add autotrim class to help with trimming (read the class documentation for

an explanation). There's no button/key using this by default (yet?).

Nasal/aircraft.nas

@@ -643,6 +643,76 @@ var steering = {
 
 
 
+# autotrim
+# =============================================================================
+# Singleton class that supports quick trimming and compensates for the lack
+# of resistance/force feedback in most joysticks. Normally the pilot trims such
+# that no real or artificially generated (by means of servo motors and spring
+# preloading) forces act on the stick/yoke and it is in a comfortable position.
+# This doesn't work well on computer joysticks.
+#
+# SYNOPSIS:
+#	autotrim.start();  # on key/button press
+#	autotrim.stop();   # on key/button release (mod-up)
+#
+# USAGE:
+#	(1) move the stick such that the aircraft is in an orientation that
+#	    you want to trim for (forward flight, hover, ...)
+#	(2) press autotrim button and keep it pressed
+#	(3) move stick/yoke to neutral position (center)
+#	(4) release autotrim button
+#
+var autotrim = {
+	init : func {
+		me.elevator = me.Trim.new("elevator");
+		me.aileron = me.Trim.new("aileron");
+		me.rudder = me.Trim.new("rudder");
+		me.loopid = 0;
+		me.active = 0;
+	},
+	start : func {
+		me.active and return;
+		me.active = 1;
+		me.elevator.start();
+		me.aileron.start();
+		me.rudder.start();
+		me._loop_(me.loopid += 1);
+	},
+	stop : func {
+		me.active or return;
+		me.active = 0;
+		me.loopid += 1;
+		me.update();
+	},
+	_loop_ : func(id) {
+		id == me.loopid or return;
+		me.update();
+		settimer(func { me._loop_(id) }, 0);
+	},
+	update : func {
+		me.elevator.update();
+		me.aileron.update();
+		me.rudder.update();
+	},
+	Trim : {
+		new : func(name) {
+			var m = { parents : [ autotrim.Trim ] };
+			m.trimN = props.globals.getNode("/controls/flight/" ~ name ~ "-trim", 1);
+			m.ctrlN = props.globals.getNode("/controls/flight/" ~ name, 1);
+			return m;
+		},
+		start : func {
+			me.last = me.ctrlN.getValue();
+		},
+		update : func {
+			var v = me.ctrlN.getValue();
+			me.trimN.setDoubleValue(me.trimN.getValue() + me.last - v);
+			me.last = v;
+		},
+	},
+};
+
+
 # HUD control class to handle both HUD implementations
 # ==============================================================================
 #
@@ -709,6 +779,7 @@ _setlistener("/sim/signals/nasal-dir-initialized", func {
 	props.globals.getNode("/sim/time/delta-realtime-sec", 1).setDoubleValue(0.00000001);
 	HUD.init();
 	data.init();
+	autotrim.init();
 
 	if (getprop("/sim/startup/save-on-exit")) {
 		data.load();