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I can easily grasp the idea of tanks of fuel inside the fuselage of an aircraft, and the ease of sealing such tanks in a relatively standard shape. However, I'm having a hard time understanding how wing tanks could be fit around the internal structure of a wing, as I envision the inside of a wing (especially nearer to the fuselage) to be a honeycomb of structural pieces that would be very difficult to seal around or find any large volumes of contiguous space for fitting tanks. Is the wing structure itself the fuel tank, or are there separate fuel tank bladders that are somehow fit into the wing voids?

For my question, I am not concerned about military fuel tanks which might have special materials in the walls which could seal off a leak due to combat damage, but rather just non-military aircraft.

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    $\begingroup$ youtube.com/watch?v=giEMzfmQoqs $\endgroup$
    – mins
    Nov 9, 2017 at 22:57
  • $\begingroup$ The gist is that there isn't a tank in the wing, the wing itself is the tank. All the internal structure is simply immersed in the fuel. $\endgroup$
    – Jan Hudec
    Nov 10, 2017 at 9:32

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The tanks are generally sealed with some kind of putty/resin/epoxy like material. Keep in mind that only the joints between exterior panels need to be sealed. On many aircraft this is done through inspection panels. The material is, in a manner of speaking painted/applied.

Here is an example from a Mooney, which are pretty infamous for having in wing tanks

enter image description here (source)

Tanks on larger aircraft are not all that different, much like this one off a 747

enter image description here (source)

its worth noting that tanks don't need to be continuously open, they can have structures with in them as long as the fuel is free to flow around. Large and small tanks alike may very well have structural elements inside with flow holes drilled to allow fuel to move around. These elements also help reduce the effects of large quantities of fuel sloshing around as the aircraft changes attitude.

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  • $\begingroup$ Indeed, continuously open tanks are often bad -- the fuel sloshes around, which can be a bad thing for a vehicle that's so sensitive to the position of its centre of gravity. I believe road tanker trailers usually have baffles installed to prevent partial loads surging forwards under braking. $\endgroup$
    – David Richerby
    Nov 10, 2017 at 11:46
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The wing of large airliners gets its strength ffrom the wing box structure: the skin on top and bottom of the wing take compressiona nd tension loads, two spars mainly keep them apart. Stringers are attached to the skin to keep them in shape.

enter image description here

That means that the wing has a hollow inside, which can be used as a fuel tank. The wing box is the tank. It is sealed at the edges by resin; some anti-slosh structures are put in place as required.

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Composite aircraft also have integral tanks. Even gliders have them, only for water ballast. A composite fabric will never be completely tight, so the tank walls need a waterproof coating. This also helps to reduce water absorption by the epoxy matrix - wet epoxy has lower strength.

Below is a photo from the wing assembly of the composite flying wing SB-13. This is the moment when the mold with the lower left wing shell is placed on the opposite half with the upper left wing shell. The light grey area is the water tank coating, and it contains two ribs mounted in lateral direction to reduce water sloshing. In a flying wing with its limited pitch control, longitudinal sloshing is quite undesirable.

The coating is simple swimming pool paint. After the wing was bonded together, more paint was poured into the tank and the wing was moved around to ensure that the sloshing paint covers all remaining surfaces, especially those that had to be left unpainted to ensure proper bonding.

SB-13 left wing assembly

Note the pushrods: There are four (two for the elevons, one for the winglet rudder, one for the speed brakes), three of which enter the wing at the root ahead of the spar. Rocker levers transfer the motion to the outer wing pushrods which are behind the spar. The spar itself has three degrees less sweep than the wing to improve aeroelastic damping.

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The existing wing structure of front and rear spars and upper and lower wing skins act as an integral wing fuel tank. When the wing is assembled sealant is applied between all parts to make the wing water-tight.

While fuselages are primed with green primer paint, integral wing tanks are primed with special yellow primer which is made to be a barrier against the fuel.
An airliner wing will have a "dry bay" just over the engine for safety in the event the engine were to explode and throw turbine blades into it. On a 777 the Dry bay area is blocked out by a composite piece that is secured and sealed in place. Each wing will usually have one dividing wall in it in the vicinity of the engine pylon (everything inboard from that point is considered the center wing tank, including the center wing section/tank).

Airliners with 4 engines will have each wing divided into at least 2 sections (one to feed each engine).

Perfectly sealing the inside of the wing is very important. There are lots of QA inspections and a final test.

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