It should be possible to model trim tab effects on the stabilizer using
function definitions in the <aerodynamics> section and in the <system> or
<flight_control> section. The trim tab setting could be used to calculated
the stabilizer pitching moment, and it should then be possible to calculated
the equilibrium stabilizer position. The flight control components would be
in charge of moving the aerosurface to that position. Subsequently, the lift
and drag contributions would be calculated.
I think that could work .
Jon
From: Leonardo Fabian Grodek [mailto:fabian.gro...@gmail.com]
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
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