SITL: don't enable wind until we are off the ground

this makes it possible to test with much larger wind speeds, without
the sim crashing due to the wind flipping us over on the runway

Tools/autotest/jsbsim/runsim.py

@@ -257,7 +257,9 @@ def main_loop():
                 paused = False
                 jsb_console.send('resume\n')
 
-        if tnow - last_wind_update > 0.1:
+        # only simulate wind above 5 meters, to prevent crashes while
+        # waiting for takeoff
+        if tnow - last_wind_update > 0.1 and fdm.get('agl', units='meters') > 2:
             update_wind(wind)
             last_wind_update = tnow