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National Advisory Committee for Aeronautics, Technical Notes - Tank Tests of a Model of a Flying Boat Hull Having a Longitudinally Concave Planing Bottom

The portion of a flying—boat hull or seaplane float
forward of the step supports a large part of the total
load during take—off and receives the major portion of the
impact in alighting. Its'propér form is a compromise re~
sulting from considerations of water resistance, drag'fi"
flight, and shock—absorbing qualities. The most common
solution is fundamentally a V—bottom planing surface having
a straight or slightly convex profile near the step.

As a part of a compre_hensive study of planing phenom-
ena. Sottorf (reference 1) investigated two planing sur-
faces that were transversely flat but longitudinally cone
cave. The surfaces were tested at various angles of trim
for a load coefficient CA of 0.65 and a Speed coefficient-
of 5.55.

The ratios of longitudinal radius to beam
were 38.3 and 20, and the load—resistance ratios at the
best angle of trim were found to be 10 and 16 percent high~
er, respectively, than_that of a plane surface. The trims
ming moment about the trailing edge and the height of the
spray were reduced.

Tank tests of N.A.G.A. model ll-A demonstrated the
superiority of a straight profile for the planing bottom
of a flying boat over the convex profile of N.A.C.A. mod—
el.ll in regard to water resistance (reference 2). Tests
of a model of the U.S. Navy PH~1 flying boat (reference 3)
showed that a small downward drop or "hook" at the step
caused the resistance of this hull to be slightly lower.

A logical extension of these tests is the investigation oi
a model in which a generally concave profile of the plan-
ing bottom would be compared with a straight profile. Ac-
cordingly, a concave planing bottom was introduced into
N.A.C.A. model 11—0 and the resulting form was tested in
the N.A.C.A. tank as model ll-B.