Flight Sim Control

Hi everyone, I'm trying to script basic flight controls for a flight sim I would like to make. I can't seem to get the object to move forward. I want the player to be able to control the speed using the keypad + and - keys. Making the speed increase or decrease and stop moving. This is the script I have, I'm not getting any errors but it just doesn't seem to work at all.

Any help would be greatly appreciate or even suggestions to make it better.

 using UnityEngine;
 using System.Collections;
 
 public class PlayController : MonoBehaviour 
 {
     //Variables for speed control
     public float curSpeed;
     public float maxSpeed = 1200;
     public float minSpeed = 0;
 
 
 
     void Start () 
     {
         CurSpeed ();
     }
     
 
     void FixedUpdate () 
     {
         if (Input.GetKeyDown (KeyCode.KeypadPlus)) 
         {
             rigidbody.AddForce (Vector3.forward * curSpeed);
         }
         if (Input.GetKeyDown (KeyCode.KeypadMinus)) 
         {
             rigidbody.AddForce (Vector3.forward * curSpeed);
         }
 
         transform.Rotate (Input.GetAxis("Vertical"), 0.0f,

-Input.GetAxis("Horizontal")); }

     void CurSpeed()
     {
          float incSpeed = 5;
          float decSpeed = 5;
 
         if (curSpeed > 0 && curSpeed < maxSpeed) 
         {
             curSpeed += incSpeed;
         } 
         else if (curSpeed == maxSpeed) 
         {
             curSpeed = maxSpeed;
         }
 
         if (curSpeed <= maxSpeed && curSpeed > minSpeed) 
         {
             curSpeed -= decSpeed;        
         }
         else if (curSpeed == minSpeed)
         {
             curSpeed = minSpeed;
         }
     }
 }