Limit speed on x axis when applying force with gravity

 I have a 2dSprite with rigidbody2d gravity.
 when pressing the "down" key, I constant relative force.
 I want to limit the speed of the sprite, this works great when the movement is only on the Y axis (rotation=0).
 But when the sprite has a rotation, the gravity makes it fall down.
 
 Code: 
 
     void FixedUpdate () {
         float MAX_SPEED = 3;
         if (Input.GetKey ("down")) {
             Vector2 power_vector = new Vector2(0,0.5f);
             if (r_2d.velocity.magnitude<MAX_SPEED)
                 ApplyRelativeForce (power_vector, ForceMode2D.Impulse);
         }
     }
 
 Suggestions?