Kcas Calculator

This free calculator estimates the number of driving miles between two locations in the United States, and also provides written directions. Use this in conjunction with the gas mileage calculator to plan the gas cost of a road trip, or explore hundreds of other calculators addressing health, finance, math, fitness, and more. The last guest posting is correct. Indicated airspeed compared to miles per hour is akin to comparing apples to camels. The calculatiion that does make sense is to compare groundspeed in knots to miles per hour. Calibrated airspeed is expressed in knots and is abbreviated KCAS. Normally it doesn't differ much from IAS. True airspeed (TAS) - IAS cor rected for instrument installation error, compressibility error, and errors due to variations.

Calibrated airspeed (CAS) is indicated airspeed corrected for instrument and position error.

When flying at sea level under International Standard Atmosphere conditions (15 °C, 1013 hPa, 0% humidity) calibrated airspeed is the same as equivalent airspeed (EAS) and true airspeed (TAS). If there is no wind it is also the same as ground speed (GS). Under any other conditions, CAS may differ from the aircraft's TAS and GS.

Calibrated airspeed in knots is usually abbreviated as KCAS, while indicated airspeed is abbreviated as KIAS.

In some applications, notably British usage, the expression rectified airspeed is used instead of calibrated airspeed.^[1]

Practical applications of CAS[edit]

CAS has two primary applications in aviation:

• for navigation, CAS is traditionally calculated as one of the steps between indicated airspeed and true airspeed;
• for aircraft control, CAS (and EAS) are the primary reference points, since they describe the dynamic pressure acting on aircraft surfaces regardless of density, altitude, wind, and other conditions. EAS is used as a reference by aircraft designers, but EAS cannot be displayed correctly at varying altitudes by a simple (single capsule) airspeed indicator. CAS is therefore a standard for calibrating the airspeed indicator such that CAS equals EAS at sea level pressure and approximates EAS at higher altitudes.

With the widespread use of GPS and other advanced navigation systems in cockpits, the first application is rapidly decreasing in importance – pilots are able to read groundspeed (and often true airspeed)directly, without calculating calibrated airspeed as an intermediate step. The second application remains critical, however – for example, at the same weight, an aircraft will rotate and climb at approximately the same calibrated airspeed at any elevation, even though the true airspeed and groundspeed may differ significantly. These V speeds are usually given as IAS rather than CAS, so that a pilot can read them directly from the airspeed indicator.

Calculation from impact pressure[edit]

Since the airspeed indicator capsule responds to impact pressure,^[2] CAS is defined as a function of impact pressure alone. Static pressure and temperature appear as fixed coefficients defined by convention as standard sea level values. It so happens that the speed of sound is a direct function of temperature, so instead of a standard temperature, we can define a standard speed of sound.

For subsonic speeds, CAS is calculated as:

${\displaystyle CAS=a_0\sqrt{5\left[{\left(\frac{q_c}{P_0}+1\right)}^{\frac{2}{7}}-1\right]}}$

where:

• ${\displaystyle q_c}$ = impact pressure
• ${\displaystyle P_0}$ = standard pressure at sea level
• ${\displaystyle a_0}$ is the standard speed of sound at 15 °C

For supersonic airspeeds, where a normal shock forms in front of the pitot probe, the Rayleigh formula applies:

${\displaystyle CAS=a_0{\left[\left(\frac{q_c}{P_0}+1\right)\times {\left(7{\left(\frac{CAS}{a_0}\right)}^2-1\right)}^{2.5}/\left(6^{2.5}\times {1.2}^{3.5}\right)\right]}^{(1/7)}}$

The supersonic formula must be solved iteratively, by assuming an initial value for ${\displaystyle CAS}$ equal to ${\displaystyle a_0}$.

These formulae work in any units provided the appropriate values for ${\displaystyle P_0}$ and ${\displaystyle a_0}$ are selected. For example, ${\displaystyle P_0}$ = 1013.25 hPa, ${\displaystyle a_0}$ = 1,225 km/h (661.45 kn). The ratio of specific heats for air is assumed to be 1.4.

These formulae can then be used to calibrate an airspeed indicator when impact pressure (${\displaystyle q_c}$) is measured using a water manometer or accurate pressure gauge. If using a water manometer to measure millimeters of water the reference pressure (${\displaystyle P_0}$) may be entered as 10333 mm ${\displaystyle H_{2}0}$.

At higher altitudes CAS can be corrected for compressibility error to give equivalent airspeed (EAS). In practice compressibility error is negligible below about 3,000 m (10,000 ft) and 370 km/h (200 kn).

See also[edit]

References[edit]

1. ^Clancy, L. J. (1975) Aerodynamics, pp 31, 32. Pitman Publishing Limited, London. ISBN0 273 01120 0
2. ^Some authors in the field of compressible flows use the term dynamic pressure or compressible dynamic pressure instead of impact pressure.

Kcas Calculator Math

Bibliography[edit]

Kcas To Ktas Calculator

• Blake, Walt (2009). Jet Transport Performance Methods. Seattle: Boeing Commercial Airplanes.
• Gracey, William (1980), 'Measurement of Aircraft Speed and Altitude' (11 MB), p. 15, NASA Reference Publication 1046.

External links[edit]

• JavaScript Calibrated Airspeed calculator from True Airspeed and other variables at luizmonteiro.com

Kcas To Ktas Calculator