Determine if the propellor is clockwise or counter-clockwise rotating.

Each arm is labeled with a rotation icon that shows the direction of propellor rotation.

If this icon is missing or you can't read it, you can determine the direction of rotation by examining the propellor.

A clockwise rotating propellor pushes air down while rotating clockwise, and the opposite is true for a counter-clockwise rotating propellor, and the propellor blade slopes up in the direction of rotation.