variable-speed requirement with standard types of servo equipment would have required the 
use of a relatively large motor having good speed regulation and a variable-ratio gear box for 
changing speed. To eliminate the need for a large motor and variable-ratio gear box, a special 
servo was devised which accomplished these functions by means of electrical feedback. 
2.2 POWER AND POWER CONTROL EQUIPMENT 
A great deal of study went into the selection of a drive system and its components. 
Choice of a prime mover was quickly limited to a storage battery because its energy per pound 
compares favorably to compressed gasses, wound springs, etc.; because batteries have little 
effect on the model and its contents; and because electricity is easy to control. 
Four commercially available types of batteries were considered: the lead-acid, 
silver-zinc, nickel-cadmium, and nickel-iron. Weight, terminal voltage stability, and battery 
cost per test run eliminated all but the nickel-cadmium battery. The nickel-cadmium battery 
weighs less than one-third the weight of an equivalent lead-acid type, it can be charged or 
discharged at very high current rates, and it has an extremely long life of 10 to 15 years. Its 
terminal voltage curve was not flat like that of the silver-zinc battery, but was much better 
than that of the lead-acid battery. 
When the prime mover had been selected, consideration was given to propulsion motor 
requirements. Use of a 100-cell battery, with cell switching and voltage interpolating, was 
eliminated because of certain disadvantages, which included the inability of the system to 
restrict the voltage change to 1 percent during a test run. The only way in which the speed 
accuracy requirement could be met was to use a feedback-regulated variable-voltage motor- 
generator set. Since specifications called for two independently controlled sources, two 
motor-generator sets are provided. A source of 400-cycle power for instrumentation was also 
specified, hence an alternator set is provided. 
To keep the weight of the rotating equipment to a minimum, fairly high rotational speeds 
were dictated. However, this introduced the possibility of objectionable precessional forces 
acting while the model was maneuvering or being tested in waves. The rotation of the pair of 
motor-generator sets, or of one motor-generator set and the alternator set, are opposite one 
another so that gyroscopic effects are partially canceled. 
A further advantage of motor-generator sets is that their drive motors will remain 
running once started. The propulsion motors are easily started and stopped by controlling 
the generator voltage. Hence, no starting equipment is needed for-the propulsion motors, and 
the starting equipment for the motor-generator and alternator sets is not located in the model. 
To meet the speed accuracy requirement, feedback regulation is required. Normally, 
the speed of a motor is controlled by the use of a tachometer generator and a speed reference 
voltage. In this application it is desired that the speed of the motors be constant within 
1 percent during the approach part of the run, but be permitted to change when the load on the 
propellers varies, as in turns. Furthermore, series propulsion motors were specified, which 
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