 Common-mode support interference in mounting comparisons

Common-mode support interference in mounting comparisons is an aerodynamic disturbance shared by different mounting arrangements, so that agreement between their corrected loads fails to reveal it. Repeating a wind tunnel measurement with several supports can test differences in support effects without identifying their shared absolute contribution.

Write the support-tare-corrected load for mount $j$ aswhere $F$ is the desired isolated-model force, $b$ a shared wind-tunnel support interaction residual, ${\delta }_j$ a mount-specific residual and ${\epsilon }_j$ measurement noise. Differences eliminate both $F$ and $b$:Even arbitrarily precise agreement does not separate $F$ from $b$. Replacing $(F,b)$ by $(F+c,b-c)$ leaves every predicted observation unchanged. This is shared-bias nonidentifiability in mounting comparisons.

For an original example, corrected mount results 21.9, 22.0 and 22.1 N are compatible with isolated-model force 20.0 N, common residual 2.0 N and mount-specific residuals $-0.1,0,0.1$ N. Averaging reduces their visible spread but does not remove the common 2 N. A standard error derived only from replicate spread would omit that contribution.

An independent bound adds information. If a reference experiment justifies $1.5\le b\le 2.5$ N and other residuals are negligible, an exact mean 22 N implies $19.5\le F\le 20.5$ N. Without the independent bound, agreement alone supplies no such interval. The reference must constrain the common disturbance, rather than repeat the same unvalidated subtraction.

Real support interference altered both airfoil drag coefficients and their inferred thickness dependence in the 1939 variable-density-tunnel investigation.^[ref] Multiple mounting arrangements are useful diagnostics, but their concordance establishes only the aspects to which the comparison is sensitive.