How come helicopters can have open windows?

TL;DR: Well, frankly, they don't bother about balance. They've got a rotor on their tail which helps them maintain balance. If you just look at it, you will see a huge difference between the body shape of an airplane and a helicopter. read more

There have been many airplanes which open their doors for aerial activities, for ex, parachuting etc. The trick behind it is low speed (to minimize drag) and high thrust (from the engines). Comparing this to helicopters, which travel around 150-200 kmph. So the drag caused by open windows is a very small compared to that in the airplanes. read more