The Airspeed Indicator (ASI)
What is an ASI ?
This is an airspeed indicator from a typical light helicopter, the R22.
The White needle points to the current airspeed (in this case zero kts).
The Green arc is the normal operating speed range of this helicopter.
The Red Line is the Vne (maximum airspeed) of this helicopter, in this case 102kts.
The larger numbers around the outside of the scale are in knots or nautical miles per hour.
The smaller numbers around the inner scale are in miles per hour.
How it works
The airspeed indicator measures dynamic pressure, or the difference between ambient air pressure and total air pressure.
The total air pressure (or pitot pressure) is fed into the instrument from the pitot or pressure head via pipes.
The ambient pressure is fed into the instrument from the static port via pipes.
Total pressure is the result of bringing air moving relative to the aircraft to a stop, it includes ambient pressure which is why we calculate the difference between the two which gives us dynamic pressure.
The formula for dynamic pressure is (without proof)
P = ½ρV2
From this we can calculate V if we know P rearranging gives us
V = √2P/ρ
Knowing the air density enables us to calculate our Velocity.
Airspeed indicators are calibrated for one air density and temperature, so any variation from this will cause erroneous indications.
Example Content
Web References
The Pressure Instruments
