Forces on an A.C.V. Executing an Unsteady Motion 



tion has very little effect, and that smoothing in the longitudinal di- 

 rection, simulated by a a = 5, gave reasonable agreement with ex- 

 perimental results. Since this value of «a does not produce an ex- 

 cessively steep wave system (for the usual range of p Q /pga), linear 

 theory would be valid in this case. On the other hand, actual measure- 

 ments of the pressure drop-off indicate that aa = 40 (i. e. eight times 

 as smooth) would be more realistic. Thus, we found that nonlinearity 

 and viscous action was the main cause of the discrepancy at low speeds. 

 However, we pursued the use of such large amounts of smoothing, 

 since it gave us an adequate model at low speeds, and allowed us to 

 compute the unsteady wave resistance of an ACV starting from rest. 



Finally, with regard to the sign convention during yaw motion; 

 side force on the craft and yaw to starboard were considered positive. 

 In most cases the side force had the same sign as the yaw angle, 

 indicating that this effect favourably aids the turn. In a couple of in- 

 stances at low speeds, principally subhump speeds, the opposite was 

 true. 



97 



