void Start ()
    {
    body = GetComponent<Rigidbody> ();

    forward_speed = 0f;
    forward_max_speed = .9f;
    forward_acceleration = .02f;
    forward_movement = Vector3.zero;
    rotation_multiplier = 2.5f;
    }

  //////////////////////////////////////////////////

  void FixedUpdate ()
    {
    if (key_up)
      {
      if (forward_speed < forward_max_speed) forward_speed += forward_acceleration;
      }
    else if (key_down)
      {
      if (forward_speed > -forward_max_speed) forward_speed -= forward_acceleration;
      }
    else
      {
      if (forward_speed > -.1f && forward_speed < .1f) forward_speed = 0f;
      if (forward_speed != 0f) forward_speed = forward_speed * 0.9f;
      }

    if (key_left) turning_rotation = forward_speed * -rotation_multiplier;
    if (key_right) turning_rotation = forward_speed * rotation_multiplier;

    // rotation
    transform.Rotate (0f, turning_rotation, 0f);

    // forward
    forward_movement = transform.forward * forward_speed;
    body.MovePosition (body.position + forward_movement);
    }

void Start ()
    {
    body = GetComponent<Rigidbody> ();

    forward_speed = 0f;
    forward_max_speed = 50f;
    forward_acceleration = 1f;
    forward_movement = Vector3.zero;
    rotation_multiplier = .067f;
    }

  //////////////////////////////////////////////////

  void FixedUpdate ()
    {
    if (key_up)
      {
      if (forward_speed < forward_max_speed) forward_speed += forward_acceleration;
      }
    else if (key_down)
      {
      if (forward_speed > -forward_max_speed) forward_speed -= forward_acceleration;
      }
    else
      {
      if (forward_speed > -.1f && forward_speed < .1f) forward_speed = 0f;
      if (forward_speed != 0f) forward_speed = forward_speed * 0.9f;
      }

    if (key_left) turning_rotation = forward_speed * -rotation_multiplier;
    if (key_right) turning_rotation = forward_speed * rotation_multiplier;

    // rotation
    transform.Rotate (0f, turning_rotation, 0f);

    // forward
    body.velocity = transform.forward * forward_speed;
    }

void Start ()
    {
    body = GetComponent<Rigidbody> ();

    forward_speed = 0f;
    forward_max_speed = 70f;
    forward_acceleration = 5f;
    forward_movement = Vector3.zero;
    rotation_multiplier = .02f;
    }

  //////////////////////////////////////////////////

  void FixedUpdate ()
    {
    if (key_up)
      {
      if (forward_speed < forward_max_speed) forward_speed += forward_acceleration;
      }
    else if (key_down)
      {
      if (forward_speed > -forward_max_speed) forward_speed -= forward_acceleration;
      }
    else
      {
      if (forward_speed > -.1f && forward_speed < .1f) forward_speed = 0f;
      if (forward_speed != 0f) forward_speed = forward_speed * 0.9f;
      }

    if (key_left) turning_rotation = forward_speed * -rotation_multiplier;
    if (key_right) turning_rotation = forward_speed * rotation_multiplier;

    // rotation
    transform.Rotate (0f, turning_rotation, 0f);

    // forward
    forward_movement = transform.forward * forward_speed;
    body.AddForce (forward_movement, ForceMode.Acceleration);
    }