As mentioned by Vagabond, Drones have a lift problem associated with a low atmosphere planet, with reduced levels of oxygen that would be needed for combustion engines. So, how would they remain aloft?

Bomber Drones would be purely jet-engined powered, and would burn a mixture that supplied it's only oxygen for the combustion process. They would have a limited fuel capability, that would need to be replenished while they get rearmed. This limited fuel would also mean that they would have limited flight time, and if they ran out of fuel before reaching the drone bay, they would crashland. They refuel and reload at a Drone Factory.

Assassin and Repair Drones on the other hand are equipped with a large array of batteries to power their turbo-prop engines that keep them aloft. They would have effiicient solar panels on their roofs, which would resupply power during daylight hours, but not at a rate that would allow them to remain airborne. If they run out of power, they must land and take the time necessary to restore their batteries via solar power. Alternatively, they can visit various structures, that are outfitted with battery recharge stations and restore their energy supplies quickly in this manner. Assassin and Repair Drones also have limited fuel to allow them to have faster forward movement, which can only be resupplied at a Drone Factory.