From 1d8be86335aab41c5e9bfaa5149735edf2309308 Mon Sep 17 00:00:00 2001
From: Csaba Halasz <hcs@vostro.(none)>
Date: Mon, 28 Dec 2009 18:29:10 +0100
Subject: [PATCH] Fix crab angle and ground speed calculation in AILocalTraffic
---
src/ATCDCL/AILocalTraffic.cxx | 39 ++++++++++++++++-------------------
1 file changed, 18 insertions(+), 21 deletions(-)
diff --git a/src/ATCDCL/AILocalTraffic.cxx b/src/ATCDCL/AILocalTraffic.cxx
index f96c38992..40500ee35 100644
--- 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;
}