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scale-length of the turbulence Lug , the standard deviation sug , and the (expected) mean
airspeed of the aircraft. UDRYD1 does not take into account variations of the filter coefficients
with the airspeed, as the influence of those variations is usually very small; if this
limitation is not acceptable, use UDRYD2 instead.
Equations
For a detailed discussion of the equations from the block UDRYD1 and the underlying
theory, refer to section 4.2.
• Longitudinal turbulence velocity, [ms−1]:
ug(s) = Hugw1 (s) w1(s)
where w1 is a white noise signal, that is generated internally by a white-noise generator
within the block UDRYD1, and Hugw1 is the transfer function of the longitudinal turbulence
velocity filter.
• Transfer function of the longitudinal turbulence filter:
Hugw1 (s) = sug
r
2Lug
V
1
1 + Lug
V s
Note: the value of the airspeed V, used by UDRYD1, is kept constant during simulations.
This value must be specified by the user; obviously, the most realistic result is obtained
by specifying the (estimated) mean velocity of the airplane. If very large variations in
airspeed are anticipated, use the block UDRYD2 instead.
Inputs
None.
Outputs
ug longitudinal turbulence velocity, ug
u˙ g time-derivative of longitudinal turbulence velocity, ug dot
Parameters
UDRYD1 does not require any workspace parameters to be specified. The user must specify
the scale length Lug , the standard deviation sug , and the estimated mean value of the
true airspeed for which the motions are evaluated in the mask dialog, which is opened
after double-clicking the block UDRYD1.
Connections
in: no connections.
out: ug and u˙ g are usually combined with other turbulence velocities and their timederivatives
in a single vector that can be connected on the input side of the aircraft
model (through the uwind port).
Type browse udryd1 at the command-line for on-line help.
9.1. The wind and turbulence blocklibrary 173
UDRYD2 Main FDC library / Wind and turbulence / Atmospheric turbulence models / UDRYD2
Wind and turbulence library / Atmospheric turbulence models / UDRYD2
Type
Masked subsystem block.
Description
The block UDRYD2 computes the longitudinal turbulence velocity component, which is
aligned along the XB-axis of the aircraft, together with its time-derivative, using a longitudinal
Dryden filter with coefficients that will vary with airspeed. The user must specify
the longitudinal scale-length of the turbulence Lug and the standard deviation sug , but the
filter coefficients of UDRYD2 are computed ‘on-the-fly’, as a function of the airspeed. Since
the resulting variations in filter characteristics are usually relatively small, the turbulence
library also offers the simplified block UDRYD1, which uses constant filter coefficients.
Equations
For a detailed discussion of the equations from the block UDRYD2 and the underlying
theory, refer to section 4.2.
• Longitudinal turbulence velocity, [ms−1]:
ug(s) = Hugw1 (s) w1(s)
where w1 is a white noise signal, that is generated internally by a white-noise generator
within the block UDRYD2, and Hugw1 is the transfer function of the longitudinal turbulence
velocity filter.
• Transfer function of the longitudinal turbulence filter:
Hugw1 (s) = sug
r
2Lug
V
1
1 + Lug
V s
Note: the filter coefficients are computed ‘on-the-fly’, as a function of the actual value of
the airspeed V. In practice, the transfer-function has been implemented in the form of its
block-diagram equivalent, which was determined using the theory from section 6.4.2; the
gains in this block-diagram are ‘scheduled’ as a function of V.
Inputs
V true airspeed of the aircraft, V
Outputs
ug longitudinal turbulence velocity, ug
u˙ g time-derivative of longitudinal turbulence velocity, ug dot
Parameters
UDRYD2 does not require any workspace parameters to be specified. The user must
specify the scale length Lug , and the standard deviation sug in the mask dialog, which
is opened after double-clicking the block UDRYD2.
Connections
in: the airspeed V is usually extracted from the state vector x of the nonlinear aircraft
model.
out: ug and u˙ g are usually combined with other turbulence velocities and their timederivatives
in a single vector that can be connected on the input side of the aircraft
model (through the uwind port).
Type browse udryd2 at the command-line for on-line help.
174 Chapter 9. Wind and turbulence block reference
VDRYD1 Main FDC library / Wind and turbulence / Atmospheric turbulence models / VDRYD1
　
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