Fix crab angle and ground speed calculation in AILocalTraffic

diff --git a/src/ATCDCL/AILocalTraffic.cxx b/src/ATCDCL/AILocalTraffic.cxx
index f96c38992bdd4db68b2e6b48f10725e0ba311ecd..40500ee350e73dd38f312f5417ae68e6f33d113c 100644 (file)
--- a/src/ATCDCL/AILocalTraffic.cxx
+++ b/src/ATCDCL/AILocalTraffic.cxx
@@ -1127,27 +1127,24 @@ void FGAILocalTraffic::FlyTrafficPattern(double dt) {
     }
 
        if(inAir) {
-               // FIXME - at the moment this is a bit screwy
-               // The velocity correction is applied based on the relative headings.
-               // Then the heading is changed based on the velocity.
-               // Which comes first, the chicken or the egg?
-               // Does it really matter?
-               
-               // Apply wind to ground-relative velocity if in the air
-               vel = IAS - (cos((_hdg - wind_from) * DCL_DEGREES_TO_RADIANS) * wind_speed);
-               //crab = f(track, wind, vel);
-               // The vector we need to fly is our desired vector minus the wind vector
-               // TODO - we probably ought to use plib's built in vector types and operations for this
-               // ie.  There's almost *certainly* a better way to do this!
-               double gxx = vel * sin(track * DCL_DEGREES_TO_RADIANS); // Plane desired velocity x component wrt ground
-               double gyy = vel * cos(track * DCL_DEGREES_TO_RADIANS); // Plane desired velocity y component wrt ground
-               double wxx = wind_speed * sin((wind_from + 180.0) * DCL_DEGREES_TO_RADIANS);    // Wind velocity x component
-               double wyy = wind_speed * cos((wind_from + 180.0) * DCL_DEGREES_TO_RADIANS);    // Wind velocity y component
-               double axx = gxx - wxx; // Plane in-air velocity x component
-               double ayy = gyy - wyy; // Plane in-air velocity y component
-               // Now we want the angle between gxx and axx (which is the crab)
-               crab = atan2(ayy - gyy, axx - gxx) * DCL_RADIANS_TO_DEGREES;
-               //cout << "crab = " << crab << '\n';
+               // calculate ground speed and crab from the wind triangle
+               double wind_angle = GetAngleDiff_deg(wind_from + 180, track);
+               double wind_side = (wind_angle < 0) ? -1.0 : 1.0;
+
+               double sine_of_crab = wind_speed / IAS * sin(fabs(wind_angle) * DCL_DEGREES_TO_RADIANS);
+               if (sine_of_crab >= 1.0) {
+                       // The crosswind component is greater than the IAS,
+                       // we can't keep the aircraft on track.
+                       // Assume increased IAS such that it cancels lateral speed.
+                       // This is unrealistic, but not sure how the rest of the sim
+                       // would react to the aircraft going off course.
+                       // Should be a rare case anyway.
+                       crab = wind_side * 90.0;
+               } else {
+                       crab = asin(sine_of_crab) * DCL_RADIANS_TO_DEGREES * wind_side;
+               }
+               vel = cos(wind_angle * DCL_DEGREES_TO_RADIANS) * wind_speed
+                       + cos(crab * DCL_DEGREES_TO_RADIANS) * IAS;
        } else {        // on the ground - crab dosen't apply
                crab = 0.0;
        }