# Ideal Airspeed Correction

Calculate equivalent airspeed (EAS), calibrated airspeed (CAS), or true airspeed (TAS) from each other

## Library

Flight Parameters

## Description

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).

`$TAS=\frac{EAS×a}{{a}_{0}\sqrt{\delta }}$`

Using the compressible form of Bernoulli's equation and assuming isentropic conditions, the last two equations for EAS and CAS are derived.

`$EAS=\sqrt{\frac{2\gamma P}{\left(\gamma -1\right){\rho }_{0}}\left({\left(\frac{q}{P}+1\right)}^{\left(\gamma -1\right)/\gamma }-1\right)}$`
`$CAS=\sqrt{\frac{2\gamma {P}_{0}}{\left(\gamma -1\right){\rho }_{0}}\left({\left(\frac{q}{{P}_{0}}+1\right)}^{\left(\gamma -1\right)/\gamma }-1\right)}$`

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.

## Parameters

Units

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

Airspeed input

Specify the airspeed input type:

 `TAS` True airspeed `EAS` Equivalent airspeed `CAS` Calibrated airspeed

Airspeed output

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)

Action for out-of-range input

Specify if an out-of-range input (supersonic airspeeds) invokes a warning, an error, or no action.

## Inputs and Outputs

InputDimension TypeDescription

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.

OutputDimension TypeDescription

First

Contains the selected airspeed in the selected units.

## Assumptions and Limitations

This block assumes that the air flow is compressible, isentropic (subsonic flow), dry air with constant specific heat ratio, γ.

## Examples

See the `aeroblk_indicated` model and the `aeroblk_calibrated` model for examples of this block.

## References

Lowry, J. T., Performance of Light Aircraft, AIAA Education Series, Washington, DC, 1999.

Aeronautical Vestpocket Handbook, United Technologies Pratt & Whitney, August, 1986.