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National Advisory Committee for Aeronautics, Technical Notes - Hovering and Low Speed Performance and Control Characteristics of an Aerodynamic Servo Controlled Helicopter Tower

An investigation has been conducted with the Langley helicopter
tower to obtain basic performance and control characteristics of an
aerodynamic-servocontrolled rotor system. Blade-pitch control is
obtained in this configuration by utilizing an auxiliary flap to twist
the blades. Rotor thrust and power required were measured fer the
hovering condition and over a range of'wind velocities from O to 30 miles
per hour. The control characteristics and the transient response of the
rotor to various control movements were also measured.

The hovering-performance data are presented as a survey of the wake
velocities and the variation of torque coefficient with thrust coeffi-
cient. The power required for the test rotor to hover at a thrust of
1350 pounds and a rotor speed of 2&0 rpm is approximately 6.5 percent
greater than that estimated for a conventional rotor of the same diameter
and solidity. It is believed that most of this difference is caused by
the flap servomechanism. The reduction in total power required for
sustentation of the configuration tested at various wind velocities and
at the normal Operating rotor thrust was found to be similar to the
theoretical and experimental results for rotors with conventionally
actuated pitch. The control effectiveness was determined as a function
of rotor speed. Sufficient control was available to give a thrust
range of 0 to 1500 pounds and a rotor tilt of iYO. The time lag
betwsen flap motion and blade-pitch response is approximately 0.02
to 0.03 second. The response of the rotor following the blade-pitch
response is similar to that of a rotor with conventionally actuated
pitch changes.

The over-all characteristics of the rotor investigated indicate
that satisfactory performance and control characteristics were Obtained.

Some type of servomechanism to control the blade pitch of a heli-
copter rotor may be desirable in certain cases, for example, in large
rotors where excessive control forces and pitching moments may be
encountered. Accordingly, tests were made on the Langley helicopter
tower to determine the performance and control characteristics of an
aerodynamic-servocontrolled helicopter rotor system. It was intended
that an investigation of this configuration would give fundamental
information of a general nature on rotors with such control systems.