Flying Machines have two main purposes: to scout, and to attack air units. If someone is making a lot of air units, use Flying Machines to attack them. Upgrade the Flying Machine to enhance it with more damage.
Flying Machines also act as detectors for invisible units.
Spells and Abilities
True Sight (Passive)
Reveals nearby invisible units.
Flying Machine Bombs (Passive)
Allows Flying Machines to attack land units.
Flak Cannons (Passive)
Upgrades the weapons on Flying Machines to give them an area effect damage attack against air units.