From 78f5db1a9e19bda93cca0d06468e271c05c602f1 Mon Sep 17 00:00:00 2001
From: Alex Selimov <alex@alexselimov.com>
Date: Wed, 12 Aug 2026 20:16:48 -0400
Subject: [PATCH] Add propeller spin direction

---
 src/constants.zig   | 5 +++++
 src/drone.zig       | 2 ++
 src/forces.zig      | 3 ++-
 src/integration.zig | 2 ++
 4 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/src/constants.zig b/src/constants.zig
index e574c41..043b397 100644
--- a/src/constants.zig
+++ b/src/constants.zig
@@ -1 +1,6 @@
 pub const g: f64 = 9.80665;
+
+pub const PropSpinDirection = enum(i8) {
+    cw = 1,
+    ccw = -1,
+};
diff --git a/src/drone.zig b/src/drone.zig
index 10e8002..039cb79 100644
--- a/src/drone.zig
+++ b/src/drone.zig
@@ -1,5 +1,6 @@
 const vec3 = @import("./vec3.zig");
 const std = @import("std");
+const constants = @import("./constants.zig");
 
 pub const Drone = struct {
     mass_kg: f64,
@@ -19,6 +20,7 @@ pub const Propeller = struct {
     angular_velocity: f64,
     thrust_coefficient: f64,
     moment_coefficient: f64,
+    direction: constants.PropSpinDirection,
 };
 
 pub const QuadCopterSim = struct {
diff --git a/src/forces.zig b/src/forces.zig
index 247d020..86171ea 100644
--- a/src/forces.zig
+++ b/src/forces.zig
@@ -23,7 +23,7 @@ pub fn propThrust(attitude: vec3.QuatF64, p: drone.Propeller) struct { vec3.Vec3
     const drag_moment = vec3.Vec3F64.init(
         0,
         0,
-        p.moment_coefficient * p.angular_velocity * p.angular_velocity,
+        @intFromEnum(p.direction) * p.moment_coefficient * p.angular_velocity * p.angular_velocity,
     );
 
     return .{ force, vec3.vec3Add(force_moment, drag_moment) };
@@ -55,6 +55,7 @@ test "thrust is correct" {
         .angular_velocity = 100,
         .thrust_coefficient = 1,
         .moment_coefficient = 2,
+        .direction = constants.PropSpinDirection.cw,
     };
 
     const expected_thrust_mag = -100 * 100;
diff --git a/src/integration.zig b/src/integration.zig
index 7bd4804..7e5cf65 100644
--- a/src/integration.zig
+++ b/src/integration.zig
@@ -1,6 +1,7 @@
 const drone = @import("./drone.zig");
 const vec3 = @import("./vec3.zig");
 const forces = @import("./forces.zig");
+const constants = @import("constants.zig");
 const std = @import("std");
 
 pub fn timestep(dt: f64, d: drone.Drone, props: []const drone.Propeller, s: *drone.State) void {
@@ -62,6 +63,7 @@ test "Validate timesteps" {
         .cg_to_prop = .init(1, 0, 0),
         .moment_coefficient = 0.1,
         .thrust_coefficient = 0.5,
+        .direction = constants.PropSpinDirection.cw,
     };
     const props: [1]drone.Propeller = .{prop};