# Dictionary Definition

airspeed n : the speed of an aircraft relative to
the air in which it is flying

# User Contributed Dictionary

## English

### Derived terms

### See also

# Extensive Definition

Airspeed is the speed of an aircraft relative to the air.
There are several different measures of airspeed: indicated
airspeed, calibrated airspeed, equivalent airspeed and true
airspeed.

It is measured within the flying aircraft with an
airspeed
indicator — a device which connects to ram air
pressure from outside the aircraft and compares it to non-moving
air pressure outside the aircraft. The ram pressure is sampled by a
device called a pitot tube,
carefully located clear of the propeller blast and other airflow
distortions. There is also typically one or more static ports
carefully located on the outside of the aircraft.

## Indicated airspeed

Indicated
airspeed (IAS) is the airspeed
indicator reading (ASIR) uncorrected for instrument, position,
and other errors. From current EASA definitions: 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.
http://www.easa.eu.int/doc/Agency_Mesures/Certification_Spec/decision_ED_2003_11_RM.pdf

Outside of the former Soviet bloc, most airspeed
indicators show the speed in knots i.e.
nautical miles per hour. Some light aircraft have airspeed
indicators showing speed in miles per hour.

An airspeed
indicator is a differential pressure gauge with the pressure
reading expressed in units of speed, rather than pressure. The
airspeed is derived from the difference between the ram air
pressure from the pitot tube, or stagnation
pressure, and the static
pressure. The pitot tube is mounted facing forward; the static
pressure is frequently detected at static ports on one or both
sides of the aircraft. Sometimes both pressure sources are combined
in a single probe, a pitot-static
tube. The static pressure measurement is subject to error due
to inability to place the static ports at positions where the
pressure is true static pressure at all airspeeds and attitudes.
The correction for this error is the position
error correction (PEC) and varies for different aircraft and
airspeeds. Further errors of 10% or more are common if the airplane
in flown in “uncoordinated” flight.

## Calibrated airspeed

Calibrated airspeed (CAS) is indicated airspeed corrected for instrument errors, position error (due to incorrect pressure at the static port) and installation errors.Calibrated airspeed values less than the speed of
sound at standard sea level (661.4788 knots) are calculated as
follows:

V_c=A_0\sqrt minus position and installation
error correction.

- Q_c \, is the impact pressure (inches Hg) sensed by the pitot tube,
- P_0 \, is 29.92126 inches Hg; static air pressure at standard sea level,
- A_0 \, is 661.4788 knots;, speed of sound at standard sea level.

Units other than knots and inches of mercury can
be used, if used consistently.

This expression is based on the form of Bernoulli's
equation applicable to a perfect, compressible gas. The values
for P_0 and A_0 are consistent with the
ISA i.e. the conditions under which airspeed indicators are
calibrated.

## Equivalent airspeed

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. An
aircraft in forward flight is subject to the effects of
compressibility. Likewise, the calibrated airspeed is a function of
the compressible impact pressure. EAS, on the other hand, is a
measure of airspeed that is a function of incompressible dynamic
pressure. Structural analysis is often in terms of incompressible
dynamic pressure, so that equivalent airspeed is a useful speed for
structural testing. At sea level, standard day, calibrated airspeed
and equivalent airspeed are equal (or equivalent), but only at that
condition. For the performance engineer, there is no practical
reason to use equivalent airspeed for anything. However, structural
analysis is often performed in terms of equivalent airspeed (since
it is a direct function of the incompressible dynamic pressure), so
the performance engineer needs to be able to convert V_e to
parameters that are more useful.:

Let q \, represent the dynamic pressure \frac
\rho V^2 = \frac \rho_0 V_e^2.

Then the relationship between the pressure
difference p_t \, - \, p sensed by a pitot-static system and the
dynamic pressure is given by:

\frac = \frac = 1 + \frac M^2 + \frac M^4 + \frac
M^6 + ...

- M \, is the Mach number,
- V \, is the true airspeed,
- V_e \, is the equivalent airspeed,
- \gamma \, is the ratio of the specific heats of air and
- \rho \, is the air density.

The ratio of the specific heats, \gamma \, , is
1.4 in air. Substituting this value gives:

\frac = \frac = 1 + \frac M^2 + \frac M^4 + \frac
M^6 +...

