 Empty-tunnel support tare

An 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 $\rho =1.2\mathrm{kg}{\mathrm{m}}^{-3}$, speed $U=30$ m/s and reference cross-sectional area $A=0.10{\mathrm{m}}^2$, the dynamic pressure and load normalization areIf the channel is drag, the support tare corresponds to drag coefficient increment $1.08/54=0.0200$, and the corrected measured coefficient is $10.92/54\approx 0.20222$. 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.^[ref] The arithmetic above is an original normalized-load example, not a reported tunnel result.