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JSBSim How to avoid sinking the landing gear?

Good sims require good FDMs (the "thing" that makes an aircraft behave like an aircraft).

JSBSim How to avoid sinking the landing gear?

Postby abassign » Sat Jun 30, 2018 11:23 pm

When our G91-R1B is parked the wheels are properly resting on the ground, but when you land or braking hard, the wheels sink into the ground as if they were in butter.
Where am I doing wrong ?!

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This is the code for landing gear:

Code: Select all
<ground_reactions>
        <contact type="BOGEY" name="NOSE">
            <location unit="IN">
                <x> 95.3 </x>
                <y> 0.0 </y>
                <z> 0.0 </z>
            </location>
            <static_friction>  0.80 </static_friction>
            <dynamic_friction> 0.60 </dynamic_friction>
            <function type="CORNERING_COEFF">
                <table>
                    <independentVar>gear/slip-angle-deg</independentVar>
                    <tableData>
                        0.0     0.0
                        2.0     0.1
                        4.0     0.2
                        6.0     0.25
                        8.0     0.29
                        10.0    0.38
                    </tableData>
                </table>         
            </function>
            <rolling_friction> 0.03 </rolling_friction>
            <spring_coeff unit="LBS/FT">       10000.0 </spring_coeff>
            <damping_coeff unit="LBS/FT/SEC">  3000.0 </damping_coeff>
            <damping_coeff_rebound unit="LBS/FT/SEC">2600.0</damping_coeff_rebound>
            <max_steer unit="DEG"> 45.00 </max_steer>
            <brake_group>NONE</brake_group>
            <retractable>1</retractable>
            <relaxation_velocity>
                <rolling unit="FT/SEC"> 1.0 </rolling>
                <side unit="FT/SEC"> 1.0 </side>
            </relaxation_velocity>
        </contact>
       
        <contact type="BOGEY" name="LEFT_MAIN">
            <location unit="IN">
                <x> 219.61 </x>
                <y> -54.4 </y>
                <z> 0.0 </z>
            </location>
            <static_friction>  0.80 </static_friction>
            <dynamic_friction> 0.60 </dynamic_friction>
            <rolling_friction> 0.05 </rolling_friction>
            <spring_coeff unit="LBS/FT">      25000.0 </spring_coeff>
            <damping_coeff unit="LBS/FT/SEC">  4500.0 </damping_coeff>
            <damping_coeff_rebound unit="LBS/FT/SEC">5000.0</damping_coeff_rebound>
            <max_steer unit="DEG">0</max_steer>
            <brake_group>LEFT</brake_group>
            <retractable>1</retractable>
            <relaxation_velocity>
                <rolling unit="FT/SEC"> 1.0 </rolling>
                <side unit="FT/SEC"> 1.0 </side>
            </relaxation_velocity>
        </contact>
       
        <contact type="BOGEY" name="RIGHT_MAIN">
            <location unit="IN">
                <x> 219.6 </x>
                <y>  54.4 </y>
                <z> 0.0 </z>
            </location>
            <static_friction>  0.80 </static_friction>
            <dynamic_friction> 0.60 </dynamic_friction>
            <rolling_friction> 0.05 </rolling_friction>
            <spring_coeff unit="LBS/FT">      25000.0 </spring_coeff>
            <damping_coeff unit="LBS/FT/SEC">  4500.0 </damping_coeff>
            <damping_coeff_rebound unit="LBS/FT/SEC">5000.0</damping_coeff_rebound>
            <max_steer unit="DEG">0</max_steer>
            <brake_group>RIGHT</brake_group>
            <retractable>1</retractable>
            <relaxation_velocity>
                <rolling unit="FT/SEC"> 1.0 </rolling>
                <side unit="FT/SEC"> 1.0 </side>
            </relaxation_velocity>
        </contact>...
Developer of the program https://wiki.flightgear.org/Julia_photoscenery_generator
FDM developer of the G91R1B aircraft https://wiki.flightgear.org/FIAT_G91R1B
JSBSim collaborator
abassign
 
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Re: JSBSim How to avoid sinking the landing gear?

Postby Alant » Sat Jun 30, 2018 11:42 pm

The stiffness of the undercarriage leg has to be matched to the model animation.

The stiffeness term for your nose gear is is <spring_coeff unit="LBS/FT"> 10000.0 </spring_coeff>

You need to animate the nose wheel and the suspension compoents with code such as this from my TSR2 (the object names are probably not correct for your .ac model and <factor>0.4914</factor> will need to tuned to match your spring coeff ):-
And also put the correct motion to the oleo struts.
<!-- Nose Gear Compression -->
<animation>
<type>translate</type>
<object-name>NG-Tyre-L</object-name>
<object-name>NG-Tyre-R</object-name>
<object-name>NG-Hub-L</object-name>
<object-name>NG-Hub-R</object-name>
<object-name>NG-Axle</object-name>
<object-name>NG-SustainingRam</object-name>
<object-name>NG-OleoLowerLink</object-name>
<property>gear/gear[0]/compression-norm</property>
<!-- <factor>0.508</factor> -->
<factor>0.4914</factor>
<axis>
<x>0</x>
<y>0</y>
<z>1</z>
</axis>
</animation>
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Re: JSBSim How to avoid sinking the landing gear?

Postby AndersG » Sat Jun 30, 2018 11:57 pm

There may well be a /gear/gear[X]/compression-ft property too (I use it on some of my aircraft). It is sometimes easier to use for animation.
Callsign: SE-AG
Aircraft (uhm...): Submarine Scout, Zeppelin NT, ZF Navy free balloon, Nordstern, Hindenburg, Short Empire flying-boat, ZNP-K, North Sea class, MTB T21 class, U.S.S. Monitor, MFI-9B, Type UB I submarine, Gokstad ship, Renault FT.
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Re: JSBSim How to avoid sinking the landing gear?

Postby Alant » Sun Jul 01, 2018 12:55 am

Thanks Anders,

I was obviously not aware of that when I wrote that bit of code. Converting ft to meter is much simpler than converting norm to meter. I hate Flightgears's norm properties with a passion, as "norm "has a different real value in every case that it is used.

Alan
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