Ensure non-zero acceleration of N1/N2 for windmilling when N1/N2 = 0 (J…

…SBSim-Team#509)

Fixes issue JSBSim-Team#494.

src/models/propulsion/FGTurbine.cpp

@@ -190,8 +190,9 @@ double FGTurbine::Off(void)
   FuelFlow_pph = Seek(&FuelFlow_pph, 0, 1000.0, 10000.0);
   // some engines have inlets that close when they are off. So, if a flag is true disable windmilling
   if (disableWindmill == false) {
-    N1 = Seek(&N1, in.qbar/10.0, N1/2.0, N1/N1_spindown);
-    N2 = Seek(&N2, in.qbar/15.0, N2/2.0, N2/N2_spindown);
+    // Need a small non-zero increment for acceleration otherwise acceleration will be 0 if N1 = 0
+    N1 = Seek(&N1, in.qbar/10.0, N1/2.0 + 0.1, N1/N1_spindown);
+    N2 = Seek(&N2, in.qbar/15.0, N2/2.0 + 0.1, N2/N2_spindown);
   } else {
     N1 = Seek(&N1, 0, N1/2.0, N1/N1_spindown);
     N2 = Seek(&N2, 0, N2/2.0, N2/N2_spindown);