Besoh and Liu 



vative at low mass ratio for torsional flutter. It appears that the 

 most significant difference in the two types of calculations is the in- 

 clusion of sweep angle as a parameter which couples structural and 

 hydrodynamic effects. The present calculation should reliably indicate 

 the presence of a flutter instability throughout the mass ratio range 

 shown in Figure 14. Additional comparisons with existing flutter data 

 can help to determine the accuracy of flutter speed prediction to be ex- 

 pected as a function of mass ratio. 



Improvement in the accuracy of flutter speed prediction will 

 require improvement in the hydrodynamic loading formulation. The 

 sensitivity of calculated damping to small changes in loading, parti- 

 cularly for bending -type struts, suggests that hydrodynamic loading 

 must be very accurately described in order to obtain accurate flutter 

 speed predictions. Possible sources of inaccuracy in the loading for- 

 mulation are the presence of cavitation, real fluid effects involving the 

 boundary layer and wake, and inexact modification of the two-dimension- 

 al loading for three-dimensional flow. Rowe [_21j has shown that large 

 changes in calculated flutter speed result when the loading applied to 

 struts is modified to simulate cavitation. Available observations are 

 insufficiently detailed to confirm the existence of the assumed distri- 

 butions of cavitation. It has been shown [_22j that altering boundary 

 layer characteristics with disturbance wires affects agreement bet- 

 ween theoretical and experimental loading in two-dimensional flow. 

 However, the results of such modification on flutter characteristics 

 have not been investigated. Reliable measurements of three-dimension- 

 al strut loading which could be used to assess the accuracy of the strip 

 theory employed in the present flutter analysis are not available. 



The existence of two different unstable hydroelastic modes 

 implies that future flutter experiments and calculations must be carri- 

 ed out in sufficient detail to distinguish between the modes. This will 

 require measurement or calculation of hydroelastic mode characte- 

 ristics as a function of speed. Measurements of damping characteristics 

 at zero speed are important, particularly for struts which undergo tor- 

 sional flutter, so that calculated damping can be adjusted to include 

 structural damping. 



Flutter research will be incomplete until hydroelastic mode 

 characteristics of full-scale strut systems are measured. These mea- 

 surements will provide comparisons with model data and calculations 

 as well as indicate the stability of the actual struts. 



V. DESIGN PROCEDURES 



Design of inverted-T strut-foil systems to operate in subcavi- 



364 



