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This question already has an answer here:

Some of the supersonic fighter jets (such as F-4 Phantom and F-5) have narrow waist on their fuselage.

for F-4

and for F-5

Why do these fighter jets have narrow waist?

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marked as duplicate by Simon, Farhan, fooot, CGCampbell, vasin1987 Jan 25 '15 at 19:09

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It is likely due to application of the Area rule, which helps to reduce drag due to shock waves in transonic and supersonic flight.

It basically is an attempt to keep the cross-sectional area of the craft from varying rapidly as you move along the length. So at a point where the wings area is increasing a lot, the area of the fuselage may be reduced to compensate.

This reduction of area is mainly intended when these airplanes fly at transonic speeds (0.8-1.2) but not when flying at supersonice speeds where a front shock wave is created.

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  • $\begingroup$ I was not aware of the area rule for transoceanic fly. Thanks for detailed answer. $\endgroup$ – albin Sep 28 '18 at 19:10

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