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月六級真題（第二套）閱讀 Section A
柯林斯高階英漢雙解學習詞典釋義
N-COUNT
（輪船或飛機上的）螺旋槳，推進器
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.