful. $\endgroup$ – Daniele Procida Jun 19 '16 at 9:28
  • $\begingroup$ Wings are usually inflated with liquid fuel, and the fuselage must be pressurized at a level comfortable for occupants, so I'm not understanding what part of the structure might benefit from being inflatable. $\endgroup$ – Michael Hall Feb 19 at 22:26
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We now have experimental use of inflatables in space as well as for boats.

If an aircraft's engines are responsible for the inflation of any aerodynamic or structural part of the aircraft, however, one would have to answer what happens if the engines stop. A conventional aircraft whose engines fail at a sufficiently high altitude can glide for some time, giving the crew a chance to restart the engines; it may even be landed without engine power if necessary. This is a very bad time for the aircraft to lose aerodynamic performance or structural strength--for example, for the winglets to deflate just when the crew need the best possible glide slope.

Unlike boats or spacecraft, moreover, most conventional aircraft are intended to descend from altitudes of low pressure to altitudes of high pressure. If the amount of gas in an inflatable structure is held constant, one has to balance structural rigidity at low altitudes against the stress on the fabric from the much greater pressure difference at high altitudes. If the amount is not constant, it has to be replenished during descent. The engines cannot be relied on to do this, since the reason for descent may be engine failure. Gas could be supplied from high-pressure cylinders, but those can be heavy.

For an aircraft not intended to climb very high, these concerns are less severe.

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It has been done: https://en.wikipedia.org/wiki/Goodyear_Inflatoplane It was intended as something that could be dropped to pilots behind enemy lines for escape purposes. Never got popular as if the pressure dropped the structure failed.

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  • $\begingroup$ Not very practical as an airplane, but lack of space in fighters was the bigger issue than damage suceptibility. The bundle was fairly bulky and heavy. $\endgroup$ – Zeiss Ikon Feb 19 at 17:42
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A paraglider (or powered parachute or powered paraglider) is an example of a popular style of inflatable aircraft.

Their behavior in failure modes is well-known-- in the paraglider case, many pilots have had the pleasure of experiencing it first-hand.

(Ok, I see you specifically said LARGE aircraft design. Sorry for the side-track... )

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In a way, aeroplane structures have pioneered using pressure differential for structural benefit. The fuselage of an airliner is under pressure and functions as a pressure vessel. In flight, the fuselage is supported by the wings - nose and tail want to sag down from gravity forces, creating tension in the upper section and compression in the lower section.

Compression stresses require reinforcements against buckling, and structures can be constructed lighter if compression forces are reduced. And this is what the internal air pressure does: it pre-loads the fuselage structure under universal tension stress. Compression stresses at the lower fuselage segment are considerably reduced, or replaced by a reduction in tension forces.

Inflatable structures are beneficial because of the elimination of buckling: flimsy plastic that buckles in all directions will obtain universal stiffness when under tension stress from the inflation pressure, and this is where potential weight gains could be obtained. But without the air pressure the structure would collapse, and this is where the concept might not be very suitable for aeronautical purposes. Safety.

The overpressure in an airliner fuselage is an essential design demand for the passengers to be able to survive the conditions at cruise altitude in reasonable comfort. But introducing a potential failure point only to save weight is a different matter - I'm not saying it could not be done, but it would require a large effort in safety analysis.

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