How are fuel tanks fixed inside aeroplane wings, allowing the wing to flex in flight, without flexing the tank

Question

If the fuel tanks are load bearing members of the wing, I can see how it's done, but if the tank is just a "passenger" within the wing, I can't see how it's done. ( I have only flown a few times, so I am totally in awe of how these big aeroplanes work)

Answer 1

In many larger airplanes, the fuel tanks are "wet wings" where the skin of the wing and the structural ribs and stringers inside the wing are all part of the fuel tank. The joints and seams are sealed with a flexible sealant from the inside of the tank. There are manholes on the bottom of the wing that allow maintenance personnel to actually crawl up inside the wing spaces to reseal leaking fuel tanks after the tank has been drained. (You can't be claustrophobic if you want to be an aircraft mechanic!)

Some older airplanes DO use rubber fuel bladders inside the wing spaces, or even individual metal or plastic fuel tanks, but modern designers prefer a wet wing because it is lighter and easier to build and maintain.

Answer 2

Most fuel tanks in larger airplanes are integrated (wet wings). They use a flexible fuel-resistant rubber sealant called PRC to seal all the joints between the metal to make a fuel tank.

While the wings can flex an enormous amount, the amount of flex in an area say, a foot long, is almost imperceptible.

These tanks almost universally develop leaks (sometimes right from the manufacturer) so some poor sap has to crawl in with a sealant gun and fix them.

Non integrated solid fuel tanks are usually mounted with some shock-absorbing material (like cork or rubber) which will allow the wing to flex without disturbing the tank too much (and also prevents chaffing damage to the tank, for the most part).

Then there are also rubber bladders that are held in place with lacing or snaps or something similar. These are free to flex with the wing.

Edit: Oh, how nice. Someone gave me a thumbs down but had nothing better to say.

Answer 3

I've built planes for the last 28 years.
MD-80/90/95 (AKA 717) DC-10 and C-17's do not have a separate tank bolted into the wing, but rather the empty area in the wing is a "wet wing", creating the fuel area.
Some MD-80's had an Aux (auxiliary) fuel tank in the aft and center cargo holds, which WERE separate tanks, bladder lined that bolted into the cargo.

Answer 4

This is a good engineering question, and Im glad to see that you can realize that this is a problem that needs to be addresed.

Some planes have rubber bladders as fuel tanks, others the tank is designed as part of the wing. But the wing needs to be able to flex up and down and so then does the fuel tank.

Planes that go very fast like the SR-71 have another problem.
They go so fast that they heat up and heat causes metal to expand. So they design the plane with gaps so the metal can move and expand when in flight. Without this design feature the plane would just worp out of shape once it had reached its operating speed.
The SR-71 is known to leak alot of fuel when it on the ground because of this.