Abstract
This paper reviews the different flight mechanisms of UAVs. First of all, we divided UAVs into four main categories; Horizontal Takeoff and Landing(HTOL), Vertical Takeoff and Landing(VTOL), Hybrid and Bio-Based. Then each category is divided into some sub-categories in order to have a better review. in this article the focus is on the flight mechanism and scheme, so space UAVs and Amphibians are neglected due to the use of the similar mechanism. Similarly, Smart Dusts (which are explored in some similar activities) are ignored.