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National Advisory Committee for Aeronautics, Technical Notes - A Complete Tank Test of a Flying Boat Hull with a Pointed Step - NACA Model No. 22

The results of a complete tank test of a model of a
flying—boat hull of unconventional form, having a deep
pointed step, are presented in this nete.’ The advant_age
of the pointed-step type over the usual forms cf flying~
boat hulls with respect to resistance at high speeds is
pointed out.

A take—off example using the data from these tests is
worked out, and the results are compared with those of an
example in which the test data for a hull of the type in
general use in the United States are applied_to a flying
boat having the same design specifications. A definite
saving in take—off run is shown by the pointed—step type.

Typical‘curves of the take—off characteristics of a
flying boat show two regions in which the excess thrust
available for acceleration is notably low. The first oc—
curs at the “hump" of the resistance curve, in the lpw~__
speed.part of the planing range, usually at about 30 per~
cent of the get~away speed. The second occurs hear the
get—away speed. A large part of the take—off time is
spent in accelerating through these regions Qf_low excess
thrust. The high speed_obtaining during the scoond period
of low acceleration causes the distance run during the
last few seconds before get—away to be excessively great.

A decrease in the high—speed resistance consequently
causes a pronounced reduction in the length of the take:
off run, and reduces the probability of damage to the hull
when a take-off is made in rough water.

The designer has some control over the relative magni~
tude of the resistan6e .3n the twc3 cr3tiEa1 regions, as was
pointed out in reference 1. ,Using a small hull for a given
load is favorable to low resistance at high speeds, but unu
favorable in t.he. .hump region. The. resistance'at the hump,
however, is more critically dependent upon hull loading
than that at high speeds. If a design shows a t-ondoncy to
"stick" near get~away itIIcan be improved to some extent by
decreasing the hull. size, thus increasing the value of the
load coefficient and hence the ratio of load to resistance.