A helicopter rises from rest on the ground vertically upwards with...

$⇒$For upward motion of helicopter

${v}^{2} = {u}^{2} + 2as$

${v}^{2} = 0 + 2gh$

$v = \sqrt{2gh}$

$⇒$ Now particle will start moving under gravity.

$s = ut + \dfrac{1}{2}a{t}^{2}$

$- h = \sqrt{2gh}t - \dfrac{1}{2}g{t}^{2}$

then $t = \dfrac{\sqrt{2gh} \pm \sqrt{2gh + 4 \times \dfrac{g}{2} \times h}}{2 \times \dfrac{g}{2}}$

$t = \sqrt{\dfrac{2gh}{g}} ( 1 + \sqrt{2} )$

$t = \sqrt{\dfrac{2h}{g}} ( 1 + \sqrt{2} )$