depressor tension response. A sine wave with a period of 28 sec and a nominal flap 

 amplitude of + 17 deg was selected as a severe test; most operational conditions 

 will not require this range of flap angles to obtain the desired cyclic depth 

 variation. 



RESULTS 



Without active control, the depressor towed in an acceptable manner at all 

 speeds and stabilizer settings. Initially some erratic yawing motions of the de- 

 pressor, apparently generated by oscillations of the passive roll-trim device, were 

 noted. This was corrected by Increasing the chord of the trim tab slightly to shift 

 the center of hydrodynamic pressure aft relative to the tab pivot axis. Results 

 of the steady towing experiments are presented in Table 2 which shows the cable 

 tension (corrected to standard sea conditions) and the coefficient of tension C 

 for the range of stabilizer and control flap incidence angles investigated. The 

 coefficient of tension C is defined as 



T 



C = ° o (3) 



l/2pV S 



where T is cable tension at the depressor, 

 o 



p is fluid density, 



V is towing speed, and 



S is wing projected area. 

 The coefficient of tension as a function of control flap deflection is plotted 

 in Figure 9 for various speeds and stabilizer incidence angles. This figure indi- 

 cates that a stabilizer incidence angle of -8 deg (leading edge down) will provide 

 the widest range of depressor tension coefficients. This angle, therefore, was 

 selected as the most suitable. 



Response of the depressor to a square wave control input is shown in Figure 10 

 for two flap gain values. The damping of the response at the lower tension condi- 

 tion appears to be improved with the lower flap gain value. At the higher tension 

 condition, the response is critically damped for both flap gain values. These re- 

 sponses are influenced by towline length and therefore will change somewhat with a 

 longer towline; however, the trends probably are indicative. 



19 



