Besoh and Liu 



in Figure 6 after normalization by the factor b go a . This normalizat- 

 ion was successful for values of mass ratio between 2.0 and 6. 2, 

 but large variations still exist at values below 2.0. Parameters that 

 differ among the lower mass ratio models include the elastic axis 

 location, profile, sweep angle, and submergence of the struts, and 

 the size and inertial characteristics of pods attached to the struts. 

 The effects of strut profile have recently been investigated at NSRDC, 

 and results for three different profiles are indicated in Figure 6. At 

 speeds high enough to produce ventilation over the entire chord of the 

 strut, a ventilated cavity originating from a blunt leading edge on a 

 strut substantially destabilizes the system. The effects of strut sub- 

 mergence will be discussed later. 



The reduced flutter speeds for torsional flutter exhibit the 

 characteristics found in classical hydrofoil flutter. The flutter speed 

 parameter gradually decreases to a minimum value as mass ratio de- 

 creases, and then increases rapidly for related series of strut models 

 at lower values of mass ratio. Minimum values occur approximately 

 between mass ratios of 2. to 3. . The effect of mass ratio on 

 torsional flutter speeds is similar to that predicted by classical two- 

 dimensional flutter theory and also to that predicted in the higher 

 mass ratio region in finite sweep angle analyses [6,7,8] . 



II. 4. Strut Submergence 



The effects of strut submergence on flutter speed are closely 

 related to the effects of generalized mass ratio. When the simplified 

 forms of mass ratio are used, the two parameters are inversely pro- 

 portional to one another. The close relationship is evident in experi- 

 mental flutter results in which submergence has been varied without 

 changing other strut characteristics. These results, shown in Fig. 7, 

 constitute a replotting of data contained in Figures 4 through 6 but are 

 given to illustrate the effects of submergence directly. 



Flutter speeds for bending-type struts decrease as strut sub- 

 mergence increases, with minimum flutter speeds occurring at full 

 submergence. The increase in submerged length produces a decrease 

 in mass ratio and therefore a decrease in flutter speed. Torsion-type 

 struts show a local minimum in flutter speed at approximately 50 % 

 submergence. This local minimum would be expected to occur if the 

 strut configuration passed through intermediate values of mass ratio, 

 and will not necessarily correspond to 50 percent submergence. An 

 increase in flutter speed will of course occur as the submergence 

 becomes very small regardless of the mass ratio. The effect of sub- 

 mergence on the strut model with pod and foils is similar to that ob- 

 served for struts without foils in the bending flutter region and at high 



350 



