How to Rotate A Plane Normal 90 degree downward?

Help! I can’t find a solution to rotate a plane normal 90 degree downward.

To put it this way, what I’m trying to achieve is to let a actor slide downward on a angled plane. I can’t use plane constraint normal cause I’m not using character movement. I was trying to find the slide direction and add force to the actor to that direction.

I tried to calculate the slide direction using plane normal but I can’t figure out how to do it.

Someone please help! Thanks.

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