From 180a7905e58c54c5044626e858d513dde31c4d56 Mon Sep 17 00:00:00 2001
From: Andrew Tridgell <andrew@tridgell.net>
Date: Fri, 22 Apr 2016 10:45:55 +1000
Subject: [PATCH] SITL: make Z down in motors

---
 libraries/SITL/SIM_Frame.cpp | 2 +-
 libraries/SITL/SIM_Motor.cpp | 2 +-
 libraries/SITL/SIM_Motor.h   | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/libraries/SITL/SIM_Frame.cpp b/libraries/SITL/SIM_Frame.cpp
index 17f2ff717d..36ddf64f9f 100644
--- a/libraries/SITL/SIM_Frame.cpp
+++ b/libraries/SITL/SIM_Frame.cpp
@@ -174,7 +174,7 @@ void Frame::calculate_forces(const Aircraft &aircraft,
         thrust += mthrust;
     }
 
-    body_accel = -thrust/mass;
+    body_accel = thrust/mass;
 
     if (terminal_rotation_rate > 0) {
         // rotational air resistance
diff --git a/libraries/SITL/SIM_Motor.cpp b/libraries/SITL/SIM_Motor.cpp
index eae3c0f967..85a3c4221e 100644
--- a/libraries/SITL/SIM_Motor.cpp
+++ b/libraries/SITL/SIM_Motor.cpp
@@ -33,7 +33,7 @@ void Motor::calculate_forces(const Aircraft::sitl_input &input,
     rot_accel.x = -radians(5000.0) * sinf(radians(angle)) * motor_speed;
     rot_accel.y =  radians(5000.0) * cosf(radians(angle)) * motor_speed;
     rot_accel.z = yaw_factor * motor_speed * radians(400.0);
-    thrust(0, 0, motor_speed * thrust_scale); // newtons
+    thrust(0, 0, -motor_speed * thrust_scale); // newtons NED
     if (roll_servo >= 0) {
         float roll;
         uint16_t servoval = input.servos[roll_servo+motor_offset];
diff --git a/libraries/SITL/SIM_Motor.h b/libraries/SITL/SIM_Motor.h
index fd289d0c93..8171d47bd4 100644
--- a/libraries/SITL/SIM_Motor.h
+++ b/libraries/SITL/SIM_Motor.h
@@ -68,5 +68,5 @@ public:
                           float thrust_scale,
                           uint8_t motor_offset,
                           Vector3f &rot_accel, // rad/sec
-                          Vector3f &body_thrust) const; // newtons
+                          Vector3f &body_thrust) const; // Z is down
 };