BCE0031: Language feature not implemented: Ambiguous(Void AddRelativeForce(Vector3, ForceMode), Void AddRelativeForce(Vector3), Void AddRelativeForce(Single, Single, Single), Void AddRelativeForce(Single, Single, Single, ForceMode)).

hey I'm pretty new to unity and javascript and all I want to do is apply force on its local y axis for a helicopter and I keep getting the same error over and over please help

error : Assets/heli.js(42,34): BCE0031: Language feature not implemented: Ambiguous(Void AddRelativeForce(Vector3, ForceMode), Void AddRelativeForce(Vector3), Void AddRelativeForce(Single, Single, Single), Void AddRelativeForce(Single, Single, Single, ForceMode)).

the variables :

 var rotor : Vector3;
 var rb : Rigidbody;
 var rpm : float;
 var maxrpm : float;
 var minrpm : float;
 var pitchtorque : float;
 var rolltorque : float;
 var collective : float;
 var maxcollective : float;
 var mincollective : float;
 var enginepow : float;
 var health : float;
 var enginestate : boolean;
 var engine : boolean;
 var bladedrag : float;
 var lift : float;

the code with the error:

 if (rpm > 10) {
 
     this.rb.AddRelativeForce = (transform.forward * lift);
 }