= Common-mode support interference in mounting comparisons
\b[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 $j$ as
$$
z_j=F+b+\delta_j+\varepsilon_j,
$$
where $F$ is the desired isolated-model <force>, $b$ a shared <wind-tunnel support interaction residual>, $\delta_j$ a mount-specific residual and $\varepsilon_j$ measurement noise. Differences eliminate both $F$ and $b$:
$$
z_j-z_k=\delta_j-\delta_k+\varepsilon_j-\varepsilon_k.
$$
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 $1.5\leq b\leq2.5$ N and other residuals are negligible, an exact mean 22 N implies $19.5\leq F\leq20.5$ 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.<Airfoil Section Data Obtained in the NACA Variable-Density Tunnel as Affected by Support Interference and Other Corrections by Eastman N. Jacobs and Ira H. Abbott>[NTRS abstract, support-interference correction and airfoil-thickness trend]{ref} Multiple mounting arrangements are useful diagnostics, but their concordance establishes only the aspects to which the comparison is sensitive.
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