Re: [Flightgear-devel] [Flightgear-commitlogs] [FlightGear] 02/02: YASim gear: Clamping of _frac replaced

[Julian S. <ju...@op...>]

On Fri, 21 Jul 2023 14:04:25 +0100
James Turner <ja...@fl...> wrote:

> > On 21 Jul 2023, at 13:32, Julian Smith <ju...@op...> wrote:
> > 
> > 
> > For reference, this all started with
> > https://sourceforge.net/p/flightgear/codetickets/1509/.
> > 
> > It's a bit curious because the original reproducer says:
> > 
> >    reduce spring constant so, when plane is stationary, nose gear
> >    reaches full compression.
> > 
> > It's hardly surprising that things go wrong if one deliberately
> > mis-configures things like this, so this on its own would not be a
> > reason to change anything, IMHO.  
> 
> Right, no argument there.
> 
> > 
> > However a later comment says:
> > 
> >    But I tested the my above linked merge request at TF-G2 model,
> >    where light spring constants are needed to be able to land on a
> >    grass.
> > 
> > So i think this is suggesting some particular aircraft may need a
> > modified spring function that gives greater force than normal when
> > highly compressed. (I wonder whether there's any information on the
> > real aircraft having such non-linear suspension?)  
> 
> Also agreed with your understanding here : there are particular
> aircraft (rough field aircraft?) which need light spring constants
> for other reasons. It would be good to hear from more aircraft
> developers as you say, if these are non-linear mechanical systems or
> have secondary springs when the ‘main’ spring is at full compression.
> Since intuitively, very soft springs will lead to the gear often
> being at full compression, when travelling over plausibly bumpy
> ground.
> 
> > But we've recently learned that we must not allow ridiculously high
> > compression to cause ridiculously high spring forces, because the
> > ground level can momentarily leap upwards by thousands of metres.  
> 
> Right, although it might be separately god to see if we can address
> that would some filtering in the ground intersection code.

Yes, definitely.

> 
> > Maybe we could simply add a quadratic term scaled by a new
> > dimensionless constant `spring2` in the XML:
> > 
> >    force = _spring * compression * (_frac + spring2 * _frac**2)
> > 
> > [One could view `spring2` has modifying the maximum force of the
> > spring at full compression, 
> > 
> >    force_max -> force_max * (1 + spring2).
> > 
> > For example `spring2=10` would result in 11x the force at maximum
> > compression compared to the default `spring2=0`.]
> > 
> > I'm hopeful that this could address the original problem without the
> > danger of pathological behaviour. I think it would be pretty easy to
> > implement too.  
> 
> Definitely worth a shot, as far as I’m concerned. Again I’d be

Ok, i might have a look at this, unless someone else gets there first.

> happiest when we have some unit-test driving this code that can try
> different spring constants, since I think that’s the only way to
> capture different aircraft FDM configs in a robust way, if we make
> changes to this code again. But, I appreciate in the absence of any
> existing tests in this area, writing the first test will be a
> non-trivial amount of work.

Actually i added a Yasim Gear unit test when i implemented torus tires,
to check i hadn't changed default behaviour - see:
flightgear/test_suite/unit_tests/FDM/testYASimGear.cxx

- Jules

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