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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Airspeed Measurement