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Why do we correct airspeed for installation error?

Asked by: 3771 views Aerodynamics, Aircraft Systems, Commercial Pilot, Flight Instructor, General Aviation, Private Pilot, Student Pilot

As far as I know, CAS is the IAS corrected for instrument and installation error, but I do not know why we correct for them.

Any credible sources and ideas or comments are welcome !

 

Steve.

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2 Answers

  1. Best Answer


    John D Collins on Apr 20, 2016

    CAS is what you would wish your airspeed to indicate if your system had no error. For most airspeeds and configurations, the difference is small between CAS and IAS. However, particularly at slow airspeeds and configurations with flaps deployed, there can be a substantial difference. For a 1981 C172, flaps up at 100 Kts indicated, the CAS is 98 Kts or within 2%. But with the flaps down at an indicated speed of 40 Kts, the CAS is 47 Kts or an 18% error.

    When you calculate aerodynamic equations or values, CAS is used, not IAS. The airplane may stall at 60 Kts according to the POH, but since this is a CAS value, the actual IAS may be 50 Kts.

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  2. Steve on Apr 21, 2016

    Sir, then what is the reason we correct for k strument and position error?

    Is that because of flap settings that cause the turbulent aurflow or position of pitot static port?

    Then, what is instrument error?

    Steve.

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