Somehow I need to incorporate distance (from the transmitter) into the the
nav1 heading control, or find another way to solve the same problem. As you
get closer to the target, the cone gets narrower, but more important is that
the size of adjustments to correct for a give angle error become smaller.
In the old autopilot I corrected for this with the following code:
// clamp closer when inside cone when beyond 5km...
if (current_radiostack->get_navcom1()->get_nav_loc_dist() > 5000) {
double clamp_angle =
fabs(current_radiostack->get_navcom1()->get_nav_cdi_deflection()) * 3;
if (clamp_angle < 30)
SG_CLAMP_RANGE( adjustment, -clamp_angle, clamp_angle);
}
This cut down the roll angle target dramatically during the last couple
minutes of the approach. This was just a hack that worked, and I suspect
there is a much better solution.
Basically the problem is that the large aircraft carry too much momentum and
thus tend to oversteer especially in the zone where smaller and smaller
corrections are required. And a given distance off course will produce larger
and larger error angles. Thus the heavy aircraft will produce "S" patterns of
increasing amplitude rather than actually zeroing in on the glideslope.
Playing with the gain doesn't help because of the response times involved (and
the fact that you don't want to shake up the passengers too much).
One idea I had was to base the error not on an angle from the radial but on
the distance from the center of the cone. The required response to correct
this value would be consistant all the way down the cone.
Best,
Jim
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