Hi,
I have no experience in FGFS capabilities in terms of autopilot and
trimming, but I do have some in JSBSim, so I will refer only to that FDM,
and I hope this will be relevant to this discussion.
First, I think it would be good to have this in mind: there are two types of
trim: 1. Aircraft pitching moment trim, which refers to making the aircraft
pitching moment zero, and 2. stick force trim, which, obviously, refers to
zeroing the pilot stick forces.
As Heiko has already pointed, airliners and most business jets use the
stabilizer for pitching moment trim. In JSBSim this CAN be modeled as
follows:
<channel name="Pitch">
:
:
<summer name="Stabilizer Trim Sum">
<input>fcs/pitch-trim-cmd-norm</input> <!-- Note that here is the
trim variable that comes from the trimming algorithm -->
<clipto>
<min>-1</min>
<max> 1</max>
</clipto>
</summer>
:
:
This will yield a fcs/stabilizer-trim-sum that can be mapped into a
fcs/stabilizer-pos-deg (using an aerosurface_scale) which can be then used
to calculate the tail angle of attack (alpha+stab-downwash) and from there
the tail lift and/or pitching moment (of course this aerodynamic data is not
always easy to achieve).
Regarding the second type of trim (stick force), as far as I know JSBSim
does not have the capability for this yet, although it may be possible to
build that on the base of existing trimming options). For manually operated
elevators, the elevator angle for zero force will depend on hinge moments
(affected by alpha_tail, dynamic pressure, etc.), and will normally will be
different from 0 degrees. For powered elevators (as in modern or high
performance aircraft) zero elevator deflection will give zero stick force
(by means of an "artificial feeling" system).
For how all said above can be applied in FGFS...well, somebody familiar with
that part would need to jump in.
I hope this was not more confusing than helpful...
Fabian
On Sun, Mar 7, 2010 at 7:29 AM, Innis Cunningham <inn...@hotmail.com> wrote:
> Hi Guys
> I think you will find on all Boeing aircraft if the elevator is out of
> neutral
> for a set amount of time the horizontal stabilizer runs to return the
> elevator to
> the null position.Hence retriming the aircraft.Hence the name stabilizer
> trim
> I think also you will find that the tab on the elevator is not
> a trim tab but moves in the opposite direction to the elevator movement.
> So until we get a working FDM for the horizontal stabilizer in FG anything
> else will be a hack.
>
> Cheers
> Innis
>
>
> >
> > Hi,
> >
> > I just tried with the SenceaII and both Ti and elevator filter influence
> this.
> >
> > Great, thanks to all for the help!
> >
> > Cheers
> > Heiko
> >
> > > If you have a pid-simple-controller,
> > > try increasing Ti, it slows down
> > > the action.
> > > > Hi Torsten,
> > > >
> > > > That sounds like it could be done without nasal just
> > > in the autopilot file. I guess the speed of the
> > > trim-pitch-channel can be changed with the filter? (The
> > > 737-300 has a speed-trim at certain situations)
> > > >
> > > > Kind regards
> > > > Heiko
> > > >
> > > > > Hi Heiko,
> > > > >
> > > > > the auto-pitch-trim channel moves the elevator
> > > trim to
> > > > > return the
> > > > > elevator into the neutral position while the
> > > pitch-channel
> > > > > moves the
> > > > > elevator to change attitude.
> > > > > Or a bit more technical:
> > > > > The auto-pitch-trim-channel compares the
> > > elevator-pos-norm
> > > > > to a
> > > > > reference of zero and adjusts elevator-trim
> > > > > The pitch-channel compares e.g. current pitch
> > > with a
> > > > > reference of some
> > > > > calculated target pitch and adjusts elevator-pos
> > > > > Usually the pitch-trim channel is much slower
> > > than the
> > > > > elevator channel.
> > > > > HTH, Torsten
> > > > >
> > > > >
> > > > >
> > >
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