NAVWEPS 00-807-80  BASIC AERODYNAMICS  PITOT-STATIC SYSTEM  w  /  :%  .  I.  q  PITOT WITH SEPARATE  STATIC SOURCE  PRESSURE INDICATED BY GAUGE IS  DIFFERENCE BETWEEN TOTAL AND  STATIC PRESSURE, H-p= q  Figure. 1.5. Airspeed Measurement  0.05 psi position error is an airspeed error  of 10 knots.  A typical variation of air-  speed system position error is illustrated in  figure 1.6.  (3) The equivalent airspeed (PAS) is the  result of correcting the (CAS) for compressi-  bility effects. At high flight speeds the  stagnation pressure recovered in the pitot  tube is not representative of the airstream  dynamic pressure due to a magnification  by compressibility. Compressibility of the  airflow produces a stagnation pressure in  the pitot which is greater than if the flow  were incompressible. As a result, the air-  speed indication is given an erroneous mag-  nihcation.  The standard airspeed indicator  is calibrated to read correct when at standard  sea level conditions and thus has a com-  pressibility correction appropriate for these  conditions. However, when the aircraft is  operating above standard sea level altitude,  instrument and errors due to position or lo-  cation of the installation.  The instrument  error must be small by design of the equip-  ment and is usually negligible in equjpment  which is properly maintained and cared for.  The position error of the installation must  be small in the range of airspeeds involving  critical performance conditions.  Position  errors are most usually confine,d  to the static  source in that the actual static pressure  sensed at the static port may be different  from the free airstream static pressure.  When the .,aircraft is operated through a  large range’ of angles of attack, the static  pressure distribution  varies  ‘ quite greatly  and it becomes quite difficult to ’ minimize  the static source error.  In most instances a  compensating group of static sources may  be combined to reduce the position error.  In order to appreciate the magnitude of this  problem, at flight speed near 100 knots a  11 