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List:       flightgear-devel
Subject:    Re: [Flightgear-devel] YASim ignores spin?
From:       "Edward d'Auvergne" <true.bugman () gmail ! com>
Date:       2015-09-24 8:06:26
Message-ID: CAED9pY8Ur=TReCo3dgtHQmbV_TV8-UESuzGpBfZStn3q+fszHA () mail ! gmail ! com
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-On 24 September 2015 at 02:17, Riot <rain.backnet@gmail.com> wrote:
> I'm going through the source of yasim again, and found something that I
> think merits a separate email thread.
>
> The RigidBody class has a member called _spin, and this is used in
> calculations of acceleration at a point, as well as angular acceleration (in
> RigidBody::getAngularAccel) - a pretty critical core component of the
> simulation.
>
> However, I cannot see that _spin is actually ever set to anything other than
> zero.  It is not set within that class, and the only setter for it,
> setBodySpin, is never called anywhere in the source code.  It should
> presumably be updated every tick from State::rot (the rotational velocity),
> but this does not appear to ever happen.
>
> I hope I'm mistaken in my reading of the code here, because if I'm not, it
> means all gyroscopic effects on rotational calculations in yasim are wrong -
> and have possibly been this way through its entire history.  Andy, could you
> please clarify whether that is in fact the case?

Hi Riot,

This might explain why people have had issues with the non-existent
gyroscopic effects, which is especially noticeable in pre-1920
aircraft.  For example, see Lewis' message at:

    [forum] Re: Looking for aircraft with 'paper cut' bugs (
http://forum.flightgear.org/viewtopic.php?f=4&t=26183&p=243077#p242888
)

There are a few discussions later in that thread (unfortunately the
Gmane mail archive is down at the moment, so some of the links will
temporarily be unreadable).  You can test this and see the lack of the
effect with the Sopwith Camel in FGAddon:

$ fgfs --aircraft=sopwithCamel    # UIUC FDM, no gyro effect.
$ fgfs --aircraft=sopwithCamel-YASim    # YASim, no gyro effect.

However Brent Hugh (a.k.a. Flug) has implemented the huge gyroscopic
effect using the JSBSim FDM, so this might be a good reference test:

$ fgfs --aircraft=sopwithCamel-JSBSim

See this video and forum post:

https://www.youtube.com/watch?v=0w0UrzCRlTk
http://forum.flightgear.org/viewtopic.php?f=4&t=19584&start=30#p182946

However, according to Lewis:  "Flug tried to do this with an extremely
complex (external) systems/nasal fix for the Camel as a JSB sim FDM.".
On a different note, there also appears to be still open bugs in
JSBSim with regard to broken gyroscopic effects (
https://sourceforge.net/p/jsbsim/bugs/110/ ).  Maybe something for Jon
to hopefully look at ;)

It would be great if the YASim FDM would eventually correctly support
the gyroscopic effect!  Hopefully somehow setting values in the _spin
vector is all that is needed, and that this does not require a lot of
missing code to be written.  This will allow a lot of the pre-1920
aircraft in FlightGear to be made far more realistic.

Cheers,

Edward

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