But all these concepts are massively expensive, require safety certification standards for road and air, need dual controls, involve complex folding wings and propellers,and have to be flown from air-strips.
2019年6月六级真题（第二套）阅读 Section A
柯林斯高阶英汉双解学习词典释义
N-COUNT
（轮船或飞机上的）螺旋桨，推进器
A propeller is a device with blades which is attached to a boat or aircraft. The engine makes the propeller spin round and causes the boat or aircraft to move.