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.