Abstract: An improvement of a vertical fin (18) attached to a pylon strut (14) that is cantilevered from a wing rear spar (16) of a supersonic aircraft (9), the improved pylon (10) for mounting heavy assemblies such as an engine (8) to an aircraft wing (12). The fin (18) is a thin, elongate member having a ratio of fin height (36) to fin width (38) of about between 3:1 to 5:1. The ratio of fin length(34) to fin width (38) is preferably equal to or greater than 30:1. The fin at its lower surface (24) is attached to an upper surface (20) of the strut (14). The fin (18) preferably extends forward over a portion of the upper surface (22) of the wing (12) and is additionally attached thereto. One embodiment of a fin construction includes a U-shaped lower channel (40), an inverted U-shaped upper channel (42), a pair of substantially parallel upright sidewalls (32), an arcuate upper closeout member (46), and aerodynamic seals 48 as required.