Machmeter

Wg Cdr Rajagopal

Machmeter

Index

Local Speed of Sound

  • Speed of sound is not constant but varies with temperature
  • Sound travels slower in colder air and faster in warm air
  • Speed of sound decreases with increase in altitude
  • Local speed of sound (LSS) at sea level, in ISA (+ 15 deg C) is 661 knots
  • Local speed of sound (LSS) at 30,000 ft, in ISA (-45 deg C) is 589 knots

Calculation of Local Speed of Sound

  • Formula for Local speed of sound in knots =
  • 38.95 x Square root of Absolute Temperature In Kelvin

Definition of Mach Number

  • Mach Number = True Air Speed / Local Speed of Sound
  • Mach number is the ratio between the speed of aircraft to local speed of sound (or) ratio between the dynamic and static Pressures
  • 0.85 mach means speed of aircraft is 85% of the local speed of sound

Importance of Mach Number

  • Shock waves are formed when aircraft reaches local speed of sound
  • These shock waves would present difficulties in handling
  • Lift decreases
  • Drag increases
  • Pitch changes
  • Cause buffeting
  • Loss of controls

Construction of Machmeter

  • Machmeter displays the True Air Speed of the aircraft as mach number by determining ratio between dynamic & static pressure
  • Machmeter has two capsules
  • Dynamic pressure airspeed capsule
  • Static fed in altitude capsule
  • Ratio of static and dynamic is compared
  • Moves ratio arm and in turn ranging arm which moves pointer

Instrument and Position errors

  • Instrument errors are unavoidable due manufacturing imperfections
  • High error at high altitudes due to smaller pressure changes
  • Position or Pressure error is caused due to turbulence creating suction around static source
  • Error is reduced with static vents
  • Advanced pitot tube on high speed aircraft reduce error

Manoeuvre Induced errors

  • Manoeuvre induced error causes short term fluctuations due to changes in pitch or yaw

Density and Temperature Errors

  • Machmeter presents the speed as a ratio
  • Hence, it does not suffer from density and temperature errors
  • Variation affect both static and dynamic pressures
  • Cancel out each other

Compressibility Errors

  • Machmeter calculates the ratio between the speed of aircraft and that of sound
  • Therefore, it does not suffer from compressibility errors
  • Compressibility is a ratio of dynamic and static pressures
  • Machmeter is designed to read this ratio

Blockage in Pitot Line

  • Blockage in pitot line blocks input pitot pressure
  • Machmeter will not register any change of speed
  • Indication remains constant in level flight
  • In a constant speed climb or descent, the mach meter would
  • Under read in descent due to decrease in static component in pitot capsule (numerator)
  • Over read in climb due to increase in static component in pitot capsule (numerator)

Blockage in Static Vent

  • Blockage in static system makes the static (denominator) to remain constant
  • Mach number will be correct in level flight
  • Blockage in static system makes the static (denominator) to remain constant
  • Over read during constant speed descent due to incorrect low static (denominator)
  • Under read during constant speed climb due to incorrect high static (denominator)

Leaks in Static Line outside Pressurised Cabin

  • Leak in pitot line would reduce the pitot pressure (numerator) sensed by the system makes the machmeter to under read
  • Leak (outside pressurised hull) in static line will cause no error in the machmeter since the static will be sensed correctly

Leaks in Static Line inside Pressurised Cabin

  • Leak (inside pressurised hull) in static line will cause the cabin pressure to be detected as static pressure (denominator)
  • Cabin pressure is expected to be higher than actual static pressure
  • Under reads in climb
  • Over read in descent

Constant CAS Climb in Standard Conditions

  • Constant CAS climb increases TAS at higher altitude
  • Reduced density at higher altitude
  • Density affected by atmospheric pressure

Constant TAS Climb in Standard Conditions

  • Constant TAS climb increases Mach number at higher altitude
  • LSS reduces with altitude
  • Speed of sound is lower in cold air (altitude)

Climb Planning for High performance Aircrafts

  • High performance aircrafts would climb at
  • Constant CAS at lower level
  • Constant Mach number at higher levels

Descent in Standard Conditions

  • Constant mach number descent increases TAS at lower altitude
  • TAS will have to increase to match with increase in LSS
  • CAS will increase due to higher density at lower levels

Descent Plan for High Performance Aircrafts

  • High performance aircrafts would descend at
  • Constant mach number at higher levels
  • Constant CAS at lower level
  • In ISA conditions CAS < TAS < Mach Number

Mach Number and TAS in Isothermal Layer

  • Mach Number remains constant with height
  • Temperature remains constant with height
  • Ratio of TAS & LSS would remain constant

Constant TAS Climb and Descent in an Isothermal Layer

  • At a constant TAS
  • CAS will reduce during climb due to reducing density
  • CAS would increase during descent due to increasing density

Constant CAS Climb and Descent in an Isothermal Layer

  • At a constant CAS
  • TAS will increase during climb due to reducing density
  • TAS would reduce during descent due to increasing density

Local Speed of Sound in an Inversion Layer

  • Local speed of Sound increases with height due to higher temperature
  • Constant TAS climb decrease Mach number at higher altitude
  • Constant TAS descent increase Mach number at higher altitude

Constant CAS Climb in Inversion Layer

  • Constant CAS climb increases TAS at higher altitude
  • Reduced density at higher altitude
  • Density affected more by atmospheric pressure

Summary of CAS and TAS

  • Relation between CAS and TAS
  • During a constant CAS climb, TAS will always increase
  • During a constant TAS climb, CAS will always reduce
  • Pressure has a greater effect on air density than temperature.

Summary of CAS, TAS and Mach Number

  • Pressure has a greater effect on air density than temperature.
  • Relation between CAS and Mach Number
  • During a constant Mach No climb, CAS will always reduce
  • During a constant CAS climb, Mach No will always increase

Mach – Air Speed Indicator

  • Combines functions of Mach meter and Air Speed Indicator
  • Analogue instrument indicates Air speed on fixed scale and Mach number on a movable scale
  • Digital readout Instrument displays Mach No and Air speed from an Air Data Computer

Use of Mach – Air Speed Indicator

  • Provides Calibrated (Rectified) Airspeed instead of Indicated Air Speed
  • Speed limiter pointer is a manually controlled needle for speed limits
  • Maximum operating Mach number M no
  • Maximum operating speed V no

Errors of Mach – Airspeed Indicator

  • Errors of both Air Speed Indicator and Mach Meter
  • Instrument errors
  • Position (pressure)
  • Manoeuvre induced
  • Density
  • Temperature
  • Compressibility

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