What conditions can lead to retreating blade stall?

Retreating blade stall occurs when the rotor blades of a helicopter no longer generate sufficient lift as the rotor system's forward speed increases. The key factor in this phenomenon is the relationship between the rotor RPM and forward airspeed.

When a helicopter experiences too much forward airspeed in relation to rotor RPM, the retreating blades—those that are moving away from the direction of flight—begin to lose lift due to a combination of increased relative airflow and a decrease in angle of attack. As forward airspeed increases, the airflow over the retreating blade becomes more horizontal, which can result in a stall if the blade's angle of attack becomes too low for effective lift generation. This condition can be exacerbated in certain helicopters that have specific design characteristics regarding their rotor systems, leading to a situation where managing airspeed and rotor RPM becomes crucial to prevent stall.

Other options present factors that might influence flight but do not directly relate to the critical condition of retreating blade stall as significantly as the relationship of forward airspeed to rotor RPM does. Factors like high rotor RPM with low forward airspeed or environmental considerations such as terrain and density altitude may affect flight dynamics, but do not specifically lead to the retreating blade stall condition in the same direct way as the