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Indicated airspeed (IAS) is the airspeed read directly from the airspeed indicator (ASI) on an aircraft, driven by the pitot-static system.[1] It uses the difference between total pressure and static pressure, provided by the system, to either mechanically or electronically measure dynamic pressure. The dynamic pressure includes terms for both density and airspeed. Since the airspeed indicator cannot know the density, it is by design calibrated to assume the sea level standard atmospheric density when calculating airspeed. Since the actual density will vary considerably from this assumed value as the aircraft changes altitude, IAS varies considerably from true airspeed (TAS), the relative velocity between the aircraft and the surrounding air mass. Calibrated airspeed (CAS) is the IAS corrected for instrument and position error.[1] An aircraft's indicated airspeed in knots is typically abbreviated KIAS for "Knots-Indicated Air Speed" (vs. KCAS for calibrated airspeed and KTAS for true airspeed). The IAS is an important value for the pilot because it is the indicated speeds which are specified in the aircraft flight manual for such important performance values as the stall speed. These speeds, in true airspeed terms, vary considerably depending upon density altitude. However, at typical civilian operating speeds, the aircraft's aerodynamic structure responds to dynamic pressure alone, and the aircraft will perform the same when at the same dynamic pressure. Since it is this same dynamic pressure that drives the airspeed indicator, an aircraft will always, for example, stall at the published indicated airspeed (for the current configuration) regardless of density, altitude or true airspeed.[2] Furthermore, the IAS is specified in some regulations, and by air traffic control when directing pilots, since the airspeed indicator displays that speed (by definition) and it is the pilot's primary airspeed reference when operating below transonic or supersonic speeds.

Contents

1 Calculation 2 IAS vs CAS 3 IAS and V speeds 4 IAS and navigation 5 Other airspeeds 6 See also 7 Notes 8 References

Calculation[edit] Indicated airspeed measured by pitot-tube can be expressed by following equation delivered from Bernoulli's equation.

u =

2 (

p

t

p

s

)

ρ

displaystyle u= sqrt frac 2(p_ t -p_ s ) rho

NOTE: The above equation applies only to conditions that can be treated as incompressible. Liquids are treated as incompressible under almost all conditions. Gases under certain conditions can be approximated as incompressible. See Compressibility. The compression effects can be corrected by use of Poisson constant. This compensation corresponds to Equivalent airspeed.

u =

2 γ

γ − 1

p

s

ρ

[

(

p

t

p

s

+ 1

)

γ − 1

γ

− 1

]

displaystyle u= sqrt frac 2gamma gamma -1 frac p_ s rho left[left( frac p_ t p_ s +1right)^ frac gamma -1 gamma -1right]

where:

u

displaystyle u

is indicated airspeed in m/s;

p

t

displaystyle p_ t

is stagnation or total pressure in pascals;

p

s

displaystyle p_ s

is static pressure in pascals; and

ρ

displaystyle rho

is fluid density in

k g

/

m

3

displaystyle kg/m^ 3

.

  γ

displaystyle gamma ,

is the Poisson constant
Poisson constant
of a gas at a constant pressure to heat at a constant volume (1.4 for air).

IAS vs CAS[edit] The IAS is not the actual speed through the air even when the aircraft is at sea level under International Standard Atmosphere
International Standard Atmosphere
conditions (15 °C, 1013 hPa, 0% humidity). The IAS needs to be corrected for known instrument and position errors to show true airspeed under those specific atmospheric conditions, and this is the CAS (Calibrated Airspeed). Despite this the pilot's primary airspeed reference, the ASI, shows IAS (by definition). The relationship between CAS and IAS is known and documented for each aircraft type and model. IAS and V speeds[edit] The aircraft's pilot manual usually gives critical V speeds
V speeds
as IAS, those speeds indicated by the airspeed indicator. This is because the aircraft behaves similarly at the same IAS no matter what the TAS is: E.g. A pilot landing at a hot and high airfield will use the same IAS to fly the aircraft at the correct approach and landing speeds as he would when landing at a cold sea level airfield even though the TAS must differ considerably between the two landings. Whereas IAS can be reliably used for monitoring critical speeds well below the speed of sound this is not so at higher speeds. An example: Because (1) the compressibility of air changes considerably approaching the speed of sound, and (2) the speed of sound varies considerably with temperature and therefore altitude; the maximum speed at which an aircraft structure is safe, the never exceed speed (abbreviated VNE), is specified at several differing altitudes in faster aircraft's operating manuals, as shown in the sample table below.

