airship_added_mass.xml

<!--

  Airship added/virtual mass model for JSBSim.

    Copyright (C) 2008 - 2023  Anders Gidenstam  (anders(at)gidenstam.org)

    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Lesser General Public
    License as published by the Free Software Foundation; either
    version 2.1 of the License, or (at your option) any later version.

    This library is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301
    USA

-->
<system name="added-mass">

 <description>
  This system computes forces and moments due to added/virtual mass.
  The model is based on
  [Max M. Munk, "Aerodynamic forces on airship hulls", NACA report 184, 1924]
  and the introduction in
  [Christopher J. Atkinson, Robert G. Urso, "Modeling of Apparent Mass Effects
   for the Real-Time Simulation of a Hybrid Airship", AIAA Modeling and
   Simulation Technologies Conference, Aug 2006].

  Usage:

  Define the following properties:
    aero/constants/volume-ft3
    aero/constants/length-diameter-ft2          (length^2 + diameter^2)
    aero/constants/added-mass/k-axial           (all three are positive but
    aero/constants/added-mass/k-transverse       can be entered as negative
    aero/constants/added-mass/k-rotational       for backwards compatibility)
    aero/constants/added-mass/tweak-factor      (1.0 is the standard value)

    aero/constants/added-mass/k-traverse        (old misspelling, can be given
                                                 to work with older FlightGear)

  Define the following external forces and moments:

    <force name="added-mass-bx" frame="BODY">
     <location unit="">
      { AERORP }
     </location>
     <direction>
      <x> 1.0 </x>
      <y> 0.0 </y>
      <z> 0.0 </z>
     </direction>
    </force>
    <force name="added-mass-by" frame="BODY">
     <location unit="">
      { AERORP }
     </location>
     <direction>
      <x> 0.0 </x>
      <y> 1.0 </y>
      <z> 0.0 </z>
     </direction>
    </force>
    <force name="added-mass-bz" frame="BODY">
     <location unit="">
      { AERORP }
     </location>
     <direction>
      <x> 0.0 </x>
      <y> 0.0 </y>
      <z> 1.0 </z>
     </direction>
    </force>

    <moment name="added-mass-pitch" frame="BODY" unit="LBSFT">
     <direction>
      <x> 0.0 </x>
      <y> 1.0 </y>
      <z> 0.0 </z>
     </direction>
    </moment>
    <moment name="added-mass-roll" frame="BODY" unit="LBSFT">
     <direction>
      <x> 1.0 </x>
      <y> 0.0 </y>
      <z> 0.0 </z>
     </direction>
    </moment>
    <moment name="added-mass-yaw" frame="BODY" unit="LBSFT">
     <direction>
      <x> 0.0 </x>
      <y> 0.0 </y>
      <z> 1.0 </z>
     </direction>
    </moment>

  The old deprecated force based external moment implementation below is still
  supported.

    <force name="added-mass-pitch[0]" frame="BODY">
     <location unit="">
      <x> AERORP X </x>
      <y> AERORP Y </y>
      <z> AERORP Z - 0.3048 </z>
     </location>
     <direction>
      <x> 1.0 </x>
      <y> 0.0 </y>
      <z> 0.0 </z>
     </direction>
    </force>
    <force name="added-mass-pitch[1]" frame="BODY">
     <location unit="M">
      <x> AERORP X </x>
      <y> AERORP Y </y>
      <z> AERORP Z + 0.3048 </z>
     </location>
     <direction>
      <x> 1.0 </x>
      <y> 0.0 </y>
      <z> 0.0 </z>
     </direction>
    </force>

    <force name="added-mass-yaw[0]" frame="BODY">
     <location unit="M">
      <x> AERORP X </x>
      <y> AERORP Y - 0.3048 </y>
      <z>  0.0 </z>
     </location>
     <direction>
      <x> 1.0 </x>
      <y> 0.0 </y>
      <z> 0.0 </z>
     </direction>
    </force>
    <force name="added-mass-yaw[1]" frame="BODY">
     <location unit="M">
      <x> AERORP X </x>
      <y> AERORP Y + 0.3048 </y>
      <z>  0.0 </z>
     </location>
     <direction>
      <x> 1.0 </x>
      <y> 0.0 </y>
      <z> 0.0 </z>
     </direction>
    </force>

 </description>

 <channel name="startup">

  <!-- Allow the vehicle state to stabilize. -->
  <fcs_function name="aero/added-mass/active">
   <function>
    <min>
     <property> aero/constants/added-mass/tweak-factor </property>
     <max>
      <value> 0.0 </value>
      <difference>
       <property> simulation/sim-time-sec </property>
       <value> 2.0 </value>
      </difference>
     </max>
    </min>
   </function>
  </fcs_function>

 </channel>

 <property>aero/constants/added-mass/k-transverse</property>
 <property>aero/constants/added-mass/k-traverse</property>
 <channel name="backward-compatibility">

