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      FDC help: FIXSTATE
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    <h2>
      The utility <i>FIXSTATE</i>
    </h2>
    <p><i>FIXSTATE</i> is a Matlab program, which can be used to artificially fix
    state variables of the non-linear aircraft model (in this case the Simulink
    <i><a href="beaver.htm">Beaver</a></i> model).</p>
    <p><i>FIXSTATE</i> sets the
    value of the gain vector <i>xfix</i>, which is a multiplication factor for
    the time-derivative of the state vector (<i>xdot = dx/dt</i>) within the
    non-linear aircraft model. The corresponding gain block <i><a href=
    "xfix.htm">XFIX</a></i> can be found in the subsystem <i><a href=
    "eqmotion.htm">Aircraft equations of motion</a></i> within the <i>Beaver</i> model.</p>
    <p>The gain-factor <i>xfix</i> is applied as follows: the
    time-derivative of the state-vector, used by the Simulink integrator, is
    equal to: </p>
<pre>
  xdot_used = xdot_true .* xfix
</pre>
    <p>(where the <code>.*</code> symbol corresponds with the Matlab operator for
    element-by-element of vectors). The elements of <i>xfix</i> are either
    equal to one or zeros. If <i>xfix = ones(1,12)</i>,
    <i>xdot_used</i> will be equal to <i>xdot_true</i>, which means that none
    of the state variables will be artificially fixed. If one or more elements
    of <i>xfix</i> is (are) equal to zero, the time-derivatives of the
    corresponding states will remain zero, hence, these states will remain
    equal to their initial condition.</p>
     <p>For instance: if the true airspeed (that is: the first element of the
    state vector <i>x</i>) needs to be kept constant, <i>xfix</i> has to be set equal to
    <code>[0&nbsp;1&nbsp;1&nbsp;1&nbsp;1&nbsp;1&nbsp;1&nbsp;1&nbsp;1&nbsp;1&nbsp;1&nbsp;1]</code>. This example may, for instance, be useful if one wants to compare
    a speed hold autopilot mode with the &#39;ideal&#39; response with
    artificially fixed airspeed. Another practical purpose for <i>FIXSTATE</i>
    is the determination of purely longitudinal or lateral dynamics, without
    cross-coupling effects.</p>
     <p><b>Note:</b> the variable <i>xfix</i> needs to be defined in the workspace
    if the system <i>Beaver</i> (or a subsystem equivalent thereof) is
    evaluated for simulation, trimming, and/or linearization purposes. If this variable can't be found when such a task is started, the default value <i>xfix = ones(1,12)</i> will be used automatically.</p>
    <h3>
      Definition of the state-vector
    </h3>
<pre>
  x     = [V alpha beta p q r psi theta phi xe ye H]&#39;

  V     = true airspeed [m/s]
  alpha = angle of attack [rad]
  beta  = sideslip angle [rad]
  p     = roll-rate [rad/s]
  q     = pitch-rate [rad/s]
  r     = yaw-rate [rad/s]
  psi   = yaw angle [rad]
  theta = pitch angle [rad]
  phi   = roll angle [rad]
  xe    = X-coordinate of aircraft&#39;s c.g. [m]
  ye    = Y-coordinate of aircraft&#39;s c.g. [m]
  H     = altitude of aircraft&#39;s c.g. [m] (above sea level)
</pre>
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