During the Vietnam War (American War if you're Vietnamese) how did the Viet Cong go about attacking American helicopters? Specifically, what weapons were most commonly used, and how effective were they? Would the downwash of a helicopter's rotors have a significant effect on incoming projectiles?
While you’re waiting for a response, you might find this recent thread including /u/tlumacz’s response about Vietnam-era anti-aircraft weaponry relevant.