private void CalculateTandSw()
{
if (cmb_Wto.Text == "lbs")
{
S_w = Math.Round(Wto / Double.Parse(txt_W_S.Text), 1);
Thr = Math.Round(Wto * Double.Parse(txt_T_W.Text), 1);
txt_Thrust.Text = Thr.ToString();
txt_SurfaceArea.Text = S_w.ToString();
}
}