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UDRYD2 = Longitudinal Dryden filter #2. This masked subsystem
block, which is contained in the library WINDLIB from the sub-
directory WIND, implements a longitudinal Dryden filter with
coefficients that vary with the true airspeed V. The user must
specify the scale lenght Lug and the standard deviation sigma_ug.
The airspeed V is used as inputsignal to the block UDRYD2. Often,
the changes in V due to atmospheric turbulence are not very large.
In such cases, you may wish to use a Dryden filter with constant
coefficients, computed under the assumption that V is constant
and equal to a user-specified value, in stead. The block UDRYD1
contains such a filter for longitudinal turbulence (see
UDRYD1.HLP for more info).

The longitudinal gust velocity is obtained by passing white
noise through a Dryden filter, which is defined by the following
transfer function:

               K
  Hug  =  -----------          K   = sigma_ug * sqrt(2*Lug/V)
           1 + tau*s           tau = Lug/V

where:

  V       : true airspeed [m/s]
  sigma_ug: standard deviation of longitudinal gust velocity [m/s]
  Lug     : scale length for longitudinal gust [m]

Here, V is the INPUTSIGNAL to the masked subsystem block. V it-
self has to be obtained from the aircraft model considered, e.g.,
V may be an outputsignal from the system BEAVER, which uses the
turbulence velocity ug and its time-derivative as inputsignals!

Input to the block UDRYD2:
==========================
  V       : true airspeed [m/s]

Outputs from the block UDRYD2:
==============================
  ug      : longitudinal gust velocity [m/s]
  d(ug)/dt: time derivative of ug [m/s^2]

The gust velocity and its time derivative have to be added to the
total wind velocity component uw and its time derivative uwdot in
the vector uwind.

More info.
==========
See VDRYD2.HLP, WDRYD2.HLP, and TURB2.HLP for more info about
other Dryden filters with coefficients that vary with the air-
speed V. See UDRYD1.HLP, VDRYD1.HLP, WDRYD1.HLP, and TURB1.HLP
for more info about Dryden filters with constant coefficients,
assuming V = constant.

See CWIND.HLP and BLWIND.HLP for more information about blocks
to compute the components of wind along the aircraft's body-axes
(constant wind and a wind profile in the Earth's boundary layer,
respectively). The use of the vector uwind within the aircraft
model is outlined in the helpfiles LEVEL1.HLP, FWIND.HLP, and
XYHDOT.HLP, which refer to blocks within the system BEAVER.

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The FDC toolbox, Copyright Marc Rauw 1994-1998.