The free-to-play game C-evo has only one predesigned air unit: the Glider, which reverts to being a ground unit at the end of each turn. All others are designed by the player. Even the most primitive require Science; interestingly, even the most advanced (using Smart Weapons) do not require Steel. Air units stop blinking when they reach one of their owner's cities (unless just passing through on a "Go" order), but they can be restarted if they have any movement points remaining.