Diving below mph IAS kmh IAS

30,000 ft 370 595

25,000 ft 410 660

20,000 ft 450 725

15,000 ft 490 790

10,000 ft 540 870

Ref: Pilot's Notes for Tempest V Sabre IIA Engine - Air Ministry A.P.2458C-PN IAS and navigation[edit] For navigation, it is necessary to convert IAS to TAS and/or ground speed (GS) using the following method:

correct IAS to calibrated airspeed (CAS) using an aircraft-specific correction table; correct CAS to true airspeed (TAS) by using Outside Air Temperature (OAT), Pressure-altitude and CAS on an E6B
E6B
flight computer or equivalent functionality on most GPSs; convert TAS to ground speed (GS) by allowing for the effect of wind.

With the advent of Doppler radar
Doppler radar
navigation and, more recently, GPS receivers, with other advanced navigation equipment that allows pilots to read ground speed directly, the TAS calculation in-flight is becoming unnecessary for the purposes of navigation estimations. TAS is the primary method to determine aircraft's cruise performance in manufacturer's specs, speed comparisons and pilot reports. Other airspeeds[edit] From IAS, the following speeds can also be calculated:

convert CAS to equivalent airspeed (EAS) by allowing for compressibility effects (not necessary at slow speed or low altitude); EAS is used by aircraft engineers and some very high-altitude flying aircraft such as the U-2 and the SR-71; convert EAS to true airspeed (TAS) by allowing for differences in density altitude.

On large jet aircraft the IAS is by far the most important speed indicator. Most aircraft speed limitations are based off IAS, as IAS closely reflects dynamic pressure. TAS is usually displayed as well, but purely for advisory information and generally not in a prominent location. Modern jet airliners also include ground speed (GS) and Machmeter. Ground speed shows the actual speed that the aircraft uses compared to the ground. This is usually connected to a GPS
GPS
or similar system. Ground speed is just a pilot aid to estimate if the flight is on time, behind or ahead of schedule. It is not used for takeoff and landing purposes, since the imperative speed for a flying aircraft always is the speed against the wind. The Machmeter
Machmeter
is, on subsonic aircraft, a warning indicator. Subsonic aircraft must not fly faster than a specific percentage of the speed of sound. Usually passenger airliners do not fly faster than around 85% of speed of sound, or Mach 0.85. Supersonic
Supersonic
aircraft, like the Concorde
Concorde
and military fighters, use the Machmeter
Machmeter
as the main speed instrument with the exception of take-offs and landings. Some aircraft also have a taxi speed indicator for use on the ground. Since the IAS often starts at around 40-50 knots (on jet airliners), pilots may need extra help while taxiing the aircraft on the ground. Its range is around 0 to 50 knots. See also[edit]

Acronyms and abbreviations in avionics True airspeed Equivalent airspeed Calibrated airspeed

Notes[edit]

^ a b Clancy, L.J. (1975), Aerodynamics, Section 3.9, Pitman Publishing Limited, London. ISBN 0-273-01120-0 ^ Kermode, A.C.,Mechanics of Flight, 8th Edition – page 64. Longman Group Limited, London ISBN 0-582-23740-8

References[edit]

Gracey, William (1980), "Measurement of Aircraft Speed and Altitude" (11 MB), NASA Reference

.