Someone mentioned once about the zerg not really having air superiority in the game, even when char is being invaded. The reason for that is simple, because the zerg forces had to be arranged in a beatable way. The maps were setup with solid anti air, OR anti ground defenses, meaning there was always a hole to exploit. Even when they were combined, it was always light enough to muscle through in the end. Watch the cinematic of the start to the invasion of char. The entire battlefleet was nearly wiped out by the zerg anti air. The ground troops were scattered and slaughtered. Only our resident hero raynor managed to keep the entire debacle from turning into a total loss. Without him, its likely every human on char would have been slaughtered.