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]
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
IAS vs CAS[edit]
The IAS is not the actual speed through the air even when the aircraft
is at sea level under
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
With the advent of
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.
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 |