We know that humidity degrades engine performance. Is there any charts or tables which quantify effect on engine performance due humidity? The question is specific to helicopters.
Figure 11 in GE Gas Turbine Performance Characteristics estimates the impact of humidity on some of their turbine engines. It shows a power loss of less than 1% over a large range of humidity.
Additionally, I've read that the effect of humidity on helicopter turboshaft engine performance is negligible. This was in flight manuals for specific helicopters, which I unfortunately can't share here. What I can say is that these manuals provide charts specifying the maximum power or torque available at various air temperatures and pressures. For humidity, however, they tell you to ignore it.
This article on helicopter engines also indicates that humidity effects are small compared to temperature and pressure.