Adept at Depth 



Special modes of operation are 

 possible such as the antimud mode 

 shown in Fig. 5 wherein the cyclic 

 pitch is augmented with a counter - 

 thrusting collective pitch to provide a 

 relatively undisturbed field below the 

 rescue vehicle. The final criteria for 

 choice might well be the necessary 

 number of main propulsion motors re- 

 quired for the two systems. It is ob- 

 vious that a given horsepower will be 

 obtainable for less weight in two mo- 

 tors than in five. 



SPEED 



Fig. 4 - Comparison of the 

 energy consumption of the 

 tandem propeller and of the 

 ducted thruster 



Being agile or adept at depth is 

 not merely a result of providing direc- 

 tionality to force. One must consider 

 force magnitude and response times 

 as well as damping and stability fac- 

 tors. It is clear, for instance, that if 



a designer chooses to provide stability in pitch and roll through the use of meta- 

 centric height rather than through the control system, he imposes burdens upon 

 the control systems engineer. Likewise, if he attains arrow stability in the 



Fig. 5 - Antimud mode of operation 

 309 



