Hovercar Script, object slowly descends instead

I'm trying to write a script using various examples with the end-goal of making an object hover.

However, my code simply makes the object slowly descend to a plane underneath it.

There are 4 thrusters that are placed within the car's hierarchy and are within the script's array in the inspector tab.

The code is as follows:

 using UnityEngine;
 using System.Collections;
 
 public class ThrusterScript : MonoBehaviour {
 
     //DEFINITIONS
 
     //FLOATS - independent variables
     public float desiredHeight;
     public Rigidbody rb;
 
 
     //ARRAY - containing all vehicle thrusters
     public Transform[] thruster_group;
 
     
     //FORCES - fN (normal force) and fG (gravitational force)
     Vector3 fN;
     Vector3 fG;
 
     void Start()
     {
 
         //Defining rb as Rigidbody; rb.mass as its mass
         rb = GetComponent<Rigidbody>();
 
         //Defining global forces for simplicity
         fN = -1 * (Physics.gravity * rb.mass);
         fG = (Physics.gravity * rb.mass);
 
     }
 
     void FixedUpdate()
     {
 
         //RAYCASTING from all thrusters in array
         RaycastHit hit;
         foreach (Transform thruster in thruster_group)

         {

             Vector3 fThrust;
             float HDifCo;
 
             //SIMPLICITY - refer to desiredHeight as dH
             float dH = desiredHeight;
 
             if (Physics.Raycast (thruster.position, thruster.up * (-1), out hit, desiredHeight)) {
 
                 //DEFINITION - Height Difference Coefficient
                 HDifCo = 1 - (hit.distance / dH);
   
                 fThrust = fG * HDifCo;
                 rb.AddForceAtPosition(fThrust, thruster.position);
             }
         }
     }
 }

Can anyone pinpoint the issue? Thank you.