Andy,
The original code assumed that "dt" was in milliseconds. The variable name
"velocity_rpms" presumably stood for "revs per millisecond".
Your patch assumes that "dt" is in 1/120ths of a second. Is this linked to your frame
rate?
The meaning of "dt" needs to be consistent across the various instances of
FGSubsystem, and I don't think it is: see my post "Propellor speed error due to
FGSubsystem::update(multi_loop)", reproduced below.
- Julian
Andy Ross wrote:
>
> Here's a patch to model.cxx that restores the proper rotation speed of
> the propeller spin.
>
> It just replaces the single 1/60000 constant with one that is
> (hopefully) more clearly derivable from basic principles. The
> original number was off by a factor of 3/25; maybe something got
> double-factored? One thing I didn't do is grab the 120 Hz update rate
> from a real source; it's hardcoded instead. I forget where it comes
> from.
>
> Andy
>
> diff -u -r1.10 model.cxx
> --- model.cxx 20 Apr 2002 15:07:47 -0000 1.10
> +++ model.cxx 30 Apr 2002 23:17:55 -0000
> @@ -527,8 +527,9 @@
> void
> FG3DModel::SpinAnimation::update (int dt)
> {
> - float velocity_rpms = (_prop->getDoubleValue() * _factor / 60000.0);
> - _position_deg += (dt * velocity_rpms * 360);
> + float velocity_rpms = _prop->getDoubleValue() * _factor;
> + // 120Hz update, 60 sec/min, 360 degrees/revolution
> + _position_deg += (dt * (1/120.0) * (1/60.0) * velocity_rpms * 360);
> while (_position_deg < 0)
> _position_deg += 360.0;
> while (_position_deg >= 360.0)
>
> --
On 21 April Julian Foad wrote:
>
> The error in the speed of the propellor, and other things that we might not have
>noticed yet, is caused by the handling of the variable "multi_loop" in
>main.cxx:fgUpdateTimeDepCalcs(). After being used to update the FDM, this
>"multi_loop" value (or the value 1 in freeze mode) is passed to the various subsystem
>"update" methods:
>
> globals->get_model_mgr()->update(multi_loop);
> globals->get_aircraft_model()->update(multi_loop);
> globals->get_viewmgr()->update(multi_loop);
> current_radiostack->update(multi_loop);
>
> The model manager, at least, is expecting to receive the number of milliseconds that
>have elapsed since the last call. The "multi_loop" number does not seem to be that.
>Nor does the definition of FGSubsystem::update help:
> /**
> * Update the subsystem.
> *
> * <p>FlightGear invokes this method every time the subsystem should
> * update its state. If the subsystem requires delta time information,
> * it should track it itself.</p>
> */
> virtual void update (int dt) = 0;
>
> That looks wrong to me. Surely "it should track it itself" applied before the "dt"
>parameter existed, and now the "dt" parameter is intended to provide the time
>difference and should be defined here.
>
> Anyone prepared to sort this out? If "dt" is to represent milliseconds, it would be
>a good idea to rename it "dt_ms" (in the derived classes too). I haven't checked how
>other classes interpret it.
>
> - Julian
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