= Empty-tunnel support tare
An \b[empty-tunnel support tare] is the wind-on aerodynamic load measured with the support arrangement present but the test model absent. Subtracting that load from the model-plus-support measurement removes a specified support contribution under matched conditions. It does not automatically remove the <wind-tunnel support interaction residual> created by their combination.
The word empty here means empty of the test model, not empty of its mounting hardware. A wind-off zero or orientation tare addresses other contributions, such as instrument offset or the changing component of <gravitational weight>. Neither measurement establishes the support's aerodynamic load in moving air.
An original scalar example uses model-plus-support <force> 12.00 N and matched support-only <force> 1.08 N. Their difference is 10.92 N. With air <density> $\rho=1.2\,\mathrm{kg\,m^{-3}}$, <speed> $U=30$ m/s and reference <cross-sectional area> $A=0.10\,\mathrm{m^2}$, the <dynamic pressure> and load normalization are
$$
q=\frac12\rho U^2=540\,\mathrm{Pa},\qquad qA=54\,\mathrm N.
$$
If the channel is drag, the support tare corresponds to <drag coefficient> increment $1.08/54=0.0200$, and the corrected measured coefficient is $10.92/54\approx0.20222$. Those numbers describe the subtraction; they do not certify an isolated-model <drag coefficient> when interference remains.
At the same normalization, a 0.10 N tare error contributes about 0.00185 to the coefficient error. A tare measured at the wrong <speed>, orientation or support geometry can therefore matter even when the force offset appears small. Document conditions and propagate <measurement uncertainty>, rather than treating a stored tare as exact for every run.
The variable-density-tunnel support-interference investigation demonstrates why direct support load and interference require separate attention.<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>[NACA-TR-669, abstract: support interference changed drag data and airfoil-thickness trends]{ref} The arithmetic above is an original normalized-load example, not a reported tunnel result.
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