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From Boeing Jet Transport Performance Methods, Measurement of Airspeed

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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 ).

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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.

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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).

http://i.imgur.com/shbcEhR.gif?1

Good stuff for performance engineers.

what is the legend in those graphs, exactly?

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Real air has impact pressure. The more impact pressure, the more Lift, the more Drag, etc.

Then why don't we use an indication of that magnitude to fly an airplane?

Why do we use two instruments (altimeter and ASI) instead of one?

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what is the legend in those graphs, exactly?

The curves labeled alpha=3 through 7 (degrees) show cL/cL0, the ratio between the lift coefficient at a Mach to that at M=0 for the particular AoA or, if you like, the lift coefficient in real air divided by that in incompressible flow.

The red line is qc/q0, the ratio between impact pressure (pt - ps) and dynamic pressure ½ρV².

Perhaps you should ignore the thin blue (highest) line, it is the pressure correction pc/p0 according to Prandl-Glauert for small perturbations, e.g. thin airfoils at small angles of attack.

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Then why don't we use an indication of that magnitude to fly an airplane?

Well, the ASI provides just that, it indicates CAS, which is aerodynamically more relevant than EAS, as illustrated in the graph.

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Why do we use two instruments (altimeter and ASI) instead of one?

You're not interested in altitude?

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.

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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.

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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?

No, it doesn’t. Tradition and convenience dictates that aerodynamic coefficients are referenced to dynamic pressure, in expressions like: Lift=cL*½ρV²*S. Because of that arbitrary definition of (lift) coefficient, ½ρV² must be used to calculate (lift) force from that coefficient. What many simplistic introductions don’t explain, is the variation of that coefficient with Mach number (and sometimes with Reynolds number).

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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 …

Actually, the airflow is slowed to zero at the stagnation points, such as the nose of the radome or the leading edge of a straight wing. But that is really unimportant, because the forces on the airplane are the result of pressures all around the airplane, and in real air all changes of pressure from the undisturbed static pressure are subject to compressibility.

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I don't think the effects would be nearly as drastic as the difference between CAS and EAS.

Have another look at my graph and think about it. For the data shown in this example, the effect is greater than the difference between CAS and EAS when Mach is greater than about 0.6.

EDIT: Below M.6 the effect is only 50% of that predicted by the Prandtl-Glauert model.
EDIT 2: To borrow from another thread: If your theory doesn't fit the experiment (flight test evidence) - it's WRONG!