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# Wind Shear Model

Calculate wind shear conditions

Environment/Wind

## Description

The Wind Shear Model block adds wind shear to the aerospace model. This implementation is based on the mathematical representation in the Military Specification MIL-F-8785C [1]. The magnitude of the wind shear is given by the following equation for the mean wind profile as a function of altitude and the measured wind speed at 20 feet (6 m) above the ground.

where uw is the mean wind speed, W20 is the measured wind speed at an altitude of 20 feet, h is the altitude, and z0 is a constant equal to 0.15 feet for Category C flight phases and 2.0 feet for all other flight phases. Category C flight phases are defined in reference [1] to be terminal flight phases, which include takeoff, approach, and landing.

The resultant mean wind speed in the flat Earth axis frame is changed to body-fixed axis coordinates by multiplying by the direction cosine matrix (DCM) input to the block. The block output is the mean wind speed in the body-fixed axis.

## Dialog Box

Units

Define the units of wind shear.

UnitsWindAltitude
Metric (MKS)Meters/secondMeters
English (Velocity in ft/s)Feet/secondFeet

English (Velocity in kts)

KnotsFeet

Flight phase

Select flight phase:

• Category C Terminal Flight Phases

• Other

Wind speed at 6 m (20 feet) altitude (m/s, f/s, or knots)

The measured wind speed at an altitude of 20 feet (6 m) above the ground.

Wind direction at 6 m (20 feet) altitude (degrees clockwise from north)

The direction of the wind at an altitude of 20 feet (6 m), measured in degrees clockwise from the direction of the Earth x-axis (north). The wind direction is defined as the direction from which the wind is coming.

## Inputs and Outputs

InputDimension TypeDescription

First

Contains the altitude in units selected.

Second

3-by-3 direction cosine matrix

OutputDimension TypeDescription

First

3-by-1 vector Contains the mean wind speed in the body axes frame, in the selected units.

## Examples

See Wind Shear ModelWind Shear Model in aeroblk_HL20aeroblk_HL20 for an example of this block.

## Reference

U.S. Military Specification MIL-F-8785C, 5 November 1980.