That's simple: Helicopter is a word, and chopter isn't. Copter and Chopper are common slang abbreviations for Helicopter. I guess chopter may be too, but it's not one I have seen. If you want to use a slang word for a helicopter, it's probably better to stick with one of the more common ones. But like any slang - be very careful using it in any formal situation.

I've never heard of "chopter." Googling suggests it might be a word used by on-line gamers, so maybe it will spread from them, to young people in general, to the rest of us, and thence to the dictionary--but it's not there yet. "Chopper" is common, but it is a kind of nickname, and it's not VERY common. For the most part, people simply say the word "helicopter" in full. For some reason it comes easily to the tongue and it's almost fun to say. Even on news reports, announcers say "And now we take you live to the channel 5 helicopter for a view of the traffic jam on Route 128..." " 'Copter" is at least as common as "chopper." Phil's "whirlybird" is familiar, but I'm not sure I've heard it used in real life. I've heard "eggbeater," but I think it refers to some specific kind of helicopter, with two counter-rotating propellers, not to helicopters in general. Military people tend to use the specific name for whatever kind of helicopter they are referring to: "HueyCobra." When talking about aircraft as a group, ordinary airplanes are distinguished from helicopters by calling the "fixed-wing aircraft."

I agree with Gary, but if you insist on using slang, here's a really fun word for you: http://i.word.com/idictionary/whirlybird

I agree with Gary and Phil. 'Chopter' isn't a proper word. Don't waste your time on it.