 Aircraft component drag interaction

An 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, thenThe 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 $\mathbf{d}=\mathbf{\mu }+Z1+\mathbf{ϵ}$, with a shared zero error $Z$ independent of four independent residual errors of standard deviation $0.5\mathrm{N}$. Because ${\mathbf{a}}^{T}1=0$, the shared zero cancels exactly and $u_{\Delta }=1.0\mathrm{N}$, 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.^[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.