You can quickly estimate the coefficient of drag Cd by selecting Cdrag from the menubar by clicking or using the Alt+C shortcut key. Click the appropriate dots 
for your plane. Feel free to modify the number you get. Of course you can
enter a calculated number directly.
If you select Accept, you will return to the main screen and the coefficient you just estimated will be placed in the Cdrag text box.
If you select Cancel, you will return without changing any data.