A <a href="https://en.wikipedia.org/wiki/quadrotor" class="extiw" title="w:quadrotor">quadrotor</a> hovers or vertically takes off/lands by applying equal thrust to each of the four rotors. In this diagram, the thickness of the arrows indicate the relative amount of thrust applied.
<a href="//commons.wikimedia.org/wiki/User:Purpy_Pupple" class="mw-redirect" title="User:Purpy Pupple">Purpy Pupple</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>