HazelNuts,
You say that CAS takes into account compression of the air which affects aerodynamic forces at certain speeds and is therefore a better indication of aerodynamic performance. Is that a correct interpretation? I see your point, and it makes sense, but I do have other questions.
When computing aerodynamic forces, you’ll use dynamic pressure frequently. For airspeed measurement, however, which must cover both the lower speeds where there’s no compressibility effect and the faster airspeeds where there IS some compression, we’ll use the term “impact pressure” to denote ( pt - ps ).
That quote you included states that when computing aerodynamic forces you use dynamic pressure. That seems to contradict what you're writing, doesn't it?
I agree that compressibility does affect the aerodynamic forces on the airplane, which is why we need to take it into consideration when flying at high speeds and/or altitudes. However, when measuring airspeed, the pitot tube measures total pressure which means that it slows the air down to a speed of zero, relative to the airplane. The more you slow the air down, the more the air is compressed. The difference between the pitot tube and the airplane is that the air flow around the airplane doesn't get slowed to zero, excluding the boundary layer. It makes sense that there would be compressibility, and expansion, of the air depending on which location it's at on the airplane but I don't think the effects would be nearly as drastic as the difference between CAS and EAS.