Originally Posted by CPMcGraw

Probably several. Check the flight conditions dialog, and you'll see things like temperature, wind, humidity, etc...

Each flight of a real model is different, no two being exactly alike. RockSim tries to simulate real-world conditions, so each simulation run has changes in those environmental parameters that mimic what you might find at your local field. In fact, you can go in and make some direct changes to this section that very closely match the typical pattern of your local field. I don't usually change anything here, being lazy, but the option is there to really fine-tune the simulation for your local patch of sky.