Determination of Aerohydro dynamic Performance of ACVs 



the main hydrodynamic characteristics of ACV s. In this case the mo- 

 del is usually subjected to the action of both the hydro-and-aerodyna_ 

 mic forces. Then, depending on the test conditions, account is made 

 of either the results directly obtained by measurements, or the aero. 

 dynamic components are to be excluded with the use of data on the blo- 

 wings c£ the model in the wind tunnel. 



The most typical tests carried out in the towing tanks are 

 those with the towed models exhibiting the freedom of heaving and trim; 

 during these tests the resistance and kinematic parameters are mea- 

 sured in the longitudinal plane in calm water and in waves. The pur- 

 pose of such tests is not only in obtaining the propulsive performance 

 data but it is largely connected with the evaluation of dynamic proper- 

 ties of these vehicles. Thus the measurements carried out at diffe- 

 rent positions of the centre of gravity along the model make it possi- 

 ble to plot the positional curves against the trim angle and to judge 

 about the static stability depending upon the conditions of motion. The 

 same type of towing tests is the basis for determining the regions of 

 steady motion in the longitudinal plane defined by the influence of the 

 waves and speed. 



Such experiments were carried out specifically on a series 

 of models with the particulars given in Table 2. In this case the re- 

 sistance and kinematic parameters were changed up to the critical 

 conditions preceding the development of plough-in. The curves in 

 figure 6 show the effect of the bow planeform on the relative resis- 

 tance — - — depending upon the running trim angle which is defined by 

 a given position of the center of gravity. It is seen from the curve 

 plotted for a cruising regime that the effect in question is observed 

 only with trim by the bow; in this case model N° 2 appears to be pre- 

 ferable. The advantages of a semi-round bow planeform manifest 

 themselves in waves too, as is seen from the curve of figure 7 where 

 the relative gain in resistance is presented for all three models in 

 waves. 



The air flow rate effect examined on model N° 2 is typical 

 for this experiment. The curve in figure 8 shows the effect of the 

 dimensionless factor of air rate upon the relative resistance in waves 



q~ Q 



in y[ 



2 Pn 



p 

 where Q = air rate, m/sec; 



Sn = area of the air cushion, m 



Pn = pressure in the cushion, kg/m 



267 



