Flight measurements of the vibration experienced by a tandem helicopter in transition, vortex-ring state, landing, approach, and yawed flight

John E. Yeates
naca-tn-4409
SEP 1958

As part of a general program of helicopter vibration research, a flight investigation involving several flight maneuvers that are critical with respect to vibration has been conducted. The maneuvers include transition near the ground, landing approaches, vortex-ring state, and yawed forward flight.

An Adobe Acrobat (PDF) file of the entire report:
http://naca.central.cranfield.ac.uk/reports/1958/naca-tn-4409.pdf