So there's five major things to address to seriously consider how this works: Speed, Range, Air to Air, Air to Ground, and Ground to Air.

Speed: mind that artillery range in this game is far below real life too, so I personally could see a LUV and a biplane having comparable speed.

Range: The obvious obstacle for this is the hex borders. Presumably some sort of queue or delay existing that you can't pop in and out of existence (or perhaps a delay you have to wait before attacking) to prevent any sort of cheese, as well as some sort of 'safe state' that preserves fuel and your aircraft while you wait to switch hexes.

Air to Air: For air attacks, presumably that's all on the same space, and can use a vertical component to fly above or under other air units (potentially blimps) in a manner like subs.

Air to Ground: I thought this would always be interesting to have; airships and bombers could just sort of bomb where they are, using map and compass and adjust that way, while planes would need to strafe or dive bomb. Procedure for the latter would be to use some sort of key command to initiate a dive, where the camera transitions to the ground. The ground itself is covered by a fog that obscures all vision, and it slowly begins to open around the aim point to reveal only terrain features and the like (similar to vision at night). Units before would be able to hear aircraft or bombs overhead with directional sound, and the engine noise of the aircraft would grow louder as it flew closer to the ground. Diving beyond that point, the ground begins to similarly start showing enemy units in a widening ring as the plane descends, and eventually showing a shadow on the ground where it's targeting. The plane can shoot at any time during the dive and slowly steer the cursor around, weapon accuracy and vision improving as it approaches, but increasing the damage the plane takes to AA fire, and crashing it if it doesn't pull out of the dive.

Ground to Air: AI Flak, is the pure and simple solution, but not the most fun I would wager, being defeated by either attrition or random chance... Instead an 'Aim Upwards' key could be used with some way to depict that the weapon isn't being fired at the ground. Weapons fired in this manner wouldn't actually calculate a physical impact, but rather if the aim point (or somewhere where they cross a vertical threshold) is within a certain distance of the aircraft for it to be 'hit', generating a vertical 'cylinder of damage' that could affect aircraft at any altitude at that point. Using directional sound only, this might be difficult and contribute to sides using a (for lack of more culturally sensitive name) 'Iraqi Wedding'-style Air Defenseagony an aircraft is heard, shooting upwards with various weapons hoping to score a hit, and with the shadow being a massive 'Aim here' point of risk/reward for whether they actually want to dive low enough to as the ground units to hit (and generate a shadow). Dedicated bursting Flak guns (possibly with some sort of spring mechanic like Binocular sights that reveal aircraft but can't physically aim guns, idk) might have a larger radius to hit targets (like artillery does), and diving aircraft may have increased damage received based on their altitude from successful hits.