Hydrofoils Running Near a Free Surface 189 
[3] Plesset, M.S., and Parkin, B.R., “Hydrofoils in Noncavitating and Cavitating Flow,” 
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Auftriebsverteilung von Tragflugeln,” Schiffstechnik 4:117 (1957) 
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DISCUSSION 
P. Kaplan (Technical Research Group, Inc., Syosset, New York) 
I noticed, or it appeared to me, that the influence of roll was only static. I wonder if 
you have considered or done any work, not reported here, as to the effect of the roll damp- 
ing, that is, what happens when the foil is performing not just a static roll, deflection, but 
is actually performing a roll angular motion? This is important since it is the roll damping 
that determines the major behavior of the roll motion. According to the results of the free 
motion studies, there was quite different behavior of the system as compared to an aircraft, 
so I wonder just what influence a dihedral will have on the roll damping and if any consid- 
eration was given to this problem. 
S. Schuster 
Our results given here of the tests and formulas are valid only for the steady state. 
Even the test for finding the point of breaking down of the lift shows only the beginning of 
the motion. Damping forces and moments can only be found during the motion itself. We are 
beginning the investigation for the unsteady problems, especially for hydrofoils in waves, 
but at present we are not able to give you an answer to these special questions. I think that 
damping is higher for rolling than for yawing, but for the whole system of two or more hydro- 
foils, we have to consider sideslip motion too. Perhaps there will be something to report 
after finishing our investigations on oscillations of a propeller in the axial direction and in 
the rotational direction. 
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