- The ASI is an instrument that measures how much is the KE of a parcel of moving air.
- It's a pressure instrument.
- The PITOT tube collects a sample of the passing airstream and guides it into a capsule which is a dead end. Thus the air comes to a full stop and so KE is released as a build up of pressure within this capsule. This dynamic pressure is simply the pressure caused by wind.
- A STATIC vent is plaved so that its face is parallel to the airflow, it is exposed only to the static pressure of the airstream. This static pressure is trasmitted to the interior of the instrument case which contains the capsule.
Total pressure = dynamic pressure + static pressure is thus acting on the interior surfaces of the capsule and the static pressure alone is acting on the exterior surfaces. The capsule therefore expands due to the pressure difference. This is the dynamic pressure of the outside airflow.
So the ASI measures dynamic pressure of the airstream INDIRECTLY by subtracting static pressure from total pressure there are some errors as a reult of positional errors due to the static pressure at the static port being different at different a/c attitudes. There maybe small instrument errors due to internal components of the ASI.
Calibrated Air Speed CAS is the indicated air speed IAS corrected for these errors.
For VFR general aviation a/c the IAS is taken as the same as CAS.
TAS is the air speed of the a/c at altitude taking into account the ambient density of the airflow which is lower than standard Sea Level density. Lower density at height means the velocity of airflow will be higher and therefore a higher actual airspeed. hence the TAS is higher than CAS. The ASI has been calibrated for S.L. standard conditions.
- It's a pressure instrument.
- The PITOT tube collects a sample of the passing airstream and guides it into a capsule which is a dead end. Thus the air comes to a full stop and so KE is released as a build up of pressure within this capsule. This dynamic pressure is simply the pressure caused by wind.
- A STATIC vent is plaved so that its face is parallel to the airflow, it is exposed only to the static pressure of the airstream. This static pressure is trasmitted to the interior of the instrument case which contains the capsule.
Total pressure = dynamic pressure + static pressure is thus acting on the interior surfaces of the capsule and the static pressure alone is acting on the exterior surfaces. The capsule therefore expands due to the pressure difference. This is the dynamic pressure of the outside airflow.
So the ASI measures dynamic pressure of the airstream INDIRECTLY by subtracting static pressure from total pressure there are some errors as a reult of positional errors due to the static pressure at the static port being different at different a/c attitudes. There maybe small instrument errors due to internal components of the ASI.
Calibrated Air Speed CAS is the indicated air speed IAS corrected for these errors.
For VFR general aviation a/c the IAS is taken as the same as CAS.
TAS is the air speed of the a/c at altitude taking into account the ambient density of the airflow which is lower than standard Sea Level density. Lower density at height means the velocity of airflow will be higher and therefore a higher actual airspeed. hence the TAS is higher than CAS. The ASI has been calibrated for S.L. standard conditions.
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