An aeroplane has to reach a certain speed before there is enough lift to get it off the ground.
飞机得在达到一定的速度以后，才能有足够的提升力从地面起飞。
《柯林斯英汉双解大词典》
2
Surface proximity and swirl rotational speed are the two basic factors leading to flame lift-off asymmetry, but their influences on the flame lift-off length is not strong.