Impact pressure

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Impact pressure is the difference between pitot pressure (also known as stagnation pressure) and static pressure. In aerodynamics notation, this quantity is denoted as qc or Qc.

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[edit] Significance

Impact pressure measurements are used to measure airspeed on aircraft. The resulting quantity, indicated airspeed, can be found as the positive square root of the following application of Bernoulli's equation for compressible fluids:

IAS = \sqrt{\frac{2q_c}{\rho}}

where (using SI units):

IAS = indicated airspeed in m/s
qc = impact pressure in pascals
ρ = standard air density in kg/m3

[edit] More information

Measurements of impact pressure are typically imperfect due to the variation of static pressure readings in different locations of the aircraft. This is why aircraft-specific correction tables are used to eliminate this error, producing calibrated airspeed.

Note that for an incompressible fluid, impact pressure is equal to dynamic pressure. For compressible fluids (such as air), the resultant airspeed can be further corrected for compressibility effects and variation of air density with altitude. These corrections yield equivalent airspeed and true airspeed, respectively.

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