How to fly in the game
To fly if you are on the Viet Cong team you have to crawl up to the top of helicopter and crawl off to the right. You should then fall through the ground instantly. You will have to press [Ctrl] as soon as you fall through the ground (or else you'll keep falling and it will take for ever to get back up). Once you've stopped falling press [space]. You should keep going up and through the ground,then you have to press CTRL again to stop going up and voila your flying.
The controls of flying are hard to master. So it will be awhile for you to master them. To go up press [space], but be careful once you go up you can't stop. To stop press [Ctrl], but once you've pressed [Ctrl] you keep going down to you hit the ground. The way i do it is hold [Ctrl] until i get to the place i want to go then let go.
P.S. If you've set the controls different from default (the way the controls were when you got the game), you should use the words LIE and JUMP instead of how you do them.
Still dont know if this is for single or multiplayer
well enjoy the trick