Aeronautical Engineering/Merging two different drag coefficients into one airplane
Imagine an airplane with a drag area of 3 square feet and a drag coefficient of 0.035. Now imagine you attach a 1 square foot flat plate to the airplane. I believe the drag coefficient for a flat plate is around 2.0. I understand that to get a truly accurate drag coefficient you would probably need to analyze the fluid dynamics, but just for a general idea, how would you calculate the new drag coefficient of the airplane after attaching the flat plate?
Well, you certainly can't add the drag coefficients because the flow around the airplane would be affected by the flat plate and vice versus. I would look in Hoerner's Fluid-Dynamic Drag book and search for experimental data as close as possible to your situation. For example, Hoerner lists the drag coefficient of a flat plat perpendicular to flow at 1.17 based on frontal area; Reynolds number between 10^4 and 10^6 (fig. III.32 in my book). Putting a cone in front of the plate reduces the drag coefficient to 0.5. Putting a half sphere behind the flat plate did not change much. Hoerner also has aircraft drag data with various bluff bodies attached to the aircraft. In other words, these things are configuration dependent, and one has to consider how the plate and airplane are configured. Then you either have to test it, find some good data, or do a really good computational fluid mechanic analysis. There is no easy answer.
Chances are the new drag will be less than that of a flat plate and more than that of the airplane. But because the drag coefficients you are using are normalized by different areas, the new drag needs to be computed and a new drag coefficient has to be found using one reference area, probably the wing area.