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The pressure capsule of an airspeed indicator is sensitive to the difference:
  • A
    (total pressure) – (static pressure).
  • B
    (total pressure) – (dynamic pressure).
  • C
    (dynamic pressure) – (static pressure).
  • D
    (dynamic pressure) – (total pressure).
The ASI indicates dynamic pressure. Dynamic pressure is the difference between total pressure (static + dynamic) and static pressure. An open-ended tube, called the Pitot tube, facing the relative airflow will transmit the total pressure (Ps + Pd) to a diaphragm (pressure capsule) situated in the instrument case. Another pressure sensor, usually situated on the side of the fuselage and called the static port, measures the static atmospheric pressure. The static pressure is fed to the ASI case and will act on the diaphragm ‘filled’ with total pressure. The static pressure acting on the outside of the diaphragm will cancel the static pressure from inside the diaphragm. The resultant of this differential pressure will be the dynamic pressure (Pt – PS = Pd) which will be mechanically transmitted to a pointer moving over a dial calibrated in knots (kn) or km/h.
 

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