Which factor is a limiting factor in determining forward speed of a helicopter?

The determination of a helicopter’s forward speed is significantly impacted by the phenomenon known as retreating blade stall. In a helicopter, the rotor blades have different airflow characteristics depending on their position in relation to the direction of travel. The advancing blade experiences higher relative wind speeds, resulting in increased lift, while the retreating blade experiences lower relative wind speeds.

As the speed of the helicopter increases, the retreating blade can reach a critical angle of attack where the airfoil can no longer generate enough lift. This condition leads to a stall on the retreating blade, which causes a loss of control and can result in a dangerous flight situation. Hence, retreating blade stall is a crucial limiting factor, particularly at higher forward speeds, as it directly influences the helicopter's ability to maintain lift and stability.

Understanding this concept is vital for pilots and engineers, as they must account for this limitation when designing flight maneuvers and when considering the helicopter's maximum operational speed.