= Aircraft component drag interaction
An \b[aircraft component drag interaction] is the part of a combined <drag> measurement that is not accounted for by an additive model of the specified separate components. It is one measurable consequence of <aerodynamic interference>; its sign and magnitude depend on operating conditions and on what counts as the common baseline.
Use the <factorial contrast for aerodynamic interference>, $\Delta=D_{11}-D_{10}-D_{01}+D_{00}$. With readings ordered as $\mathbf d=(D_{00},D_{10},D_{01},D_{11})^T$, put $\mathbf a=(1,-1,-1,1)^T$. If $\Sigma$ is their <covariance matrix>, then
$$
\Delta=\mathbf a^T\mathbf d,\qquad u_\Delta^2=\mathbf a^T\Sigma\mathbf a.
$$
The second expression propagates the <standard uncertainty> of the linear contrast; it requires the actual covariance model, not an assumption that every reading is independent.
For a stipulated instrument model, let $\mathbf d=\boldsymbol\mu+Z\mathbf1+\boldsymbol\epsilon$, with a shared zero error $Z$ independent of four independent residual errors of <standard deviation> $0.5\,\mathrm N$. Because $\mathbf a^T\mathbf1=0$, the shared zero cancels exactly and \b[$u_\Delta=1.0\,\mathrm N$], regardless of its variance. This <common-zero cancellation in a drag interaction contrast> fails for a changing zero or for a shared gain error. At the illustrative contrast $5\,\mathrm N$, a common relative gain error $g$ leaves the factor $(1+g)5\,\mathrm N$.
<Kenneth E. Ward> distinguished attached-component measurements from isolated-strut measurements in his <wind tunnel> study, rather than attributing the entire assembly drag increase to the struts alone.<Interference Effects and Drag of Struts on a Monoplane Wing by Kenneth E. Ward>[Discussion of Results, comparison with isolated strut drag]{ref} In a modern repetition, interleave baseline and assembly runs, retain all raw balance channels, and compare after consistent <measurement calibration>. Repeatability alone does not bound a systematic support interaction shared by every run.
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