there was an interesting suggestion to implement flying as a special defense mode, but I don't think anyone could work out good mechanics. Think closest was while in the defense mode, you couldn't be melee'd, and if you were already engaged in melee you got normal defense mode instead. (this means once the flyer has attacked on a turn, it has "landed" and thus vulnerable to melee and you can use industrious arbalest to "force units to last" by breaking defense mode). Trouble is then it feels pointless, because if it only works while defending, its a complicated mechanic that will be of limited use and probably OP for units with a skill that puts them in defense mode anyway, like hero skill defensive master.