What are the pods with the sharp trailing edges underneath the wings of large airliners, as shown in the image below?
My best guess would be fuel tanks
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$\begingroup$ 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). $\endgroup$– slebetmanAug 1, 2014 at 3:39
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$\begingroup$ Thanks @slebrtman! I knew that the main tanks were in the wings though, thought it might be extra tanks (-: $\endgroup$– JonnyAug 1, 2014 at 14:54
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$\begingroup$ They are called anti-shock bodies and they let the flaps go down and out at the same time. Inside them there are many levers and arms that let the flap do this. $\endgroup$– Mary ScottOct 20, 2017 at 15:38
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3 Answers
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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):
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):
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.
answered Jul 31, 2014 at 12:33
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$\begingroup$ The flap mechanism of the ATR is also much simpler and would not benefit from long fairings. The flaps rotate around a hinge point located far below the wing whereas the Fowler-type flaps move back along rails which extend beyond the training edge when the flap is retracted. Due to the lower speed, the flaps of turboprops need not to be as complex as those on jets. $\endgroup$ Nov 2, 2021 at 7:27
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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.
answered Jul 31, 2014 at 8:24
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2$\begingroup$ They are flap track fairings, but the anti-shock body function is their primary function. $\endgroup$ Jul 31, 2014 at 12:23
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3$\begingroup$ Are there any non popular science references to the anti-shock function of the actuator fairings? $\endgroup$– user7241Feb 23, 2015 at 16:46
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There is a lot of misinformation here, and the top answer tries to combine concepts from two different applications. These 'pods' are flap track fairings, sometimes called trailing edge fairings, and their purpose is to cover and reduce drag over the heavy duty mechanisms that extend the flaps of an aircraft wing during high-lift operating regimes. See https://en.m.wikipedia.org/wiki/Aircraft_fairing
These pods are not anti-shock bodies, despite nearly all answers trying to use it as a buzzword. Anti-shock bodies are attached to the upper surface of the trailing edge (unlike flap track fairings, which are attached to the underside). Anti-shock bodies were used to reduce the shock that occurrs over the wing while traveling at transonic speeds. They were made obsolete with the advent of supercritical airfoils, and no longer found on modern aircrafts.
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1$\begingroup$ Welcome to the site, AJ Zhang! They are indeed fairings but made larger than necessary in order to smoothen the cross section area distribution over length. So they indeed serve both purposes equally. Supercritical airfoils need them as much as the old-fashioned sort did. $\endgroup$ Nov 2, 2021 at 7:22
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$\begingroup$ But are they formed such to affect the area rule of the whole aircraft as the current "winning" answer suggests @PeterKämpf $\endgroup$– Jpe61Nov 2, 2021 at 8:26
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$\begingroup$ @Jpe61 … and at transsonic speed their Y and Z location is less important - what counts is the distribution of the total cross section over length. No "but" necessary. $\endgroup$ Nov 2, 2021 at 8:44
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$\begingroup$ Is there a definitive source that states they are equally performing the function of area ruling? @peterkämpf saying so does sorta list as a definitive source, but other than that, with all due respect? I find it odd that to offset the adverse effect of engine pods, these fairings would be necessary tens (?) of meters away from the engines. $\endgroup$– Jpe61Nov 2, 2021 at 16:51
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1$\begingroup$ @Jpe61 In this answer I use a picture from VC-10 development showing their effect at Mach 0.9. Unfortunately, the dumbing down of NASA pages has deleted the original sources (like oea.larc.nasa.gov/PAIS/Concept2Reality/area_rule.html) where this picture appeared online first. And Küchemann bodies do not "offset the effect of engine pods" but offset the sudden drop in wing thickness due to rear loading. $\endgroup$ Nov 2, 2021 at 18:46