THE FORCES ON AN AIR-CUSHION VEHICLE 

 EXECUTING AN UNSTEADY MOTION 



Lawrence J. Doctors 



University of New South Wales 

 Sydney, Australia 



ABSTRACT 



This paper treats the theoretical problem of an 

 air-cushion vehicle (ACV) travelling over water of 

 uniform finite or infinite depth, with an arbitrary- 

 motion. The ACV is modelled by a pressure 

 distribution applied to the free surface of an invis- 

 cid incompressible fluid and the boundary condi- 

 tions on the free surface are linearized. 



Numerical results are presented firstly for accele- 

 rated motion from rest. In deep water, the hump 

 condition is delayed to a higher Froude number, 

 while in finite depth, the hump resistance is 

 appreciably reduced - even by moderate levels of 

 acceleration. 



The effect of tank walls on these results when 

 carrying out model tests is next examined. The 

 side walls of the tank alter the resistance by less 

 than one per cent in accelerated motion, if the 

 tank width is greater than four times the model, 

 beam. 



Finally, calculations of the side force acting on a 

 yawing ACV are presented. For super-hump speeds, 

 the side force is of the same order as the wa- 

 ve resistance and favorably aids the turn. It is al- 

 so shown that the steady-state forces are realized 

 when the craft has travelled a distance of less 

 than two vehicle lengths after a manoeuver. 



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