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Howto:Modelling hydrodynamics in JSBSim: Difference between revisions
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Howto:Modelling hydrodynamics in JSBSim (view source)
Revision as of 21:31, 6 December 2016
, 6 December 2016→Modelling hydrodynamic coefficients
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[[File:Swedish Navy T21 class MTB-drag.png|thumb|600px|center|Hydrodynamic drag of a MTB after ad hoc tuning. Total (green) and parts, displacement (red), planing (cyan) and rudder assembly (purple), of the hydrodynamic drag of the [[Swedish Navy T21 class MTB]] from a single acceleration run in JSBSim/standalone. The blue line is a real tow tank result traced from a diagram in "Resultat av släpförsök med svenska patent nr. 109626", A. F. Nordström, Statens Skeppsprovningsanstalt, Göteborg, 1941-10-01 available as an image (ID 2008:25:869:1) from Sjöhistoriska museet under CC BY-SA.]] | [[File:Swedish Navy T21 class MTB-drag.png|thumb|600px|center|Hydrodynamic drag of a MTB after ad hoc tuning. Total (green) and parts, displacement (red), planing (cyan) and rudder assembly (purple), of the hydrodynamic drag of the [[Swedish Navy T21 class MTB]] from a single acceleration run in JSBSim/standalone. The blue line is a real tow tank result traced from a diagram in "Resultat av släpförsök med svenska patent nr. 109626", A. F. Nordström, Statens Skeppsprovningsanstalt, Göteborg, 1941-10-01 available as an image (ID 2008:25:869:1) from Sjöhistoriska museet under CC BY-SA.]] | ||
Firstly, the displacement drag is modelled through two functions: one for the hull and one for the rudders. | Firstly, the displacement drag is modelled through two functions: one for the hull and one for the rudders. The hull displacement drag is defined as follows: | ||
<syntaxhighlight lang="xml"> | |||
<fcs_function name="hydro/displacement-drag-lbs"> | |||
<function> | |||
<description>Drag due to displacement of hull</description> | |||
<product> | |||
<value>0.0000017</value> <!-- Base Cd per lbs buoyancy and ft^2 of hull cross section. --> | |||
<property>hydro/qbar-psf</property> | |||
<property>hydro/hull-beam-ft2</property> | |||
<property>hydro/buoyancy-lbs</property> | |||
<sum> | |||
<value>1.0</value> | |||
<product> <!-- Increase drag with beta. --> | |||
<value>10.0</value> | |||
<abs><sin><property>hydro/beta-rad</property></sin></abs> | |||
</product> | |||
</sum> | |||
</product> | |||
</function> | |||
</fcs_function> | |||
</syntaxhighlight> | |||
There are a couple of things to note: | |||
* The force is scaled by the displacement, so it will decrease as the planing forces increase and lifts the hull towards the water surface. | |||
* The force is crudely increased as the beta angle increases (the drag increases when "side slipping" through the water, but in this case there is no data to base the increment on). | |||
* The Cd per lbs displacement value has been tuned to the graph above, but it was also necessary to modify the static buoyancy/displacement lift function to decrease it faster with speed to get a decent match (among other things this simplified model computes the buoyancy based on the undisturbed water level and, further, at planing speeds the interface between the hull and the water is very different from at slower speeds). | |||
The rudder drag function, on the other hand, is simpler (the rudder angle penalty is just guessed): | |||
<syntaxhighlight lang="xml"> | |||
<fcs_function name="hydro/rudder-drag-lbs"> | |||
<function> | |||
<description>Drag due to rudders (not reduced by planing)</description> | |||
<product> | |||
<value>0.0036</value> <!-- Base Cd per ft^2 of hull cross section. --> | |||
<property>hydro/qbar-psf</property> | |||
<property>hydro/hull-beam-ft2</property> | |||
<sum> | |||
<value>1.0</value> | |||
<product> <!-- Increase drag with rudder angle. --> | |||
<value>10.0</value> | |||
<abs><property>fcs/rudder-pos-norm</property></abs> | |||
</product> | |||
</sum> | |||
</product> | |||
</function> | |||
</fcs_function> | |||
</syntaxhighlight> | |||
The third component of the drag is due to planing. | |||
''''To be continued... '''' | ''''To be continued... '''' | ||