AddForce on local transform

Hi there! I'm currently trying to make a game to put my relatively new coding knowledge to test. I figured I learn a lot quicker by just doing things and seeing what happens. I'm currently working on a plane and the last couple of days I made a functional camera system (to rotate around the player based on the mouse) and some animations of the plane. I'm currently trying to create a movement system in which the down key rotates the player up. I noticed however that my AddForce function doesn't do what I want it to do. I'd like the plane to move in the Z axis depending on the current rotation of said plane instead of it just heading into the Z angle of the world.

I'm currently working on this snippet:

         //Acceleration
         if (gameObject.CompareTag("Player") && playerRb.velocity.z > -349)
         {
             playerRb.AddRelativeForce(0, 0, -playerSpeed, ForceMode.Acceleration);
         }
         else if (gameObject.CompareTag("Player") && playerRb.velocity.z < -349)
         {
             playerRb.velocity = new Vector3(playerRb.velocity.x, playerRb.velocity.y, -350);
         }
         //
 
         //Movement
         //Up/down controls
         if (gameObject.CompareTag("Player") && Input.GetKey(KeyCode.DownArrow))
         {
             transform.Rotate((0.07f * playerSpeed) * Time.deltaTime, 0, 0);
         }
 

As you can see I already tried AddRelativeForce as another topic mentioned but that doesn't seem to work for me. The moment it hits an edge it just falls down into the abyss regardless of it's rotation.

Additionally, I was considering using another AddForce function on the rotate as it doesn't leave the ground (considering it keeps hitting the floor with its tail), is there any way to apply a force on an object from a specific position/angle? (such as the front of the plane).

Thanks in advance!