Is it crack? Snap? Wh-tch? Whop-eesh?
Alternatively, if I don't use the actual noise. Do I write: The whip rang out? Cracked?
The sound is generally referred to as the crack of a whip or a whip crack, and since crack is already onomatopoeic, you can't go wrong with it. Crack! See. That hurt a little, didn't it?