I can't believe that your Mum is truly a "helicopter parent".
If she was, she would be aware of the effect your stepfather's abuse is having on you, as helicopter parents live and breathe the experience of their children.
Either your Mum is not observant enough to realise the negative effect on you of the abuse, or she has observed it and has chosen to ignoree it.
Whichever is the case, you owe her no special consideration.
There is no "special" way to tell her that you want to leave.
Just come out with it - "Mum, I am not happy with the situation here and I am going to live with my grandparents to give myself time to restore my mental wellbeing".
Have your bags already packed before you say, this. After you say it, do not hang around for the inevitable argument.
Just pick up your bags and go.
That's what I did when I was 16.
It's hard, but you have to be strong.