(This section needs editing due to confusion
between V (TAS) and Vi (CAS) and ambiguity regarding ASI
calibration - incompressible flow equation above or compressible
flow equation under calibrated
airspeed? If the ASI is calibrated to the CAS calibration
equation which (for subsonic speeds) eliminates compressibility
error at standard sea level then the compressibility correction
above is not valid. See also link to equivalent
airspeed)

This approximation is valid up to about Mach
2.3.

Source: Aerodynamics of a Compressible Fluid.
Liepmann and Puckett 1947. Publishers John Wiley & Sons
Inc.

The difference between calibrated airspeed and
equivalent airspeed is negligible at low Mach numbers rising to 3%
at Mach 0.5 and 13% at Mach 1 depending on altitude.

The significance of equivalent airspeed is that
at Mach numbers below the onset of wave drag, all of the
aerodynamic forces and moments on an aircraft scale with the square
of the equivalent airspeed. The equivalent airspeed is closely
related to the Indicated
airspeed speed shown by the airspeed
indicator. Thus, the handling and 'feel' of an aircraft, and
the aerodynamic loads upon it, at a given equivalent airspeed, are
very nearly constant and equal to those at SL, ISA irrespective of
the actual flight conditions.

## True airspeed

True
airspeed (TAS) is the physical speed of the aircraft relative
to the air surrounding the aircraft. The true airspeed is a vector
quantity. The relationship between the true airspeed and the speed
with respect to the ground (V_g)is:

- V_t \ = \ V_g - V_w

Where:

- V_w = Windspeed vector

Aircraft flight instruments, however, don't
compute true airspeed as a function of groundspeed and windspeed.
They use impact and static pressures as well as a temperature
input. Basically, true airspeed is calibrated airspeed that is
corrected for pressure
altitude and temperature. The result is the true physical speed
of the aircraft plus or minus the wind component. True Airspeed is
equal to calibrated airspeed at standard sea level
conditions.

The simplest way to compute true airspeed is
using a function of Mach
number:

- V_t \ = \ A_0 \cdot M \sqrt

Where:

- A_0 = Speed of sound at standard sea level (661.4788 knots)
- M = Mach number
- T = Temperature (kelvins)
- T_0 = Standard sea level temperature (288.15 kelvins)

Or if Mach number is not known:

- V_t \ = \ A_0 \cdot \sqrt

Where:

- A_0 = Speed of sound at standard sea level (661.4788 knots)
- Q_c = Impact pressure (inHg)
- P = Static pressure (inHg)
- T = Temperature (kelvins)
- T_0 = Standard sea level temperature (288.15 kelvins)

The above equation is only for Mach numbers less
than 1.0.

True airspeed differs from the equivalent
airspeed because the airspeed indicator is calibrated at SL, ISA
conditions, where the air density is 1.225 kg/m³ , whereas
the air density in flight normally differs from this value.

- \frac \rho V^2 = q = \frac \rho_0 V_e^2

- \frac = \sqrt
- \rho \, is the air density at the flight condition.

The air density may be calculated from:

- \frac = \frac
- p \, is the air pressure at the flight condition,
- p_0 \, is the air pressure at sea level = 1013.2 hPa,
- T \, is the air temperature at the flight condition,
- T_0 \, is the air temperature at sea level, ISA = 288.15 K.

Source: Aerodynamics of a Compressible Fluid.
Liepmann and Puckett 1947. Publishers John Wiley & Sons
Inc.

## Groundspeed

Groundspeed is the speed of the aircraft relative to the ground rather than through the air, which can itself be moving.## See also

## References

- Glauert H., The Elements of Aerofoil and Airscrew Theory, Chapter 2, Cambridge University Press, 1947
- Liepmann H. W. and A. E. Pucket, Introduction to Aerodynamics of a Compressible Fluid, John Wiley and Sons, Inc. 1947

## External links

airspeed in Arabic: سرعة جوية

airspeed in Danish: Airspeed

airspeed in German: Fluggeschwindigkeit

airspeed in French: Vitesses
(aérodynamique)

airspeed in Korean: 대기속도

airspeed in Dutch: Luchtsnelheid

airspeed in Finnish: Ilmanopeus

airspeed in Chinese: 空速