then be used to switch to either PRESET position or to a position of zero rudder angle, The 
second type of control is WHEEL which allows the operator to select any desired rudder posi- 
tion by means of a single knob at the lower, center portion of the control panel. The third 
type of operation is STEP control. When this control is used, the operator may position the 
rudder servo in 5-degree steps covering a total range of 90 degrees. Further versatility may 
be had by presetting groups of controls and then using the SELECTOR switch to change from 
one group of controls to another. 
The control circuitry is arranged for programming the rudder from an external signal 
source, such as a Hewlett-Packard Low Frequency Generator Model 202A or equivalent. The 
only restrictions are that this input be isolated from ground, have a low impedance, and not 
have a large d-c component. Square, triangular, or sinusoidal rudder motions may be simply 
generated in this fashion. Servo followage will be limited by the frequency and amplitude of 
the applied signal and servo loading. 
The output of these control circuits is used to drive the subcarrier and RF circuitry of 
the rudder control portions of the radio-control link. In the model, the output of the rudder- 
channel discriminator is applied to the servo amplifier input. 
The rudder servo system is of special design to meet the requirements of model testing. 
It may be controlled by the radio link or cable control. When radio control is used, the external 
control voltage is compared to the output of the feed-back potentiometer k,, Figure 6. When 
the cable-control circuit is used, the d-c reference within the servo system is applied to both 
the feedback and the external control potentiometers. For either method of control, the control 
and feedback signals are mixed in the first summing network A,. 
Switch S, changes the system from standard proportional control to velocity-stabilized 
proportional control. The switch is shown in the position for standard control. Any difference 
signal produced by the summing network is converted to an a-c signal at line frequency by the 
modulator stage before amplification. Several stages of vacuum tube amplification are used 
ahead of a commercial transistor preamplifier and magnetic output stage which drives the 
servo motor. The tachometer generator is direct-coupled to the motor and a portion of its 
output is applied to the second summing network A, and used for damping. The gearbox has 
a reduction of about 1000 to 1 which provides a no-load output speed of about 56 degrees per 
second as shown in Figure 7. 
Velocity-stabilized operation requires the additional circuitry shown within the dashed 
line of Figure 6. Circuit changeover is performed by switch S,. Part of the output of the first 
stage of amplification is applied to a phase-sensitive detector and polarized relay. Detector 
sensitivity is adjusted so that the relay contacts close as soon as there is a difference of 
1 degree between the actual servo position and the commanded position. Transformer 7, and 
Potentiometer F, furnish an adjustable 400-cycle reference voltage for the velocity- 
stabilizing circuit which is compared to the tachometer output when the relay contacts are 
closed. The proportional control signal applied to the summing network A, is reduced by the 
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