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What are the pods with the sharp trailing edges underneath the wings of large airliners, as shown in the image below? enter image description here

My best guess would be fuel tanks

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FWIW, most airplanes use the entire wing as fuel tanks. The inside of the wings. They look deceptively like there's not much space in there but wings are one of the largest structures on an aircraft and they're actually very thick when designed for good aerodynamic efficiency and are therefore the largest bit of unused space. The plane in that picture specifically has fuel tanks in its wings (and maybe horizontal stabilizer/tail). –  slebetman Aug 1 at 3:39
    
Thanks @slebrtman! I knew that the main tanks were in the wings though, thought it might be extra tanks (-: –  Jonny Aug 1 at 14:54

2 Answers 2

up vote 39 down vote accepted

They are Anti-shock bodies.

In the transsonic speed range (above about Mach 0.7), aircraft drag is governed by Whitcomb area rule, which basically says that to minimize drag, the aircraft cross-sectional area must change as smoothly as possible, independent of its actual shape. It is somewhat counter-intuitive, but well established.

Compare this Junkers patent drawing (via Wikipedia):

Junkers patent drawing

which shows various combinations of positioning engines to adopt the area rule.

For the usual aircraft design the cross-section increases over the engines that hang ahead of the wing leading edge and then the thick part of the wing and wing box (the thicker part of fuselage where wings connect), but the wing tips are too thin and the cross-sectional area would decrease too quickly, so something needs to be attached to the trailing edge to make the reduction of cross-sectional area smoother. The anti-shock bodies are usually conveniently combined with flap actuator and track fairings.

There are however some aircraft that have anti-shock bodies combined with other functions, e.g. Tu-134 (and many other designs by Tupolev Design Bureau) retracts landing gear in its anti-shock bodies (which are just one large on each side):

Tu-134

The flap actuator and track fairings exist on all aircraft that have flaps that extend aft as well as down (Fowler flaps and their double-slotted variants), but on slower planes they are much thinner. Compare e.g. ATR-72, which does not need to be area-ruled with its maximum Mach 0.55.

ATR-72

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They are not fuel tanks, the fuel tanks are inside the wings.

Those are covers for the flaps actuators: since the actuators and their rails protrude beneath the wing, without a cover they would greatly increase drag. The covers are more aerodynamic and thus produce overall less drag (at the expense of a slight increase in overall weight)

See Anti-shock bodies for more information and pictures.

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They are flap track fairings, but the anti-shock body function is their primary function. –  Jan Hudec Jul 31 at 12:23

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