From Boeing Jet Transport Performance Methods,
Measurement of Airspeed
impact pressure and dynamic pressure
The difference between the total pressure and static pressures sensed on the airplane, ( pt - ps), is called impact pressure. In the chapter entitled “Physics of Air”, however, we defined ( pt - ps) as dynamic pressure, ½ρV². What’s the difference?
Impact pressure and dynamic pressure are the same at low speeds, below 200 or 250 knots, in which speed range the density of the air is considered to be unaffected by the airplane’s speed. At higher speeds, the air density around the airplane is affected by compressibility and the term ( pt - ps ) is no longer equal to ½ρV². Just remember that impact pressure is defined as ( pt - ps) whereas dynamic pressure q is defined as the kinetic energy of the airflow, ½ρV².
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 ).
Originally Posted by Microburst2002
Dynamic pressure is also an empirical fact. It is the kinetic energy per volume unit of air. The more dynamic pressure, the more Lift, the more Drag, etc... And that is why you need to know it, because the aerodynamic forces depend on it. Real air has dynamic pressure.
Real air has impact pressure. The more impact pressure, the more Lift, the more Drag, etc.
Originally Posted by Microburst2002
When compressibility effects become noticeable then you need to know your mach number, too, because it will affect aerodynamic forces and set your limits before dynamic pressure does.
The point is that compressibility affects aerodynamic forces at all speeds. The following graph shows the change of lift coefficient due to compressibility at given angles of attack (alpha) versus Mach number. The data points are from the aerodynamic data base of a large transport airplane. The ratio of impact pressure qc to dynamic pressure q0 is shown by the red line.
P.S.
The graph clearly shows that CAS (representing impact pressure) is actually a 'truer' indicator of aerodynamic performance than EAS (representing dynamic pressure).
