was never smooth, responding instead by a series of steps. This undoubtly degraded 

 the towing behavior. Also, the relatively low flap gain value of 6.6 deg/m neces- 

 sary to maintain depressor stability at low coefficients of tension compromises the 

 depth keeping performance at the higher coefficients of tension where the depressor 

 should usually operate. 



For all cable lengths and depths investigated, the maximum specified cyclic 

 depth amplitude was achieved at an oscillator amplitude of approximately one-half 

 maximum. At this oscillator amplitude, the depth response was, in all cases, very 

 close to the sine wave control input. The depth profile shaped did tend to deterio- 

 rate somewhat with the shorter towline length. This result may have been influenced 

 by increased coupling with the ship motions and by the magnitude of the ship motions, 

 which were greater during this portion of the evaluation. 



At maximum oscillator amplitude, the cyclic depth response exceeded the maximum 

 specified depth response by a factor of approximately two. Even at these large 

 amplitudes, the depth profile shape was quite good except for two notable exceptions: 

 the first case occurred during Run 16 (Figure 12d) , when the flap fully deflected 

 during the deeper depth portion of the cycle. This resulted in a sharp peak in the 

 depth profile. The second exception occurred during Run 30 (Figure 14a), when the 

 flap was operating in a region corresponding to a low coefficient of tension. At 

 this low tension condition, the depressor transient response apparently was very 

 lightly damped, which resulted in a secondary oscillation superimposed on the primary 

 depth profile. This resulted because depth response is quite sensitive to changes 

 in flap delfection at the lower limit of depressor tension, and unless the flap 

 operates in a smooth manner, depressor motions will be aggravated. As noted earlier, 

 the flap response was less than ideal; the flap often responded in steps of roughly 

 3 to 4 deg increments. 



A high-frequency (0.20 Hz) cyclic depth profile is shown in Figure 13b. For 

 this case the oscillator amplitude was set at approximately one-third maximum. The 

 resulting depth variation was + 400 mm. Since to achieve this motion, the flap de- 

 flected over most of its range, it is doubtful that a much larger depth profile can 

 be achieved at this frequency. 



The transient response of the depressor after turning off the oscillator is 

 shown in several of the recorder traces of Figures 12 through 14. The transient 

 response traces indicate that the depressor has good vertical plane damping, although 



41 



