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      FDC help: Turbulence Group 2
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    <h2>
      The turbulence block <i>Turbulence Group 2</i>
    </h2>
    <p><i>Turbulence Group 2</i> is a Simulink subsystem which is contained in the Atmospheric
    Turbulence sublibrary of the library <i><a href=
    "windlib.htm">WINDLIB</a></i>. It contains the Dryden filters <i><a href=
    "udryd2.htm">UDRYD2</a></i>, <i><a href="vdryd2.htm">VDRYD2</a></i>, and
    <i><a href="wdryd2.htm">WDRYD2</a></i> for longitudinal, lateral and
    vertical turbulence, respectively.</p>
    <p> The filters have been implemented in
    masked subsystem blocks, in which the user must specify the scale lengths
    <i>L_u<sub>g</sub></i>, <i>L_v<sub>g</sub></i>, and <i>L_w<sub>g</sub></i>,
    and the standard deviations <i>sigma_u<sub>g</sub></i>,
    <i>sigma_v<sub>g</sub></i>, and <i>sigma_w<sub>g</sub></i>.</p>
     <p>In <i>Turbulence Group 2</i>, the coefficients of the Dryden filters vary with the true
    airspeed <i>V</i>, which enters this subsystem through an <i>Inport</i>
    block. The value of the true airspeed must be obtained from the non-linear
    model of the aircraft under consideration, e.g., <i>V</i> may be extracted
    from the outputvector of the non-linear aircraft model <i><a href=
    "beaver.htm">Beaver</a></i>, which itself uses the outputs of <i>Turbulence Group 2</i>
    as input signals.</p>
     <p>In many cases, the variations of <i>V</i> due to atmospheric turbulence
    are not very large, so the resulting changes in filter coefficients may be
    neglected. In such cases, it is recommended to use <i><a href=
    "Turb1.htm">Turbulence Group 1</a></i> instead of <i>Turbulence Group 2</i>, thus eliminating the
    feedback-loop of <i>V</i> from the aircraft model to <i>Turbulence Group 2</i> within
    the resulting Simulink system. <i>Turbulence Group 1</i> uses the simpler Dryden
    implementations <i><a href="udryd1.htm">UDRYD1</a></i>, <i><a href=
    "vdryd1.htm">VDRYD1</a></i>, and <i><a href="wdryd1.htm">WDRYD1</a></i>.</p> 
    <h3>
      Inputsignal
    </h3>
<pre>
  V = true airspeed [m/s]
</pre>
    <h3>
      Outputvector
    </h3>
<pre>
  uwind = [uw vw ww uwdot vwdot wwdot]&#39;
</pre>
    (Actually, this vector should have been called <code>ugust = [ug vg wg
    ugdot vgdot wgdot]&#39;</code>, since the wind velocities and their
    time-derivatives from the subsystem <i>Turbulence Group 1</i> represent gust velocities
    only; contributions due to <i>wind</i> (-shear) still have to be added to
    this vector! See <i><a href="cwind.htm">Cwind</a></i> and <i><a href=
    "blwind.htm">BLwind</a></i> for more information on the implementation of
    constant wind and a wind-profile in the Earth&#39;s boundary layer,
    respectively.). 
<pre>
 uw   : wind (gust-) velocity along XB-axis of aircraft [m/s]
 vw   : wind (gust-) velocity along YB-axis of aircraft [m/s]
 ww   : wind (gust-) velocity along ZB-axis of aircraft [m/s]

 uwdot: time-derivative of uw [m/s^2]
 vwdot: time-derivative of vw [m/s^2]
 wwdot: time-derivative of ww [m/s^2]
</pre>
    <h3>
      Block parameters</i>
    </h3>
    Before starting a simulation of a system in which the subsystem
    <i>Turbulence Group 2</i> is involved, the parameters of the masked subsystem blocks
    <i><a href="udryd2.htm">UDRYD2</a></i>, <i><a href=
    "vdryd2.htm">VDRYD2</a></i>, and <i><a href="wdryd2.htm">WDRYD2</a></i>
    have to be defined by double-clicking these blocks and entering the correct
    values in their dialog-boxes. These parameters are:<br>
    <br>
     <i>UDRYD2</i>: 
<pre>
  Lug      = scale length for longitudinal turbulence [m]
  sigma_ug = standard deviation for longitudinal
                                           turbulence [m/s]
</pre>

    <i>VDRYD2</i>: 
<pre>
  Lvg      = scale length for lateral turbulence [m]
  sigma_vg = standard deviation for lateral turbulence [m/s]
</pre>
    <i>WDRYD2</i>: 
<pre>
  Lwg      = scale length for vertical turbulence [m]
  sigma_wg = standard deviation for vertical turbulence [m/s]
</pre>
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