Flutter of Flexible Hydrofoil Struts 



speeds, but showed almost exclusively first bending oscillations at the 

 experimental flutter speed. 



Experimental Q>J and theoretical results were also obtained for 

 the strut with a pod lighter than, and equal to, the weight of the pod 

 with foils. Torsional flutter occurred in both cases, at 20. 1 knots 

 with the smaller pod weight and at 9. 5 knots with the larger pod 

 weight. The large decrease in flutter speed when weight was added to 

 the pod would be expected at low values of torsional mass ratio, as 

 may be seen in Figure 5. Converting part of the pod mass into foils 

 has raised the flutter speed, and has had a similar effect to reducing 

 the pod mass. The foils have reduced the generalized mass ratio of the 

 strut. Calculations of generalized mass ratio are required in order to 

 correlate experimental results with values of this parameter. 



It is apparent that additional experimental and theoretical re- 

 search is needed to adequately understand flutter of struts with foils. 

 Experimental results can provide a reliable indication of the effects of 

 foil-related parameters and can lead to accurate simulation of full- 

 scale systems with reduced-scale models. Theoretical research is 

 needed to improve the accuracy of flutter speed predictions. 



IV. DISCUSSION 



The primary deficiency of the present flutter analysis is its 

 prediction of damping. This deficiency results in inaccurate predic- 

 tions of flutter speed for most struts. In the torsional flutter region, 

 the inaccuracy is strongly correlated with the value of torsional mass 

 ratio of the strut. The relationship between experimental and theore- 

 tical flutter speeds has been illustrated in Figure 14. Predictions fol- 

 low a fairly well-defined curve which is overconservative at low mass 

 ratio, quite nonconservative at high mass ratio, and which crosses 

 over experimental values at a mass ratio of slightly less than 2. It 

 should be noted that the single very nonconservative prediction was 

 strongly influenced by the presence of a large pod, and is therefore 

 not strictly representative of flutter characteristics of simple struts. 



The conservative predictions obtained at low values of torsion- 

 al mass ratio are highly significant. Many previous studies of hydro- 

 foil and airfoil flutter have shown a tendency for predicted flutter 

 speeds to become nonconservative at low values of mass ration, lead- 

 ing to a loss of confidence in flutter predictions in this region. These 

 studies have used two-dimensional loading without accounting for 

 sweep angle. The present analysis, and a similar analysis [4] made 

 previously, showed no tendency for predictions to become nonconser- 



363 



