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National Advisory Committee for Aeronautics, Technical Notes - The Effects of Various Parameters on the Load at Which Spray Enters the Propellers of a Flying Boat

The results of experiments made with a technique for
investigating the spray characteristics of flying—boat
models are presented. In the method of testing used, the
minimum load at which spray strikes powered propellers
was determined for a range of speeds and trims These
measured loads were plotted against speed with trim as a
parameter, and the resulting curves were found to have
minimum points that determined the greatest load that
could be carried without spray striking the propellers.

The forebody of-a pointed—step flying-boat hullflwas
used.for the tests, and the effeCts of varying trim,_ pro-
peller position, and amount of power (expressed in terms
of disk loading) were investigated.

Either of the two types of spray that emanate from a
forebody (pressure or velocity spray) may limit the gross
load of a flying boat, depending on the configuration.
Increasing the power reduced the load at which spray
entered the propellers. Increasing the trim increased
the minimum load at which pressure spray struck the prce
pellers but the corresponding load for velOcity spray
varied erratically with trim. The normal lateral and
longitudinal positions of the propellers tended to be near'
the positions that would give the smallest value of the
minimum load at which spray struck the propellers. For
pressure spray this minimum load increased approximately
linearly with upward movement of the propeller position.

The necessity for keeping the propellers relatively
clear of spray imposes a great handicap in reducing the
air drag of flying boats, As a_result of this require-
ment, hulls are built larger than other considerations
demand ~ except in the case of large cargo—carrying air—
nlanes in which the volume required for cargo space is
greater than the volume needed for a configuration that
would brovide adequate propeller clearance.