expression, godan verb (-bu)
fly in the air
Describes the action of flying through the sky, often used for birds, airplanes, or fantastical creatures. The phrase is a set expression combining 空 (sky) and 飛ぶ (to fly).
鳥が空を飛んでいる。
A bird is flying in the sky.
飛行機が空を飛ぶのを見た。
I saw an airplane flying in the sky.
飛ぶ is the general verb for 'to fly', while 空を飛ぶ specifically emphasizes flying through the sky.
A set phrase composed of 空 (sky) + を (object marker) + 飛ぶ (to fly). The combination is straightforward and literal.