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Generate discrete wind turbulence with Dryden velocity spectra
The Dryden Wind Turbulence Model (Discrete) block uses the Dryden spectral representation to add turbulence to the aerospace model by using bandlimited white noise with appropriate digital filter finite difference equations. This block implements the mathematical representation in the Military Specification MILF8785C and Military Handbook MILHDBK1797.
According to the military references, turbulence is a stochastic process defined by velocity spectra. For an aircraft flying at a speed V through a frozen turbulence field with a spatial frequency of Ω radians per meter, the circular frequency ω is calculated by multiplying V by Ω . The following table displays the component spectra functions:
MILF8785C  MILHDBK1797  

Longitudinal  






Lateral  






Vertical  






The variable b represents the aircraft wingspan. The variables L_{u} , L_{v} , L_{w} represent the turbulence scale lengths. The variables σ_{u} , σ_{v} , σ_{w} represent the turbulence intensities.
The spectral density definitions of turbulence angular rates are defined in the references as three variations, which are displayed in the following table:



The variations affect only the vertical (q_{g}) and lateral (r_{g}) turbulence angular rates.
Keep in mind that the longitudinal turbulence angular rate spectrum, Φ_{p}(ω) , is a rational function. The rational function is derived from curvefitting a complex algebraic function, not the vertical turbulence velocity spectrum, Φ_{w}(ω) , multiplied by a scale factor. Because the turbulence angular rate spectra contribute less to the aircraft gust response than the turbulence velocity spectra, it may explain the variations in their definitions.
The variations lead to the following combinations of vertical and lateral turbulence angular rate spectra:
Vertical  Lateral 

Φ_{q}(ω) Φ_{q}(ω) −Φ_{q}(ω)  −Φ_{r}(ω) Φ_{r}(ω) Φ_{r}(ω) 
To generate a signal with the correct characteristics, a unit variance, bandlimited white noise signal is used in the digital filter finite difference equations.
The following table displays the digital filter finite difference equations:
MILF8785C  MILHDBK1797  

Longitudinal  






Lateral  






Vertical  






Divided into two distinct regions, the turbulence scale lengths and intensities are functions of altitude.
According to the military references, the turbulence scale lengths at low altitudes, where h is the altitude in feet, are represented in the following table:
MILF8785C  MILHDBK1797 



The turbulence intensities are given below, where W_{20} is the wind speed at 20 feet (6 m). Typically for light turbulence, the wind speed at 20 feet is 15 knots; for moderate turbulence, the wind speed is 30 knots, and for severe turbulence, the wind speed is 45 knots.
The turbulence axes orientation in this region is defined as follows:
Longitudinal turbulence velocity, u_{g} , aligned along the horizontal relative mean wind vector
Vertical turbulence velocity, w_{g} , aligned with vertical.
At this altitude range, the output of the block is transformed into body coordinates.
For medium to high altitudes the turbulence scale lengths and intensities are based on the assumption that the turbulence is isotropic. In the military references, the scale lengths are represented by the following equations:
MILF8785C  MILHDBK1797 

L_{u }= L_{v }= L_{w} = 1750 ft  L_{u }= 2L_{v }= 2L_{w} = 1750 ft 
The turbulence intensities are determined from a lookup table that provides the turbulence intensity as a function of altitude and the probability of the turbulence intensity being exceeded. The relationship of the turbulence intensities is represented in the following equation: σ_{u} = σ_{v} = σ_{w} .
The turbulence axes orientation in this region is defined as being aligned with the body coordinates.
At altitudes between 1000 feet and 2000 feet, the turbulence velocities and turbulence angular rates are determined by linearly interpolating between the value from the low altitude model at 1000 feet transformed from mean horizontal wind coordinates to body coordinates and the value from the high altitude model at 2000 feet in body coordinates.
Define the units of wind speed due to the turbulence.
Units  Wind Velocity  Altitude  Airspeed 

