Helicopters even the heavily armoured ones like the Mi-24 and Apache are lightly armoured compared to vehicles. most of the apache is only armoured by Kevlar matting, the exception is the vital areas, on the Apache even the rotorblades are stressed to survive (Not withstand) 23mm HE impacts. from what I have heard, and remember this is orally, the KA-50 is better armoured in certain places like the cockpit area.
Most of the skin of helicopters are aluminium, and that offers the same protection as a motivated strand of grass at combat ranges. the best defense against damage in a helicopter is the range of the weapons and stealth.
Staffan