Add the Mach vaiable.

src/AIModel/AIBallistic.cxx

@@ -36,6 +36,7 @@ FGAIBallistic::FGAIBallistic(FGAIManager* mgr) {
     gravity = 32;
 //  buoyancy = 64;
     no_roll = false;
+    Mach = 0.4;
 }
 
 FGAIBallistic::~FGAIBallistic() {

src/AIModel/AIBallistic.hxx

@@ -67,6 +67,7 @@ private:
     bool wind;              // if true, local wind will be applied to object
     double Cd;              // drag coefficient
     double mass;            // slugs
+    double Mach;
 
     void Run(double dt);
 };