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Calculate equivalent airspeed (EAS), calibrated airspeed (CAS), or true airspeed (TAS) from each other
The Ideal Airspeed Correction block calculates one of the following airspeeds: equivalent airspeed (EAS), calibrated airspeed (CAS), or true airspeed (TAS), from one of the other two airspeeds.
Three equations are used to implement the Ideal Airspeed Correction block. The first equation shows TAS as a function of EAS, relative pressure ratio at altitude (δ), and speed of sound at altitude (a).
$$TAS=\frac{EAS\times a}{{a}_{0}\sqrt{\delta}}$$
Using the compressible form of Bernoulli's equation and assuming isentropic conditions, the last two equations for EAS and CAS are derived.
$$EAS=\sqrt{\frac{2\gamma P}{\left(\gamma -1\right){\rho}_{0}}\left({\left(\frac{q}{P}+1\right)}^{(\gamma -1)/\gamma}-1\right)}$$
$$CAS=\sqrt{\frac{2\gamma {P}_{0}}{\left(\gamma -1\right){\rho}_{0}}\left({\left(\frac{q}{{P}_{0}}+1\right)}^{(\gamma -1)/\gamma}-1\right)}$$
In order to generate a correction table and its approximate inverse, these two equations were solved for dynamic pressure (q). Having values of q by a function of EAS and ambient pressure at altitude (P) or by a function of CAS, allows the two equations to be solved using the other's solution for q, thus creating a solution for EAS that depends on P and CAS and a solution for CAS that depends on P and EAS.
Specifies the input and output units:
Units | Airspeed Input | Speed of Sound | Air Pressure | Airspeed Output |
---|---|---|---|---|
Metric (MKS) | Meters per second | Meters per second | Pascal | Meters per second |
English (Velocity in ft/s) | Feet per second | Feet per second | Pound force per square inch | Feet per second |
English (Velocity in kts) | Knots | Knots | Pound force per square inch | Knots |
Specify the airspeed input type:
TAS | True airspeed |
EAS | Equivalent airspeed |
CAS | Calibrated airspeed |
Specify the airspeed output type:
Velocity Input | Velocity Output |
---|---|
TAS | EAS (equivalent airspeed) |
CAS (calibrated airspeed) | |
EAS | TAS (true airspeed) |
CAS (calibrated airspeed) | |
CAS | TAS (true airspeed) |
EAS (equivalent airspeed) |
Specify if an out-of-range input (supersonic airspeeds) invokes a warning, an error, or no action.
Input | Dimension Type | Description |
---|---|---|
First | Contains the selected airspeed in the selected units. | |
Second | Contains the speed of sound in the selected units. | |
Third | Contains the static pressure in the selected units. |
Output | Dimension Type | Description |
---|---|---|
First | Contains the selected airspeed in the selected units. |
This block assumes that the air flow is compressible, isentropic (subsonic flow), dry air with constant specific heat ratio, γ.
See the aeroblk_indicatedaeroblk_indicated model and the aeroblk_calibratedaeroblk_calibrated model for examples of this block.