In the summer of 2006, an optics-based tip-path plane measurement system was flown onboard a Bell 206 helicopter. The system is composed of two cameras that measure the longitudinal tip-path plane angle with respect to the fuselage and an air data boom fitted with a horizontal wind vane that measures the free-stream velocity with respect to the fuselage. A global positioning system and inertial mass unit were also included to compare the measurements of the imaging system with the theoretical tip-path plane for a given drag-to-weight ratio, flight path angle, and acceleration. In general, the tip-path plane angle measurements compared well with theory and proved highly repeatable for steady-state flight maneuvers. When linear regression is performed on the data recorded by both instruments, the standard deviation for the tip-path plane angle relative to the fuselage is 0.19° and the standard deviation of the free-stream velocity angle relative to the fuselage is 1.38°. Combined, these measurements report a longitudinal tip-path plane angle with a standard deviation of 1.35°. The results of the flight test also verified the first order principles that the longitudinal tip-path plane angle varies linearly with the square of the velocity and the sum of the flight path angle and acceleration.
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