= Wind-tunnel support interaction residual
A \b[wind-tunnel support interaction residual] is the part of a measured aerodynamic load caused by placing model and support together that remains after subtracting the support's separately measured load. The support can alter the <fluid flow> around the model, and the model can alter the <fluid flow> around the support. Their combined load need not equal the sum of two isolated loads.
For an original four-state measurement model, let $y_{00}$ be the common background, $y_{10}$ the isolated-model channel, $y_{01}$ the support-only channel and $y_{11}$ the combined channel, all at matched conditions. Define
$$
I=y_{11}-y_{10}-y_{01}+y_{00}.
$$
If $F_M=y_{10}-y_{00}$ is the desired isolated-model load, support-tare subtraction gives
$$
y_{11}-y_{01}=F_M+I.
$$
The four-state expression defines an interaction contrast; it does not promise that an isolated-model state is experimentally accessible without some alternative mounting arrangement.
Agreement between two mounting corrections can be misleading. In a constructed example, let the isolated-model <force> be 20 N. Two supports have isolated loads 3 and 5 N, but each creates an interaction load of 2 N. Combined measurements are 25 and 27 N; after their respective tares both report 22 N. The perfect agreement leaves the same 10% error. This is <common-mode support interference in mounting comparisons>.
An independent reference, a justified interference model or experiments that constrain the interaction are needed to remove that remaining ambiguity. Changing a support and observing a difference establishes mounting sensitivity; observing no difference does not establish zero common interaction.
The effect has concrete experimental history. The 1939 variable-density-tunnel report found that support interference affected measured <airfoil> <drag coefficients> and their apparent dependence on thickness.<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 and revised thickness trend]{ref} A correction can therefore alter a comparative scientific conclusion, not merely the zero of one load channel.
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