DISCS 



■ CAL. TECH; 



▼ A.R.D.E. i 



PLESSET 1 SHAFFER 

 ARMSTRONG 



Figure 12. Variation of drag coefficient with cavitation number for cones. 



Plesset and Schaffer with the corresponding calculations by Armstrong and with experi- 

 mental data. It would appear from this diagram that the Plesset and Schaffer results 

 are satisfactory for cones of large apex angle but that Armstrong's results give a 

 marked improvement when the cone is slender. It is interesting to note that this 

 diagram confirms that the drag coefficient can be represented as two components, in 

 the form C D = C D(Q = 0) + a, the head drag C D( q _ 0) being practically unaffected by 

 the cavitation number for small values of Q. 



Fig. 13 gives a general comparison of the two theories with experimental meas- 

 urements for the limiting case of zero cavitation number. This diagram clearly shows 

 the difference between the Plesset-Schaffer and Armstrong methods near the origin. 

 The former curve approaches the origin with a finite gradient whereas the Armstrong 

 curve approaches the origin with zero slope and infinite curvature. 



c. Bodies with non-zero Incidence. During the steady running section of an 

 underwater trajectory, most missiles with sufficiently high velocities will have a certain 

 amount of yaw and the tail of the missile will be in contact with the wall of the cavity 

 produced by the head. In these circumstances the moment of forces acting around 

 the centre of gravity will be zero and the total lift force acting on the missile will 

 determine the radius of curvature of its trajectory. The possibility of being able to 

 calculate the forces and moments acting on both the head and tail of a yawing missile 

 in steady cavity flow is therefore of considerable practical importance. 



The amount of information concerning actual flow conditions near bodies held 

 at incidence to the flow is still very limited. In this connexion A. D. Cox and W. A. 



227 



