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 these airspeeds from one of the other two airspeeds:

• Equivalent airspeed (EAS)

• Calibrated airspeed (CAS)

• True airspeed (TAS)

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)

Method

Specify a method for computing the conversion factor. The block might generate associated lookup table data depending on the setting of the Subsonic airspeeds only check box.

Table Lookup

(Default) Generate output airspeed by looking up or estimating table values based on block inputs. If the Subsonic airspeeds only check box is selected, the Ideal Airspeed Correction block generates code that includes subsonic (Mach < 1) lookup table data. If the Subsonic airspeeds only check box is cleared, the Ideal Airspeed Correction block generates code that includes all (Mach < 5) lookup table data. Beyond Mach 5, use the Equation method. The Table Lookup method is not recommended for either of these instances: Speed of sound less than 200 m/s or greater than 350 m/s. Static pressure less than 1000 Pa or greater than 106,500 Pa. Using the Table Lookup method in these instances causes inaccuracies.

Equation

Compute output airspeed directly using block input values. Calculations involving supersonic airspeeds (greater than Mach 1) require an iterative computation. If the function does not find a solution within 30 iterations, it displays an error message. The block does not include lookup table data in generated code.

The Ideal Airspeed Correction block automatically uses the Equation method for any of these instances:

• Conversion with Airspeed input set to TAS and Airspeed output set to EAS.

• Conversion with Airspeed input set to EAS and Airspeed output set to TAS.

• Conversion with block input airspeed is greater than five times the speed of sound at sea level (approximately 1700 m/s).

Subsonic airspeeds only

Select this check box to use this block only with subsonic airspeed (airspeeds less than Mach 1) applications. Selecting this check box may improve performance.

The block generates code as follows:

• If this check box is selected, the Ideal Airspeed Correction block generates code that includes subsonic (Mach < 1) lookup table data if Method is set to Table Lookup.

  Selecting this check box displays the Action for out-of-range input parameter.

• If this check box is cleared, the Ideal Airspeed Correction block generates code that includes all (Mach < 5) lookup table data if Method is set to Table Lookup.

Action for out-of-range input

Specify action to take in case of out-of-range input, where airspeed is greater than Mach 1. This parameter displays if the Subsonic airspeeds only check box is selected.

Value	Description
None (default)	Does not display warning or error.
Warning	Displays warning and indicates that the airspeed is greater than Mach 1.
Error	Displays error and indicates that the airspeed is greater than Mach 1.

Inputs and Outputs

Input	Dimension Type	Description
TAS/EAS/CAS	double	Contains the selected airspeed in the selected units.
a	double	Contains the speed of sound in the selected units.
P0	double	Contains the static pressure in the selected units.

Output	Dimension Type	Description
TAS/EAS/CAS	double	Contains the selected airspeed in the selected units.

Assumptions and Limitations

This block assumes that the air flow is compressible 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.

Gracey, William, Measurement of Aircraft Speed and Altitude, NASA Reference Publication 1046, 1980.