Airspeed:
Air speed is the speed of aircraft relative to air.
The measurement of air speed is done by Air Speed Indicator (ASI) which infact
is connected to pitot static system. The two pressures (Pitot pressure and
static pressure) are compared by ASI to give airspeed.
Pitot
Static instrument is a system of instruments that is used
to measure air pressure gradient. It measures pressure difference and using
these values it can measure various parameters like speed, altitude etc.
Pitot
pressure:
Pitot pressure is a measure of ram air pressure i.e
the pressure created by air ramming into the tube during vehicles motion, which
is generally equals to total pressure. The pitot pressure is generally measured
by Pitot tube. The pitot tube is generally located on the wings or front
section of aircraft facing forward such that its opening is directly exposed to
the relative wind direction.
Static
Pressure:
Static pressure is measured through via static ports.
These ports are present as small holes on fuselage at the position where air
flow is accessed in undisturbed manner. For
more then one static ports, the static ports is generally located on each side
of fuselage so as to obtain accurate reading in specific flight situations.
The various airspeed measurement parameters include:
1. Calibrated
airspeeds (CAS)
2. Indicated
airspeeds (IAS)
3. True
airspeeds
4. Equivalent
airspeeds
1. Calibrated Air Speed:
Calibrated airspeed (CAS) is indicated airspeed
corrected for instrument errors, position error (due to incorrect pressure at
the static port) and installation errors.
The static weight estimation is liable to blunder
because of failure to put the static ports at positions where the weight is
genuine static weight at all velocities and dispositions. The rectification for
this mistake is the position error correction (PEC) and changes for various
aircrafts and airspeeds. Further errors of 10% or more are common if the
airplane is flown in “uncoordinated” flight.
Calibrated airspeed values less than the speed of
sound at standard sea level (661.4788 knots) are calculated as follows:
-
position and installation error correction
where
Vc =is
the calibrated airspeed,
P0 =is
29.92126 inches Hg; static air pressure at standard sea level,
A0 =is
661.4788 knots;, speed of sound at standard sea level.
1. Indicated Airspeeds:
Indicated airspeeds denotes reading uncorrected for
position, co-ordinates and other errors. Indicated airspeed means the speed of
an aircraft as shown on its pitot static airspeed indicator calibrated to
reflect standard atmosphere adiabatic compressible flow at sea level
uncorrected for airspeed system errors.
2. Equivalent Airspeeds:
Equivalent airspeed (EAS) is defined as the speed at
sea level that would produce the same incompressible dynamic pressure as the
true airspeed at the altitude at which the vehicle is flying.
It is a measure of airspeed that is a function of
incompressible dynamic pressure.
3. True Airspeeds:
True airspeed is the speed of the aircraft relative to
the atmosphere. The vector relationship between the true airspeed and the speed
with respect to the ground is
Vt = Vg - Vw
Vw = Wind speed vector
Where,
a0 = speed of sound at standard sea level
M = Mach Number
T = Temperature
T0 = standard sea level temperature
Aircraft flight instruments, however, don't compute
true airspeed as a function of groundspeed and wind speed. They use impact and
static pressures as well as a temperature input. True airspeed is equivalent
airspeed that is corrected for pressure altitude and temperature (which define
density). The result is the true physical speed of the aircraft relative to the
surrounding body of air. At standard sea level conditions, true airspeed,
calibrated airspeed and equivalent airspeed are all equal.
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