ProkhoroVj Treshohevsky,and Volkov 



DISCUSSION 



P. Kaplan 



Oceanios Inc. 

 Plainview, New York s U.S.A. 



I think my comments with regard to experiments contain 

 some helpful information and also provide a warning with regard to 

 any expectation of prediction of dynamic performance of air cushion 

 vehicles from ordinary experiments in towing tanks. For one thing, 

 referring to the particular case, in which you used forced oscilla- 

 tion, the equation given on page 12 deals with the single degree of 

 freedom of heave motion. There is a pressure variation here and if 

 you actually wrote the equations involving both pressure and heave 

 motion coupled together, you would find that there is an effective 

 mass entering the system. The effective mass is dependent upon the 

 geometry of the plenum system and certain aspects of the pressure 

 ratios, etc. The net result is that you have some reduction in the 

 effective mass. That is why you find what people are referring to as 

 a negative aided mass for ACVs, which may not be quite so. There 

 is a real physical mass, and there may be some hydrodynamic ef- 

 fective mass since the frequency dependence in Fig. 11 may be in- 

 dicative of that, but it also depends on how you look at the water sur- 

 face behaviour at low frequency and at high frequency and also at 

 low speed and at high speed. But that is not the point. The main 

 thing is that the pressure is influential, and that it should be ac- 

 counted for ; it should be measured and the equations for the system 

 should include it. Analyse that and then look at your resulting inertia 

 and the other terms. Correlate the results, and that is the way you 

 will understand what is happening with these vehicles. 



Secondly, with regard to model tests, let us look at how you 

 scale. All naval architects are accustomed to scaling dynamic phe- 

 nomena in terms of Froude scaling and that is because certain para- 

 meter scales in that way. What is the most fundamental parameter 

 that determines the behaviour of any vehicle in response to a seaway 

 disturbance ? There are two, mainly : the natural frequency and the 

 damping. The natural frequency can be derived for any type cushion 

 system for an air cushion vehicle. It involves pressure ratios, i. e. 

 atmospheric to normal pressure that supports the system, and the 

 geometric height of the plenum. 



2 88 



