Add API function to get/set a modifier function for part local forces…

… before applying them

FerramAerospaceResearch/FARAPI.cs

@@ -42,6 +42,7 @@
 	http://forum.kerbalspaceprogram.com/threads/60863
  */
 
+using System;
 using ferram4;
 using FerramAerospaceResearch.FARAeroComponents;
 using FerramAerospaceResearch.FARGUI.FARFlightGUI;
@@ -419,5 +420,28 @@ public static bool VesselVoxelizationCompletedAndValid(Vessel vessel)
 
             return !(vesselAeroModule is null) && vesselAeroModule.HasValidVoxelizationCurrently();
         }
+
+        /// <summary>
+        /// Set a modifier function for part local forces in case FAR does not correctly handle them.
+        /// It will be called before applying the forces to the part/simulation.
+        /// </summary>
+        /// <param name="part">Part to apply modifier to</param>
+        /// <param name="modifier">Modifier function of part local forces.
+        /// Signature is (part, (drag, lift, torque)) -> (new_drag, new_lift, new_torque)</param>
+        public static void SetPartAeroForceModifier(Part part, Func<Part, Vector3, Vector3> modifier)
+        {
+            part.FindModuleImplementing<FARAeroPartModule>().AeroForceModifier = modifier;
+        }
+
+        /// <summary>
+        /// Get the current part local forces modifier function
+        /// </summary>
+        /// <param name="part"></param>
+        /// <returns>Modifier function of part local forces if set and the part is valid, otherwise null.
+        /// Signature is (part, (drag, lift, torque)) -> (new_drag, new_lift, new_torque)</returns>
+        public static Func<Part, Vector3, Vector3> GetPartAeroForceModifier(Part part)
+        {
+            return part.FindModuleImplementing<FARAeroPartModule>()?.AeroForceModifier;
+        }
     }
 }

FerramAerospaceResearch/FARAeroComponents/FARAeroPartModule.cs

@@ -115,6 +115,8 @@ public class FARAeroPartModule : PartModule, ILiftProvider
         private bool updateVisualization;
         public FARWingAerodynamicModel LegacyWingModel { get; private set; }
 
+        public Func<Part, Vector3, Vector3> AeroForceModifier { get; set; }
+
         public ProjectedArea ProjectedAreas
         {
             get { return projectedArea; }
@@ -270,7 +272,7 @@ private static void IncrementAreas(ref ProjectedArea data, Vector3 vector, Matri
 
         private void Start()
         {
-            shield = new DummyAirstreamShield {part = part};
+            shield = new DummyAirstreamShield { part = part };
 
             if (waterSlowDragNew < 0)
             {
@@ -761,6 +763,7 @@ private void OnDestroy()
 
             LegacyWingModel = null;
             stockAeroSurfaceModule = null;
+            AeroForceModifier = null;
         }
 
         public struct ProjectedArea

FerramAerospaceResearch/FARAeroComponents/FARAeroSection.cs

@@ -46,6 +46,7 @@
 using System.Collections.Generic;
 using ferram4;
 using FerramAerospaceResearch.FARPartGeometry;
+using Unity.Mathematics;
 using UnityEngine;
 
 namespace FerramAerospaceResearch.FARAeroComponents
@@ -447,6 +448,24 @@ IForceContext forceContext
                     torqueVector -= dampingMoment * nonAxialAngLocalVel +
                                     axialAngLocalVel * (rollDampingMoment * axialAngLocalVel.magnitude) / 0.001f;
 
+                if (aeroModule.AeroForceModifier is not null)
+                {
+                    float velLocalForceMagnitude = Vector3.Dot(forceVector, velLocalNorm);
+                    Vector3 velParallelForce = velLocalForceMagnitude * velLocalNorm; // anti-drag
+                    Vector3 velNormalForce = forceVector - velParallelForce; // lift + side force
+                    var localForce = new Vector3
+                    {
+                        x = -velLocalForceMagnitude,
+                        y = velNormalForce.magnitude,
+                        z = torqueVector.magnitude,
+                    };
+                    float3 newLocalForce = aeroModule.AeroForceModifier(aeroModule.part, localForce);
+                    float3 multiplier = newLocalForce / localForce;
+
+                    forceVector = velParallelForce * multiplier.x + velNormalForce * multiplier.y;
+                    torqueVector *= multiplier.z;
+                }
+
                 forceVector *= data.dragFactor;
                 torqueVector *= data.dragFactor;