Which factor is least favorable for helicopter performance?

High density altitude is least favorable for helicopter performance because it significantly affects the aircraft's ability to generate lift and perform adequately under various flight conditions. At higher density altitudes, the air is thinner, meaning that the amount of lift generated by the rotor blades is reduced. This can lead to an increase in required takeoff distance, reduced climb performance, and a decreased overall safety margin.

In contrast, low altitude aids helicopter performance as there is denser air available, which contributes to better lift and maneuverability. Heavy gross weight can also adversely affect performance by requiring more lift, but it's a common condition that pilots often manage. Calm or no wind conditions generally stabilize helicopter operation, allowing for predictable control, though they may not be favorable in specific scenarios, such as when improved lift is needed from headwind. Thus, the challenges presented by high density altitude make it particularly detrimental to helicopter performance compared to the other factors presented.