= Aerodynamic interference
\b[Aerodynamic interference] occurs when nearby bodies alter one another's <fluid flow>, so their combined aerodynamic loads differ from the sum measured separately under a specified comparison. <Pressure> fields, <boundary layers> and <fluid wakes> can all participate. An <aircraft> assembly therefore needs an operating-condition definition as well as a list of parts.
A <factorial contrast for aerodynamic interference> makes that definition measurable. At the same <dynamic pressure>, model orientation and mounting convention, record a balance reading $D_{ij}$ with component A absent/present according to $i$ and component B absent/present according to $j$. The departure from additivity is
$$
\boxed{\Delta D_{\rm int}=D_{11}-D_{10}-D_{01}+D_{00}.}
$$
The empty-configuration reading $D_{00}$ matters: it prevents a common support background from being subtracted twice. For original illustrative readings $(D_{00},D_{10},D_{01},D_{11})=(2,22,7,32)\,\mathrm N$, the interaction is $5\,\mathrm N$. This <aircraft component drag interaction> is a property of the stipulated four configurations and conditions.
<Kenneth E. Ward>'s 1931 experiment supplied an early concrete comparison. A <Göttingen 387> wing was tested with struts in different upper- and lower-surface positions in a <variable-density wind tunnel>; separate strut measurements helped distinguish direct strut <drag> from interaction. Interference could be as large as the struts' own <drag>.<Interference Effects and Drag of Struts on a Monoplane Wing by Kenneth E. Ward>[Summary, Apparatus and Tests, and Discussion of Results]{ref}
The contrast does not identify the responsible flow mechanism by itself. Surface <pressure> measurements and observations of <flow separation> can test where the changes occur. Comparing at fixed <angle of attack> also differs from comparing at fixed <lift>: adding a component can change <lift>, requiring a new angle or an explicitly documented interpolation for the latter comparison. A negative contrast is possible; interaction can reduce the chosen load as well as increase it.
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