I tried it, and it has unleashed a whole new problem. The entity now falls at a speed that is dependent on the value of fps_max. If I lower that value then the thing falls slower, if I raise it, it falls faster. Since I am building a training simulator (as opposed to a normal game) I need things to be as close to reality as possible.

What is the relationship between ticks, time and frame rate? If I know this I should be able to alter the acceleration due to gravity to get things right. I think.

P.S In case this game concept looks a little strange, the simulator isn't for training human cannonballs, it's using the aiming mechanism for training a special kind of steering. I don't think human cannonballs get any kind of specialised training.