  <!-- Allow vehicles still using the old misspelled k-traverse coefficient. -->
  <switch name="aero/constants/added-mass/k-transverse-final">
   <default value="aero/constants/added-mass/k-transverse"/>
   <test logic="AND" value="aero/constants/added-mass/k-transverse">
     aero/constants/added-mass/k-transverse NE 0.0
   </test>
   <test logic="AND" value="aero/constants/added-mass/k-traverse">
     aero/constants/added-mass/k-traverse NE 0.0
   </test>
  </switch>

 </channel>

 <channel name="translational">

  <fcs_function name="aero/added-mass/force-bx-lbs">
   <function>
    <product>
     <value>-1.0</value>
     <property> aero/added-mass/active </property>
     <property> atmosphere/rho-slugs_ft3 </property>
     <abs><property> aero/constants/added-mass/k-axial </property></abs>
     <property> aero/constants/volume-ft3 </property>
     <property> accelerations/udot-ft_sec2 </property>
    </product>
   </function>
   <output> external_reactions/added-mass-bx/magnitude </output>
  </fcs_function>

  <fcs_function name="aero/added-mass/force-by-lbs">
   <function>
    <product>
     <value>-1.0</value>
     <property> aero/added-mass/active </property>
     <property> atmosphere/rho-slugs_ft3 </property>
     <abs><property> aero/constants/added-mass/k-transverse-final </property></abs>
     <property> aero/constants/volume-ft3 </property>
     <property> accelerations/vdot-ft_sec2 </property>
    </product>
   </function>
   <output> external_reactions/added-mass-by/magnitude </output>
  </fcs_function>

  <fcs_function name="aero/added-mass/force-bz-lbs">
   <function>
    <product>
     <value>-1.0</value>
     <property> aero/added-mass/active </property>
     <property> atmosphere/rho-slugs_ft3 </property>
     <abs><property> aero/constants/added-mass/k-transverse-final </property></abs>
     <property> aero/constants/volume-ft3 </property>
     <property> accelerations/wdot-ft_sec2 </property>
    </product>
   </function>
   <output> external_reactions/added-mass-bz/magnitude </output>
  </fcs_function>

 </channel>

 <channel name="rotational">

  <fcs_function name="aero/added-mass/pitch-moment-lbsft">
   <function>
    <product>
     <property> aero/added-mass/active </property>
     <value>-0.05 </value>
     <property> atmosphere/rho-slugs_ft3 </property>
     <abs><property> aero/constants/added-mass/k-rotational </property></abs>
     <property> aero/constants/volume-ft3 </property>
     <property> aero/constants/length-diameter-ft2 </property>
     <property> accelerations/qdot-rad_sec2 </property>
    </product>
   </function>
  </fcs_function>

  <fcs_function name="aero/added-mass/yaw-moment-lbsft">
   <function>
    <product>
     <property> aero/added-mass/active </property>
     <value>-0.05 </value>
     <property> atmosphere/rho-slugs_ft3 </property>
     <abs><property> aero/constants/added-mass/k-rotational </property></abs>
     <property> aero/constants/volume-ft3 </property>
     <property> aero/constants/length-diameter-ft2 </property>
     <property> accelerations/rdot-rad_sec2 </property>
    </product>
   </function>
  </fcs_function>

  <pure_gain name="aero/added-mass/pitch-moment/moment-lbsft">
   <input>aero/added-mass/pitch-moment-lbsft</input>
   <gain>1.0</gain>
   <output>external_reactions/added-mass-pitch/magnitude-lbsft</output>
  </pure_gain>
  <pure_gain name="aero/added-mass/pitch-moment/force-lbs[0]">
   <input> aero/added-mass/pitch-moment-lbsft </input>
   <gain>-0.5</gain>
   <output> external_reactions/added-mass-pitch[0]/magnitude </output>
  </pure_gain>
  <pure_gain name="aero/added-mass/pitch-moment/force-lbs[1]">
   <input> aero/added-mass/pitch-moment-lbsft </input>
   <gain>0.5</gain>
   <output> external_reactions/added-mass-pitch[1]/magnitude </output>
  </pure_gain>

  <pure_gain name="aero/added-mass/yaw-moment/moment-lbsft">
   <input>aero/added-mass/yaw-moment-lbsft</input>
   <gain>1.0</gain>
   <output>external_reactions/added-mass-yaw/magnitude-lbsft</output>
  </pure_gain>
  <pure_gain name="aero/added-mass/yaw-moment/force-lbs[0]">
   <input> aero/added-mass/yaw-moment-lbsft </input>
   <gain>-0.5</gain>
   <output> external_reactions/added-mass-yaw[0]/magnitude </output>
  </pure_gain>
  <pure_gain name="aero/added-mass/yaw-moment/force-lbs[1]">
   <input> aero/added-mass/yaw-moment-lbsft </input>
   <gain>0.5</gain>
   <output> external_reactions/added-mass-yaw[1]/magnitude </output>
  </pure_gain>

 </channel>

</system>