Metric (MKS)  Meters/second  Meters  Meters/second 
English (Velocity in ft/s)  Feet/second  Feet  Feet/second 
English (Velocity in kts)  Knots  Feet  Knots 
Define which military reference to use. This affects the application of turbulence scale lengths in the lateral and vertical directions
Select the wind turbulence model to use:
Continuous Von Karman (+q r)  Use continuous representation of Von Kármán velocity spectra with positive vertical and negative lateral angular rates spectra. 
Continuous Von Karman (+q +r)  Use continuous representation of Von Kármán velocity spectra with positive vertical and lateral angular rates spectra. 
Continuous Von Karman (q +r)  Use continuous representation of Von Kármán velocity spectra with negative vertical and positive lateral angular rates spectra. 
Continuous Dryden (+q r)  Use continuous representation of Dryden velocity spectra with positive vertical and negative lateral angular rates spectra. 
Continuous Dryden (+q +r)  Use continuous representation of Dryden velocity spectra with positive vertical and lateral angular rates spectra. 
Continuous Dryden (q +r)  Use continuous representation of Dryden velocity spectra with negative vertical and positive lateral angular rates spectra. 
Discrete Dryden (+q r)  Use discrete representation of Dryden velocity spectra with positive vertical and negative lateral angular rates spectra. 
Discrete Dryden (+q +r)  Use discrete representation of Dryden velocity spectra with positive vertical and lateral angular rates spectra. 
Discrete Dryden (q +r)  Use discrete representation of Dryden velocity spectra with negative vertical and positive lateral angular rates spectra. 
The Discrete Dryden selections conform to the transfer function descriptions.
The measured wind speed at a height of 6 meters (20 feet) provides the intensity for the lowaltitude turbulence model.
The measured wind direction at a height of 6 meters (20 feet) is an angle to aid in transforming the lowaltitude turbulence model into a body coordinates.
Above 2000 feet, the turbulence intensity is determined from a lookup table that gives the turbulence intensity as a function of altitude and the probability of the turbulence intensity's being exceeded.
The turbulence scale length above 2000 feet is assumed constant, and from the military references, a figure of 1750 feet is recommended for the longitudinal turbulence scale length of the Dryden spectra.
The wingspan is required in the calculation of the turbulence on the angular rates.
The sample time at which the unit variance white noise signal is generated and at which the discrete filters are updated.
There are four random numbers required to generate the turbulence signals, one for each of the three velocity components and one for the roll rate. The turbulences on the pitch and yaw angular rates are based on further shaping of the outputs from the shaping filters for the vertical and lateral velocities.
Selecting the check box generates the turbulence signals.
Input  Dimension Type  Description 

First  Contains the altitude, in units selected.  
Second  Contains the aircraft speed, in units selected.  
Third  Contains the NED direction cosine matrix. 
Output  Dimension Type  Description 

First  Threeelement signal  Contains the turbulence velocities, in the selected units. 
Second  Threeelement signal  Contains the turbulence angular rates, in radians per second. 
The "frozen turbulence field" assumption is valid for the cases of meanwind velocity and the rootmeansquare turbulence velocity, or intensity, is small relative to the aircraft's ground speed.
The turbulence model describes an average of all conditions for clear air turbulence because the following factors are not incorporated into the model:
Terrain roughness
Lapse rate
Wind shears
Mean wind magnitude
Other meteorological factions (except altitude)
U.S. Military Handbook MILHDBK1797, 19 December 1997.
U.S. Military Specification MILF8785C, 5 November 1980.
Chalk, C., Neal, P., Harris, T., Pritchard, F., Woodcock, R., "Background Information and User Guide for MILF8785B(ASG), `Military SpecificationFlying Qualities of Piloted Airplanes'," AD869856, Cornell Aeronautical Laboratory, August 1969.
Hoblit, F., Gust Loads on Aircraft: Concepts and Applications, AIAA Education Series, 1988.
Ly, U., Chan, Y., "TimeDomain Computation of Aircraft Gust Covariance Matrices," AIAA Paper 801615, Atmospheric Flight Mechanics Conference, Danvers, Massachusetts, August 1113, 1980.
McRuer, D., Ashkenas, I., Graham, D., Aircraft Dynamics and Automatic Control, Princeton University Press, July 1990.
Moorhouse, D., Woodcock, R., "Background Information and User Guide for MILF8785C, `Military SpecificationFlying Qualities of Piloted Airplanes'," ADA119421, Flight Dynamic Laboratory, July 1982.
McFarland, R., "A Standard Kinematic Model for Flight Simulation at NASAAmes," NASA CR2497, Computer Sciences Corporation, January 1975.
Tatom, F., Smith, R., Fichtl, G., "Simulation of Atmospheric Turbulent Gusts and Gust Gradients," AIAA Paper 810300, Aerospace Sciences Meeting, St. Louis, Missouri, January 1215, 1981.
Yeager, J., "Implementation and Testing of Turbulence Models for the F18HARV Simulation," NASA CR1998206937, Lockheed Martin Engineering & Sciences, March 1998.