Need help with my hovercraft vehicle

I have this script: using UnityEngine; using System.Collections;

 public class HoverMotor : MonoBehaviour {
     
     public float speed = 90f;
     public float turnSpeed = 5f;
     public float hoverForce = 65f;
     public float hoverHeight = 3.5f;
     private float powerInput;
     private float turnInput;
     private Rigidbody carRigidbody;
     
     
     void Awake () 
     {
         carRigidbody = GetComponent <Rigidbody>();
     }
     
     void Update () 
     {
         powerInput = Input.GetAxis ("Vertical");
         turnInput = Input.GetAxis ("Horizontal");
     }
     
     void FixedUpdate()
     {
         Ray ray = new Ray (transform.position, -transform.up);
         RaycastHit hit;
         
         if (Physics.Raycast(ray, out hit, hoverHeight))
         {
             float proportionalHeight = (hoverHeight - hit.distance) / hoverHeight;
             Vector3 appliedHoverForce = Vector3.up * proportionalHeight * hoverForce;
             if(!Input.GetKey(KeyCode.Space)){
                 carRigidbody.AddForce(appliedHoverForce, ForceMode.Acceleration);
             }
         }
         
         carRigidbody.AddRelativeForce(0f, 0f, powerInput * speed);
         carRigidbody.AddRelativeTorque(0f, turnInput * turnSpeed, 0f);
         
     }
 }

This is a hover motor. This raycasts down from the current position and determines how much force it needs to add to sustain it's current Y position. However, it will still smack into the ground and start spinning out of control. How can I fix it so that it won't smack into the ground? Thanks! (Yes, this did come from the unity hovercar physics tutorial, I just edited it a bit to fit my needs.)