We don't handle propellers turning backwards. This got clamped

implicitly in PropEngine by disallowing negative torques, but that was
removed at some point.  We really need the explosion protection here
at the source.

src/FDM/YASim/Propeller.cpp

@@ -53,6 +53,9 @@ void Propeller::calc(float density, float v, float omega,
     // calculate V (propeller "speed")
     if(v < 0) v = 0;
 
+    // The model doesn't work for propellers turning backwards.
+    if(omega < 0.001) omega = 0.001;
+
     float J = v/omega;
     float lambda = J/_j0;