Flight mechanics 2D, variables vs forces

Hi!

I am working on 2D flight sim, the only rotation that is important is the pitching up and down for the airplane. No wings are needed for this. I'm using 4 variables: Lift, Drag, Gravity, Force (which is the thrust).

Let me show you 2 of my scripts.

number 1, forces based.

 var liftValue             = 0.0;
 var liftCalc                = 0.0;
 
 var dragValue         = 0.0;
 var dragCalc             = 0.0; //Drag calculations
 
 var maxSpeed            = 0.0;
 
 var throttle            = 0.0;
 var gravityValue        = 0.0;
 var rotateSpeed      = 0.0;
 var objAngle            = 0.0;
 var xForces             = 0.0; //sum of all x forces
 var yForces             = 0.0; //sum of all y forces
 
 
 function Update(){
 
 //For increasing/decreasing speed 
 if(Input.GetButtonDown("ThrottleUp"))
 throttle++;
 
 if(Input.GetButtonDown("ThrottleDown"))
 throttle--;
 
 //For pitching up and down the airplane
 var rotationZ : float = Input.GetAxis ("Pitch") * rotateSpeed;
 transform.RotateAround(Vector3.forward,rotationZ * Time.deltaTime);
 
 }
 
 
 function FixedUpdate(){
 
 
 objAngle = transform.localEulerAngles.z;
 //This is getting the attack angle.
 var vel  = rigidbody.velocity.magnitude;
 //This is receiving the airplane speed.
 dragCalc = vel*dragValue/10;
 //This is calculating the drag based on the speed.
 liftCalc    = vel*liftValue/10;
 //This is calculating the lift based on the speed.
 
 //I've given these calculations my own kind of values.
 
 //You can ignore this, this is simply for the airplane 
 //to move straight without any effect at start
 if (objAngle == 0)  
     rigidbody.AddRelativeForce(throttle,0,0);
 
 //Pitching up.
 if (objAngle>0&&objAngle<=90) 
     {
     xForces  = Mathf.Sin(objAngle*Mathf.PI/180)*(throttle*5-liftCalc-dragCalc);
     yForces  = Mathf.Cos(objAngle*Mathf.PI/180)*(liftCalc+throttle-dragCalc)-gravityValue;
     }
     
 //Pitching down.
 if (objAngle>=270&&objAngle<360)
     {
     xForces  = Mathf.Cos(objAngle*Mathf.PI/180)*(throttle*5+liftCalc+dragCalc);
     yForces  = Mathf.Sin(objAngle*Mathf.PI/180)*(liftCalc+dragCalc-throttle)-gravityValue;
     }
 if(vel<maxSpeed)
     constantForce.relativeForce = Vector3(xForces,yForces,0);
     //I'll note that I've tried to put in different use of rigidbody
     //I tried to rigidbody.AddRelativeForce or AddForce.
 else
     vel--;
     print(vel);
     //To check up and critisize my velocity.
     
 }

My airplane never stops, the forces are always adding, how can I limit them? Or is there another way of putting them?

and my second script, variables format, basically was calculated by putting the velocity in the translate command: F = ma a = v/t v = F*t/m

(Basically the same script except this)

 function FixedUpdate(){
 
 
 objAngle = transform.localEulerAngles.z; //This is getting the attack angle.
 
 if (objAngle == 0)
     transform.Translate(0.5, 0 , 0);
 
 if (objAngle>0&&objAngle<=90)
     {
     xForces  = Mathf.Cos(objAngle*Mathf.PI/180)*(velSpeed-liftValue-dragValue);
     yForces  = Mathf.Sin(objAngle*Mathf.PI/180)*(liftValue+velSpeed-dragValue)-gravityValue;
     }
 if (objAngle>=270&&objAngle<360)
     {
     xForces  = Mathf.Cos(objAngle*Mathf.PI/180)*(velSpeed+liftValue+dragValue);
     yForces  = Mathf.Sin(objAngle*Mathf.PI/180)*(liftValue+dragValue-velSpeed)-gravityValue;
     }
     
 transform.Translate(xForces*Time.deltaTime,yForces*Time.deltaTime,0 , Space.World);
 }