Excrescence adds drag components from features that are difficult or unrealistic to model such as antennae, sensors/probes, access panels, surface roughness, ice, bugs, etc. To account for these “real world” features of a model’s parasitic drag, users may apply drag excrescence in a variety of ways. Drag counts (CD x 10^4) and drag coefficient (CD) may be added as fixed amounts to the total. Percentages may also be added as either a percent of geometric drag coefficient (% of CD_Geom) or as percent margin (% Margin) which may only be added once and will remain a fixed percentage of the Total CD rather than being relative to the geometric CD. Excrescence may also be added as a fixed drag area (D/q) which is invariant with the input flow conditions. By leveraging drag excrescence, a parasitic drag model may be tuned to very accurately match existing aircraft or test data and account for non-ideal geometric features.
Model components may be grouped with model tree ancestors of the same type in the Parasite Drag tool so that individual wetted area contributions are summed and form factors are applied to the group. Grouped components may be viewed by clicking the component name where (+) is displayed. For Wings that are generally reflected across XZ by default, the right and left sides are automatically grouped. To group a component with an ancestor, click the drop-down menu in the component row and select on of the available ancestors. The grouped component will contribute its wetted area to that of the ancestor and inherit the ancestor’s form factor, reference length, etc. An example of this would be to group a canopy with a fuselage such that the canopy is included in the fuselage parasitic drag component.
There are multiple equations available in the Parasite Drag tool to compute laminar or turbulent skin friction along with component form factors. Body- and Wing-type components will each have their own respective sets of form factor equations available and may also be set to a manual value. Using the scroll bar or expanding the Parasite Drag window width will show the entries for percent laminar flow (accepting values from 0 to 100) and interference factor (Q).
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”.
OpenVSP’s Parasite Drag tool enables very detailed parasitic drag calculations and buildups by leveraging the existing model components and a variety of atmospheric, skin friction, and form factor equations. The tool runs Comp Geom in the background to compute wetted areas and automatically populates relevant geometric parameters into the solver. Users may hover over the header labels to display a context box for each parameter.
Clicking “Calculate CD0” will populate the solver window with the resulting data which can be adjusted in real time by modifying atmosphere or drag equations and parameters. However, the solver will not update if the physical model is changed. In this case, the CD0 must be recalculated by clicking the button again. A Parasite Drag tool result file may be generated by clicking “Export”.
The Parasite Drag tool documentation page contains formulas, plots, references, and comparisons for all methods and equations used in the analysis, including atmospheric models, and serves as an excellent collection of information in its own right.