Make wheels spin down slowly once off the ground

src/FDM/YASim/Gear.cpp

@@ -282,13 +282,25 @@ float Gear::getBumpAltitude()
     return h*(1/8.)*_ground_bumpiness*maxGroundBumpAmplitude;
 }
 
+void Gear::integrate(float dt)
+{
+    // Slowly spin down wheel
+    if (_rollSpeed > 0) {
+        // The brake factor of 13.0 * dt was copied from JSBSim's FGLGear.cpp and seems to work reasonably.
+        // If more precise control is needed, then we need wheel mass and diameter parameters.
+        _rollSpeed -= (13.0 * dt + 1300 * _brake * dt);
+        if (_rollSpeed < 0) _rollSpeed = 0;
+    }
+    return;
+}
+
 void Gear::calcForce(RigidBody* body, State *s, float* v, float* rot)
 {
     // Init the return values
     int i;
     for(i=0; i<3; i++) _force[i] = _contact[i] = 0;
 
-    // Don't bother if it's not down
+    // Don't bother if gear is retracted
     if(_extension < 1)
     {
         _wow = 0;
@@ -328,7 +340,6 @@ void Gear::calcForce(RigidBody* body, State *s, float* v, float* rot)
         _wow = 0;
         _frac = 0;
         _compressDist = 0;
-        _rollSpeed = 0;
         _casterAngle = 0;
         return;
     }

src/FDM/YASim/Gear.hpp

@@ -70,6 +70,7 @@ public:
     float getBumpAltitude();
     bool getGroundIsSolid();
     float getGroundFrictionFactor() { return (float)_ground_frictionFactor; }
+    void integrate(float dt);
 
     // Takes a velocity of the aircraft relative to ground, a rotation
     // vector, and a ground plane (all specified in local coordinates)

src/FDM/YASim/Model.cpp

@@ -136,6 +136,11 @@ void Model::initIteration()
         _turb->offset(toff);
     }
 
+    for(i=0; i<_gears.size(); i++) {
+        Gear* g = (Gear*)_gears.get(i);
+        g->integrate(_integrator.getInterval());
+    }
+
     for(i=0; i<_hitches.size(); i++) {
         Hitch* h = (Hitch*)_hitches.get(i);
         h->integrate(_integrator.getInterval());