Swaan and Wahab 



Accelerations are given with the acceleration due to gravity (32.18 it/sec^) 

 as unit. They were measured at the bow and stern of the model, in the longitu- 

 dinal plane of symmetry. 



The resistance over waves was only determined as an average value. 



HOVERING PERFORMANCE 



When hovering over land the model provided with flexible trunks or rigid 

 jet exits was stable in pitch, roll and heave. The motion extinction curves are 

 given in Fig. 4. Because of the rapid extinction it is difficult to draw definite 

 conclusions. However, the results indicate that the heave and pitch motions 

 were well damped. The roll damping may be qualified as fair. 



Over water, the hovering behaviour of the model provided with rigid jet 

 exits was characterized by a sustained roll and heave oscillation, apparently 

 caused by a dynamic unstability. The rolling developed fairly slowly. It took 

 about two cycles to double the amplitude. The model appeared also to be dy- 

 namically unstable in pitch, but to a less degree than in roll. 



The most remarkable phenomenon found during the tests was that the model 

 with rigid jet exits had two modes of motion, one of which always prevailed. 

 Which of the two dominated during a test depended partly on the initial disturb- 

 ances to which the model was subjected. The model might roll considerably 

 while pitching slightly or it might pitch considerably while rolling was only 

 moderate. At the lower hovering heights the model showed a preference for the 

 mode of motion in which rolling was dominant. The Figures refer to this con- 

 dition. The behaviour described here is illustrated in Fig. 5. 



It was found that if the centre of gravity of the model was fixed at the same 

 mean rise height which the model had when it was free to heave, the roll motion 

 remained. This gave rise to the supposition that the origin of the roll motion 

 could be explained by considering the uncoupled equation of motion. When the 

 roll angle is indicated by cp , this equation is: 



Mqj + Ncp + Bcp = . 



Because the roll damping was not too large, the roll period may be approximated 



by 2rr /WB. 



The coefficient B is a measure for the static stability. The measurements 

 indicate that the value of B is larger for the vehicle hovering over water than 

 over land. This is in contradiction with the experiences of Kuhn, Carter and 

 Schade [5]. The natural roll periods overland and over water were almost equal. 

 This leads to the conclusion that the virtual moment of inertia if the model hov- 

 ering over water was larger than hovering over land, which is acceptable. 



The origin of the roll motion could possibly be explained by a non-linearity 

 in the damping coefficient N, caused by the presence of the free water surface 

 under the air cushion. A complete investigation into the cause of the dynamical 



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