Do flying monsters have to land before engaging in melee with ground targets? Are they required to move a minimum amount each round to remain in the air? Can they move part of their speed, attack, and then continue moving?
3.5 had (not very good) maneuverability rules to answer the first two questions, and the Flyby Attack feat to answer the third. ACKS has the Dive special attack, but that is an example of what some flying monters can do rather than a set of general guidelines.
The mechanics from Domains at War are that flying creatures are not threatened by any unit unless that unit has readied an attack. The flying unit attack method is basically "drop, attack, rise". Ratcheted down to the m2m scale, I guess one could imagine the giant hawk or whatever swoops down to attack as part of it's attack method, then up out of reach; re: The Birds.
I'd probably work the fly-by attack as an Overrun, but the flyer would be out of reach next to the target rather than within reach on a successful block as a land-based combatant would do.
That has always been weird - the lack of momentum, in either flying creatures or charging horses or whatever. There was a thread on charging a little while back but I can't find it.
Another Flyer thread in the same vein is here: http://www.autarch.co/forums/ask-autarchs/dive-dive-dive
Yeah, lack of momentum on charging horses makes it hella weird when a player wants to be a Knight and ride around with his Order of the Punches warriors on horses and they all charge. I cannot for the life of me think of an easy set of rules that'd make mounted combat look sane though.
The lack of momentum is just an inherent foible of turn-based systems. Any system that involves only moving on your turn and being stationary while everyone else takes their turns is going to have some weird momentum-type interactions.
The things that have been tried to solve it (requiring movement at particular rates, requiring movement at a percentage of recent movement, whatever) mostly just add complexity without actually solving the core problem that the system is turn based and is designed to essentially freeze time for anyone who is not currently taking a turn.
Hackmaster (5E, the new edition) actually does use seconds as their unit of time instead of rounds, and has no defined turns. (Obviously, if two people act on the same second, their actions are simultaneous).
It works beautifully in my opinion, the drawback is that any detailed system cannot avoid being complex. But given how much we all love spreadsheets, I do think there would be a lot of ACKS players who would enjoy the HM5 combat system (with, of course, the ACKS economy, domains, etc layered on it).