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What is required to determine indicated airspeed (IAS) using the airspeed indicator?

  • A

    The difference between total pressure and dynamic pressure.

  • B

    The difference between standard pressure and total pressure.

  • C

    The difference between total pressure and static pressure.

  • D

    The difference between dynamic pressure and static pressure.

Refer to figure.
The ASI is an air pressure sensing instrument.

It is worth mentioning that an aircraft that is stationary on the ground will be subject only to ambient atmospheric pressure or static pressure. In forward motion, the aircraft will be subject to additional or dynamic pressure.

Thus, the total pressure the aircraft will be subjected to will be the static pressure plus the dynamic pressure. This is known as total pressure or P.

Total Pressure = Dynamic + Static

The dynamic pressure experienced by the aircraft will vary with its speed through the air, and mathematically, this relationship is expressed in the formula.

Dynamic Pressure = 1/2ρV2 , where V is the true air speed, and ρ is the density of the air.

So, if dynamic pressure can be isolated from the total or P to pressure, it can be used to indicate airspeed.

Therefore, in the context of the previous information, the airspeed indicator's function is to isolate the dynamic pressure from the pitot pressure and display the dynamic pressure calibrated as airspeed.

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