When does the highest level of vibration occur in rotary wing aircraft?

The highest level of vibration in rotary wing aircraft occurs during the transitional flow effect. This phenomenon happens when the rotor blades transition from hovering to forward flight. During this transition, the airflow over the rotor blades changes significantly, which can cause variations in lift and changes in the interaction between the rotor and the generated vortex structures. This leads to an increase in vibration levels due to the complex aerodynamic effects at play.

In contrast, while hovering does produce some vibration, it is typically less intense than during the transitional flow effect because the rotor blades are operating in a stable airflow condition. During landing, the aircraft is reducing speed and returning to a hover or close to ground effect, which generally involves lower vibration levels compared to the transitional phase. High-speed flight may also introduce different flight dynamics, but it does not typically exhibit the same peaks of vibration as experienced during the transition between hovering and forward flight.