In order to support various intelligence, surveillance, reconnaissance (ISR) missions, aircraft often undergo structural changes that affect the aircraft’s aerodynamics. A computational fluid dynamics (CFD) study was performed to understand changes to the drag coefficient when two wall-mounted obstacles, representative of common ISR modifications, are positioned in a tandem configuration and exposed to fully subsonic flow. The size, shape and position of the two obstacles is varied. The CFD study used three-dimensional, steady-state, compressible, Reynolds Averaged Navier Stokes equations.

The study reported four major results. First, interference effects increase the drag of the trailing obstacle up to 45% and decreases the drag of the leading obstacle up to 27%. Second, the interference effects on both obstacles produces a 12% reduction in drag for low Mach number and a 37% increase in drag for higher Mach numbers. Third, trailing obstacles of large size help reduce the drag of the leading obstacle of a smaller size. Lastly, the region where interference effects occur becomes smaller if the fineness ratio of the trailing obstacle increases.