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In general, a vertical stabilizer connected with the fuselage will form junctions. The junctions trigger a mix of airstreams, and causes interference drag. A fairing is used the ease the sharp junction angle and reduce this type of drag. Like between the junctions of the aircraft's main wings and the fuselage.

On an A320, fairings are also installed between the junctions of the fuselage and the vertical stabilizer. enter image description here

However, on an B787 aircraft, it is not easy to discover the fairing installed for the vertical stabilizer. Does the 787 really have this fairing? If not, what stops the manufacturer from adding this component, and reducing drag? Does the 787 have this fairing? Image source:https://www.alamy.com/stock-photo/boeing-787.html?sortBy=relevant

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  • $\begingroup$ It does not necessarily reduce drag. One fuselage design goes through CFD and wind tunnel testing, and reveals a pressure delta that justifies a fairing. Another slightly different fuselage design also goes through CFD and wind tunnel testing, which reveals there isn't a need. In complex design, the answer to "why didn't they..." is almost always because "it turned out in this case they didn't need to," and very rarely "because the wanted to but couldn't." $\endgroup$
    – Max R
    Jan 18, 2023 at 20:55

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Fairings generally aren't needed if two surfaces meet at a 90 degree angle or more, because the interference drag from the two flow fields getting in each other's way is not significant unless the angle is under 90 degrees. This is why you won't see root fairings on WW2 fighters with 90 degree or greater main wing to fuselage intersection angles, like the F-4U Corsair.

Looking carefully at the Airbus in the picture, it looks as if the fairing contours at the base of the fin are to enclose a structure or components at the base of the fin that are slightly wider than the airfoil contour at the base. Like fin attachment lugs.

The fairing is not a "fillet", where a radius joins the two surfaces with a smooth curving transition between the surfaces. It's more of a flaring out of the fin at the base just to enclose bits that stick out.

Most airliners, like the 787 in the second picture, are able to contain all of the attaching structure within the fin's "mold line" (the airfoil contour surface) and all you have is a straight surface meeting a curved surface of the fuselage, such that interference drag isn't a problem and the permanent sharp skin joint does the job.

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