Flutter of Flexible Hydrofoil Struts 



tating flow can be based on the flutter -free performance of the exist- 

 ing U. S. Navy hydrofoil craft. In order to estimate the effect of va- 

 riations in configuration, it would be helpful to calculate the hydro - 

 elastic mode characteristics and the generalized mass ratio of exist- 

 ing struts for comparison with parametric trends obtained from models, 

 Further model testing may be required to establish stability criteria 

 in areas where theory and present data are inadequate, such as in the 

 presence of cavitating flow. 



Additional information about hydroelastic stability can be ob- 

 tained by flutter testing a reduced-scale model of a proposed design. 

 The model should be dynamically and geometrically scaled, except for 

 sweep angle. It has been found to be virtually impossible to obtain 

 flutter in a low density strut model at small sweep angles prior to 

 structural failure due to approaching divergence. Instead of testing the 

 model at the small sweep angle usually found on full-scale struts, the 

 model should be tested at several larger sweep angles, decreasing the 

 angle until static deflections indicate proximity to divergence. Flutter 

 speeds must then be extrapolated to the required value of sweep angle. 



Damping and frequency measurements for individual hydro - 

 elastic modes of torsion-type struts have been readily obtained at 

 NSRDC by implusive excitation. This technique involves inducing os- 

 cillation of the strut at the desired frequency, and determining damp- 

 ing and frequency from the resulting decaying oscillations. Excitation 

 was obtained from a vibration generator rapidly swept over a narrow 

 frequency interval including the desired resonance. The technique can 

 be applied at small speed increments to permit a close approach to 

 the flutter inception speed to be made safely. It has been found, how- 

 ever, that at speeds above flutter inception struts often exhibit ampli- 

 tude-limited flutter over a varying speed range before large negative 

 damping leads to large amplitude oscillations. Because of the differ- 

 ence in damping characteristics, amplitude -limited bending flutter 

 occurs over a narrow speed rarge while amplitude -limited torsional 

 flutter can occur over a wide speed range. This phenomenon probably 

 resulted in the failure of Model 2T, pod configuration D, shown in 

 Figure 3 at a speed far above flutter inception. 



Development of flutter testing techniques for full scale craft 

 would permit verification of the stability of a given design. Such tech- 

 niques should be evaluated in models and on existing craft. Future 

 designs could provide for the flutter testing system to be installed in 

 all craft during construction to make underway flutter testing routine 

 for hydrofoil vessels. 



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