A helicopter reises from rest on the ground vertically upwards with a constant acceleration g

$v^{2}=u^{2}+2 a s$

$v^{2}=2 g h+2 g h$

$v=\sqrt{4 g h}$

$\Rightarrow \sqrt{4 \mathrm{gh}}=\sqrt{2 \mathrm{gh}}+\mathrm{gt}$

$\Rightarrow \mathrm{t}=\sqrt{\frac{4 \mathrm{~h}}{\mathrm{~g}}}-\sqrt{\frac{2 \mathrm{~h}}{\mathrm{~g}}} \Rightarrow 3.4 \sqrt{\frac{\mathrm{h}}{\mathrm{g}}}$