Pressure Measurement

Wg Cdr Rajagopal

Pressure Measurement

Index

Static Pressure

  • Static pressure is the Atmospheric pressure or Ambient air pressure
  • Due to the weight of air column above the aircraft
  • Reduces with increase in altitude as air column above the aircraft reduce

Dynamic Pressure

  • Dynamic pressure is the force exerted when air is brought to rest
  • By a moving body like aircraft

Pitot Pressure

  • An aircraft in flight is affected by both static as well as dynamic pressures
  • Combined pressure is called Pitot pressure
  • Pitot pressure is a combination of static and dynamic pressures
  • Sensed by the pitot tube

Principle of Pitot Static Tube

  • Pitot Static tube consists of Pitot and Static heads
  • Pitot head senses total pressure called pitot pressure
  • Pitot pressure = Dynamic pressure + Static pressure

Position of Pitot Static Tube

  • Pitot Static tube consists of Pitot and Static heads
  • Located outside boundary layer of the aircraft
  • Avoids the effect of aircraft on the airflow
  • On the nose or wing tip or on the side of fuselage
  • Parallel to Longitudinal axis of the aircraft
  • Keeps the pitot tube parallel to the airflow in normal flight attitude

Construction of Pitot Static Tube

  • Pitot head is the open-ended tube which senses the pitot pressure
  • It brings moving air to rest in the stagnation wall of pitot tube
  • Pitot pressure sensed is a combination of Static and Dynamic pressures
  • Static head are perforations or holes with forward end sealed
  • Sealing of forward end avoids dynamic pressure

Features of Pitot Static Head

  • Heating elements are provided to prevent ice accretion
  • Drain holes in the pitot static tube removes moisture
  • Removal of moisture also causes an error due to loss in pressure
  • Both features cause error in measurement due to change the density
  • This error is corrected during calibration of pressure instruments

Position or Pressure or Static Error

  • Position error is also called Pressure error or Static error
  • Static tube is designed to sense the correct atmospheric pressure
  • Position error is caused due to the turbulent airflow near the static holes
  • Turbulence creates suction effect resulting in detection of lower static
  • Pressure instruments are provided with lesser than correct static pressure

Variation of Position, Pressure or Static Error

  • Turbulent air in the location of Pitot static tube increases static error
  • Higher speeds increase position error
  • Increase in speeds increase turbulent air flow
  • Position error also increases at high angle of attack even at lower speeds
  • Use of flaps increases angle of attack increasing position error

Static Vents for Position Error Correction

  • Position error is corrected by use of independent static vents
  • Static vents are located in area of least turbulence like sides of fuselage
  • Designed to drain water without heating unlike that of pitot tube
  • Two inter-connected static vents located on either side of fuselage
  • Inter connection removes error due side-slipping or yawing movement
  • Static vents are unsuitable in high speed aircraft due to shock waves
  • Fighter aircrafts use high speed pitot-static probe probes

Manoeuvre Induced or Time Lag Error

  • Manoeuvre induced errors cause short term fluctuations during manoeuvres
  • Manoeuvre induces temporary fluctuation of pressure in the static vents
  • This creates a time lag in readings of pressure instruments
  • The error is significant during change in configuration of flaps or slats
  • Noticeable error is also seen during change in angle of attack

Minimum and Maximun Time Lag Error

  • Manoeuvre errors are obvious during change in pitch angle
  • For example, during overshooting, take off, landing or during level out
  • Time lag error increases with increase in altitude
  • This error is seen even in Servo assisted altimeters or Air data computers
  • Time lag could vary from 3 sec at low altitudes to 10 sec at 30,000 ft

Complete Pitot Static System

  • Modern aircrafts have independent pitot & static for instruments on left & right
  • The instruments can be cross connected in case of emergency
  • Heaters with heating alert warning is provided to avoid ice accretion

Alternate Static Source

  • Alternate static source is provided for blockage of main static source
  • Alternate static may be placed inside the cabin for unpressurised aircrafts
  • It is at a non optimum location resulting in under reading of static
  • Alternate static causes over reading of Altimeter and Air speed indicator
  • Errors due to alternate static usage are tabulated in the flight manual

Share this Page