airspeed measurement
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Airspeed Measurement
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Overview! We are simply attempting to understand how an
airspeed indicator determines INDICATED airspeed
! Need an understanding of Bernoullis principles ofpressure
! We will look at the different types of airspeed andthe equations related to it.
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Indicated is what you see.Note for cross country planning you
calculate other types of airspeed first.
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Measuring airspeed! In the air, given indicated airspeed IAS, how then do
we determine CAS, EAS, and TAS?
! On the ground, given true air speed TAS, how thendo we determine EAS, CAS, and IAS?
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Ice-T mnemonic
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How airspeed indicator determinesINDICATED Airspeed
! IAS knots on airspeed indicator! 1. Measure total pressure H.!
2. Measure static pressure Ps.! 3. Calculate dynamic pressure q from Bernoullis
equation q = H - Ps.
!4. Calculate KIAS from dynamic pressure,
! V KIAS=!(295)q =17.18 !q! *Note that the airspeed indicator does not measure
dynamic pressure!
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1. Measure total pressure:
Airflow on a Symmetrical Object
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Free stream total pressure H = Ps + q.
At the stagnation point, all kinetic energy or dynamic pressure q is converted to Ps.
v=0
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Definitions! Pressure at the forward stagnation point:
= H or head pressure
= stagnation pressure
= total pressure
= ram pressure
= Pitot pressure
! The pressure at the stagnation point(total! pressure) will be greater than the ambient static pressureby! the amount of the dynamic pressureof the air stream
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2. Measuring Static Pressure (p or Ps)
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Calculating q=H-p
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3. Calculate dynamicpressure q from Bernoulli
! H = Ps + q! q = H - Ps Airspeed indicators "subtract" Ps from H
to determine q.
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4. Calculate KIAS fromdynamic pressure
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Converting H to KIAS
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Now looking at CALIBRATEDcorrection
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Remember the ICE-Tmnemonic
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"Vi + "VmCAS is IAS corrected for installation (position) andinstrument (mechanical) errors.
CAS=IAS+Vi +Vm Possible errors in our IAS
methodology:
! Installation or position or error Vi. Theinstallation or position of the pitot tube and/orstatic port may induce errors.
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Fuselage factor
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! Installation or position or error correction isprovided by manufacturer in the POH.
! Ps measured (and Vi) changes with aircraft speedand configuration.
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Mechanical Error:! Mechanical errors in the instrument. Airspeed
indicators must be checked and calibrated every 24months. Usually considered to be zero.
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