Let me ask a dumb aerodynamics question (it was related to the drag issue on Igor's airfoil with flaps).

Does the coefficient of drag c_{d} include induced drag?

Alan: That's a very GOOD question and one for which the answer isn't intuitively obvious. As best that I can explain it, SOME of the induced drag is included in those plots that you see, but on a real, 3D airplane there will be more than what you see on those plots. The plots that you see in "Theory of Wing Sections" and from computerized "wind tunnels" are supposed to represent 2 dimensional results as if the wing had infinite span.

The atmosphere can ONLY act in 2 ways on any wing surface, it can generate frictional forces and/or it can generate pressure forces. (we'll ignore the frictional forces for now). The pressure forces can ONLY act PERPENDICULAR to the surface AT THE POINT where they meet. So up near the nose of an airfoil the pressure vector is tilted a little bit forward due to the curvature of the section surface and aft of the max thickness point the pressure vector is tilted a little bit aft. Since you are a physicist, you will easily understand that the pressure vector can be resolved into two orthogonal components, one that is perpendicular to the X-axis (we'll call this lift) and the other along the Y-axis, either pointing a little bit forward or a little bit aft. We'll call that one "induced drag" since it is a force in the 'drag direction', "induced" by the production of lift. If we integrate all of these little pressure components along the X-axis we come up with the "induced drag" of the section.

NOW, if you change the angle of attack of the section, the magnitude of the pressure forces change, and also, since the chord line of the section is no longer along the X-axis you can see that the vector component of the pressure along the X-axis also increases. That is, some of the "lift" being produced is actually "tilted back" and is "dragging" the airplane backward. That's why you see the induced drag increasing as the angle of attack increases.

So, THAT part of the induced drag IS included in those 2D section plots that you see everywhere, BUT on a real, 3D airplane there is some more induced drag that comes about from the fact that real airplanes have a finite span as opposed to an "infinite" span in the 2D data. On a 3D airplane there will be drag corrections due to span-wise flow, and the energy lost in the tip vortex. On a low aspect ratio wing, it can be quite significant. THAT's why you see sailplanes with such high aspect ratios and elegantly shaped wing planforms.

So to make a long story short (too late...) Yes, the plots that you see include "most" of the induced drag, but on a real airplane the induced drag will be somewhat higher than what you see in the 2D plots.

If you remember that a "fluid" can ONLY act on a surface in two ways, friction parallel to the surface and pressure perpendicular to the surface, a lot of the aerodynamic "gobbledy gook" that we aero-types like to throw around will make a lot more sense. All those formulas are just an easy way to quantify and integrate those forces in different ways.