How to move forward (towards z), ignoring any rotations?

Hello. I have an object that moves forward along its z axis when you press w, but the object will move up or down if either x or y have any values. I want to ignore these axis and move only forward.

example, you are walking forward with your head tilted down. your body moves forward, not down towards where you are looking.

What im trying to do is make a basic arcadey style helicopter controls. When you push forward, the helicopter moves forward, but to make it looks a bit more "real" the helicopter tilts forward about 15 degrees. When this occurs, the helicopter then travels forward and down. I want to just have the helicopter move forward, not down as well.

(im using Unity5.5.0f3)

heres my code (editing out the irrelevant parts):

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class ApacheScript : MonoBehaviour{
 
     public Rigidbody helicopterRigidbody;
     
     private float leftRightRot;
     private float frontBackRot;
     private float leftRightMin;
     private float leftRightMax;
     private float frontBackMin;
     private float frontBackMax;
     private Quaternion zRotation;//leftRight
     private Quaternion xRotation;//frontBack
 
     private Vector3 temp;
 
     private float enginePower = 1000f;
     public bool isOnGround = true;
 
     void Start (){
         leftRightRot = 0f;
         leftRightMax = 30f;
         leftRightMin = -30f;
 
         frontBackMin = -15f;
         frontBackMax = 15f;
         frontBackRot = 0;
 
         helicopterRigidbody = gameObject.GetComponent<Rigidbody> ();
     }
 
     void Update () {
         temp = transform.eulerAngles;
         
         if (Input.GetKey (KeyCode.W)) {
             helicopterRigidbody.drag = 1;
             helicopterRigidbody.useGravity = false;
             transform.Rotate (2, 0, 0);
             helicopterRigidbody.AddForce(transform.forward * (70f * enginePower));
         }
         if (Input.GetKey (KeyCode.S)) {
             helicopterRigidbody.drag = 1;
             helicopterRigidbody.useGravity = false;
             transform.Rotate (-2, 0, 0);
             helicopterRigidbody.AddForce(transform.forward * (-40f * enginePower));
         } 
         
         if (temp.z > 31f && temp.z <= 180f) {
             temp.z = 30f;
             transform.eulerAngles = temp;
         } else if (temp.z > 180f && temp.z < 329f) {
             temp.z = 330f;
             transform.eulerAngles = temp;
         }
 
         if (temp.x > 16f && temp.x <= 180f) {
             temp.x = 15;
             transform.eulerAngles = temp;
         } else if (temp.x > 180f && temp.x < 344) {
             temp.x = 345;
             transform.eulerAngles = temp;
         }
 
     }
 
 
 }