Piston shall not send negative powers to thrusters

This happens when the engine is off: internal friction (static_friction_HP which is negative) is accounted for but the engine does not produce power so this internal friction is not counteracted resulting in an overall negative power. A better fix in the long term should therefore be to ignore static_friction_HP while the engine is not started.

src/models/propulsion/FGPiston.cpp

@@ -518,7 +518,10 @@ void FGPiston::Calculate(void)
   }
 
   LoadThrusterInputs();
-  Thruster->Calculate(HP * hptoftlbssec);
+  // Filters out negative powers.
+  // TODO: static_friction_HP should not be taken into account while the engine
+  // is not started.
+  Thruster->Calculate(max(HP * hptoftlbssec, 0.));
 
   RunPostFunctions();
 }