Flow conditions for OpenVSP’s Parasite Drag tool may be adjusted using two common atmosphere models (U.S. Std. Atmo. 1976 or USAF 1966); by defining any two of pressure, density, or temperature; or by directly setting the Reynolds and Mach numbers. The model length unit under the Geometry section of the Overview tab will set the default units for each of the flow condition parameters. However, the units for each parameter may be changed to fit your needs. For example, if your model is in feet but you are trying to match metric altitude and speed, simply choose the appropriate units from the parameter drop-down menu to the right of the text field. The flow conditions may be altered to compute the model drag in real time as long as no geometric changes are made to the model itself. Any model geometry changes require recalculating the wetted areas by clicking “Calculate CD0”.