(svn r25115) -Fix (r8973) [FS#5492-ish]: [NewGRF] Acceleration of NewGRF aircraft was too fast, while acceleration of default aircraft was way too slow. I.e. choose wisely who to let write the software for your orbiter.

src/aircraft_cmd.cpp

@@ -598,7 +598,14 @@ enum AircraftSpeedLimits {
  */
 static int UpdateAircraftSpeed(Aircraft *v, uint speed_limit = SPEED_LIMIT_NONE, bool hard_limit = true)
 {
-	uint spd = v->acceleration * 16;
+	/**
+	 * 'acceleration' has the unit 3/8 mph/tick. This function is called twice per tick.
+	 * So the speed amount we need to accelerate is:
+	 *     acceleration * 3 / 16 mph = acceleration * 3 / 16 * 16 / 10 km-ish/h
+	 *                               = acceleration * 3 / 10 * 256 * (km-ish/h / 256)
+	 *                               ~ acceleration * 77 (km-ish/h / 256)
+	 */
+	uint spd = v->acceleration * 77;
 	byte t;
 
 	/* Adjust speed limits by plane speed factor to prevent taxiing

src/newgrf.cpp

@@ -1618,15 +1618,9 @@ static ChangeInfoResult AircraftVehicleChangeInfo(uint engine, int numinfo, int
 				avi->max_speed = (buf->ReadByte() * 128) / 10;
 				break;
 
-			case 0x0D: { // Acceleration
-				uint acceleration = (buf->ReadByte() * 128) / 10;
-				if (acceleration > UINT8_MAX) {
-					grfmsg(1, "Acceleration property of aircraft %d is too big.", engine + i);
-					acceleration = UINT8_MAX;
-				}
-				avi->acceleration = acceleration;
+			case 0x0D: // Acceleration
+				avi->acceleration = buf->ReadByte();
 				break;
-			}
 
 			case PROP_AIRCRAFT_RUNNING_COST_FACTOR: // 0x0E Running cost factor
 				avi->running_cost = buf->ReadByte();

src/saveload/afterload.cpp

@@ -2761,6 +2761,17 @@ bool AfterLoadGame()
 		_settings_game.script.settings_profile = IsInsideMM(_old_diff_level, SP_BEGIN, SP_END) ? _old_diff_level : (uint)SP_MEDIUM;
 	}
 
+	if (IsSavegameVersionBefore(182)) {
+		Aircraft *v;
+		/* Aircraft acceleration variable was bonkers */
+		FOR_ALL_AIRCRAFT(v) {
+			if (v->subtype <= AIR_AIRCRAFT) {
+				const AircraftVehicleInfo *avi = AircraftVehInfo(v->engine_type);
+				v->acceleration = avi->acceleration;
+			}
+		}
+	}
+
 	/* Road stops is 'only' updating some caches */
 	AfterLoadRoadStops();
 	AfterLoadLabelMaps();

src/table/engines.h

@@ -580,8 +580,8 @@ static const ShipVehicleInfo _orig_ship_vehicle_info[] = {
  * @param c running_Cost
  * @param d subtype (bit 0 - plane, bit 1 - large plane)
  * @param e sound effect
- * @param f acceleration
- * @param g max_speed (1 unit = 8 mph = 12.8 km-ish/h)
+ * @param f acceleration (1 unit = 3/8 mph/tick = 3/5 km-ish/h/tick) (stays the same in the variable)
+ * @param g max_speed (1 unit = 8 mph = 12.8 km-ish/h) (is converted to km-ish/h by the macro)
  * @param h mail_capacity (bags)
  * @param i passenger_capacity (persons)
  */