In rotorcraft, the high level of vibration is most noticeable during which phase of flight due to the transverse flow effect?

In rotorcraft, the high level of vibration is most noticeable during which phase of flight due to the transverse flow effect?

• A. Cruise
• B. Hover
• C. Takeoff
• D. Landing

Answer: (C)

Transverse flow effect happens when the rotor disk does not see the same inflow across its area, which occurs as you start moving from hover into forward flight. As you transition to takeoff, forward speed increases and air interacts with the disk unevenly: the advancing part of the blade meets higher relative wind while the retreating part sees less. This creates a cyclic change in lift distribution and blade angles as the rotor turns, producing noticeable vibration. The effect is most pronounced during takeoff because that is the phase where translational inflow is changing rapidly and strongly, whereas in cruise the inflow is more uniform and in pure hover there’s little forward flow to drive the transverse flow.

Transverse flow effect happens when the rotor disk does not see the same inflow across its area, which occurs as you start moving from hover into forward flight. As you transition to takeoff, forward speed increases and air interacts with the disk unevenly: the advancing part of the blade meets higher relative wind while the retreating part sees less. This creates a cyclic change in lift distribution and blade angles as the rotor turns, producing noticeable vibration. The effect is most pronounced during takeoff because that is the phase where translational inflow is changing rapidly and strongly, whereas in cruise the inflow is more uniform and in pure hover there’s little forward flow to drive the transverse flow.