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The B-47 has six underwing GE TG-190 engines, two on each inboard pylon and one on each outboard pylon.

The inboard engines are mounted on a long pylon that extends well below and forward of the wing, such that the inboard-engine tailpipes are (longitudinally) just about even with the wing’s leading edge.

The outboard engines, in contrast, are mounted on much shorter pylons and fit snugly against the underside of the wing, with their tailpipes extending aft of the wing’s trailing edge; they’re placed so far back that their pylons even protrude beyond the trailing edge.

this is what I'm talking about

(Image originally by the United States Department of Defense, via Balcer at Wikimedia Commons; modified by me.)

Why the long pylons and forward mounting for the inboard engines, versus the short pylons and rearward mounting for the outboards?

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enter image description hereImage source

From Wikipedia:

The wing's flexibility was a concern, as it could flex as much as 17.5 ft (5.3 m) at the tip, and major effort was expended to ensure that flight control could be maintained as the wing moved up and down.

And

...with the twin inboard turbojet engines mounted in neat pods, and the outboard engines tacked under the wings short of the wingtips.... This arrangement reduces the bending moment at the wing roots, saving structural weight. The mass of the engines also acts as counter-flutter weights.

The B-47 required six engines of the type with maximum thrust that was available at the time. Four were mounted in twin pods, at about 1/3 span in order to provide decent bending and flutter relief to the thin wing. The remaining one was mounted past the fowler flaps almost at the tip - a cluster of three engines may provide cooling issues for the middle one.

In the photo it can be seen that the thin wing bends down considerably when on ground, and that there is no room for a forward extended pod at this place.

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  • $\begingroup$ I'm not seeing a bendy wing in that photo; I'm seeing a wing with considerable anhedral. $\endgroup$ – Sean Aug 9 at 0:57
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    $\begingroup$ Yes, on the ground. In air the wing tip flexed up considerably, up to 5.3 m. $\endgroup$ – Koyovis Aug 9 at 1:31
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The forward pylon mounting of most jets with wing mounted engines is to use the engines' mass to move the overall center of mass of the wing forward, closer to or ahead of the torsional axis of the wing box. Boeing discovered this helps with aerolasticity related issues with long, flexible, swept wings. A bit like mass balancing a control surface, but on the root structure.

The outermost engines, being so close to the tip where the chord is short, have to be farther aft or they would move the wing's center of mass too far forward in that area.

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    $\begingroup$ Quite so John. The B47 incorporated many new (to the US at least) design & construction elements. It is credited with establishing podded jet engines on pilons for swept wings, although the Germans had first investigated this years earlier. Swept, flexible wings on such a large aircraft was fairly unprecedented, AFAIK. The B47 had very flexible wings, & the outer engines were mounted at a relatively weaker section, particularly with regard to twisting. (early drawings had them mounted on the wing tips) $\endgroup$ – Mackk Aug 8 at 1:14
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    $\begingroup$ And the outboard engines had to be mounted on shorter pylons as the wings drooped a lot on the ground and longer pylons would have risked them dragging on the tarmac. $\endgroup$ – jwenting Aug 8 at 3:27
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    $\begingroup$ Interesting that the combined thrust of the B-47 podded twin was about half of the 737 Max LEAP fan jet. The 2 on the ends added 14,000 lbs more thrust. $\endgroup$ – Robert DiGiovanni Aug 8 at 15:06

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