Can anyone help me with this script?

Hey guys I’ve made this script for my game, its a paper plane witch gains altitude when you press down a key and when you release the key the airplane loses altitude again but i want it to arc, for example i don’t want it to fall directly when i release the key, i want it to slowly start falling.

var Speed = 10;

function Update() 


{
    transform.Translate(Vector3.forward * Time.deltaTime * Speed);

    if (Input.GetKey ("space")) {
       transform.Translate(Vector3.up * Time.deltaTime * Speed);
    }
     else
    
    {
       transform.Translate(Vector3.down * Time.deltaTime * Speed);
    }
    
    
}

I would do something like this (warning, pseudo code!):

float Acceleration = 1.0f;
float maxSpeed = 10.0f
float ySpeed = 0.0f;
....

if...
{
 if(ySpeed < maxSpeed)

  ySpeed += Acceleration;

}
else
{
 ySpeed -= Time.deltaTime;
}

Vector3 Movement = Vector3(10.0f, ySpeed, 0.0f);
GameObject.transform.position += Movement; 

Basically what you need to do, is gradually to make the y-axis more and more negative so that the object will keep moving down, but at the same time still going forward (x-axis in my example).

Look at:
http://en.wikipedia.org/wiki/Trajectory.
Maybe not really applicable in this case, but it gives you an idea on how, in time, to change the trajectory of your paper airplane.

Good luck!