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The Longitudinal Stability of Flying Boats as Determined by Tests of Models in the NACA Tank - II - Effect of Variations in Form of Hull on Longitudinal Stability

By Starr Truscott and Roland E. Olson
flflMMARY

Resultsof investigations of the longitudinal—stability
characteristics of several models are considered in an at«
tempt to arrive at general conclusions as to the effects of
variations in the form of hull on these characteristics.
Data are used from tests at constant speed, establishing the
trim limits of stability; from tests at accelerated speeds,
establishing the limits for stable positions of the center
of gravity; and from tests at decelerated speeds, establish—
ing the landing characteristics. The conclusions drawn are
not necessarily final tut the available information indicates
certain trends that are offered as a guide to future tests
and design. '

' The lower trim limit of stability is not appreciably
affected by changes in position of center of gravity, posi—
tion of step, plan form of step, depth of stop, angle of
afterbody keel, and length of afterbody. A reduction in the
angle of dead riSe decreases this limit to lower trims. An
increase in gross weight raises this limit to higher trims.

The upper trim limits of stability are not appreciably
affected by a change in position of center of gravity. Mov~
ing the step aft appears to raise the limits slightly. These
limits are raised to higher trims by an increase in gross

-weight, an increase in depth of step, an increase in angle

of afterbody keel, a decrease in length of afterbody, and

by ventilation of a shallow step. These limits are changed
by a variation in the plan form of the step in proportion to
the changes in the effective depth of step and the effective
position of the step.
The.limits for stable positions of_the center of
gravity are shifted by a distance approximately equal to
the distance the centroid of the step is moved; Increasing
the depth of step does not appreciably change these limits.
With heavier gross weights the range of stable positions
for the center of gravity is reduced.

Instability in landing at high trims is reduced or
eliminated either by increasing the depth of step or by
ventilating the step. A depth of step of the order of 8
percent of the beam has been found necessary. Large venti—
lation ducts lOcated near the keel and just aft of the step
are effective, but ventilation ducts near the chine are in—
effective. With a depth of step of 5.5—percent beam, the
landing instability of one model was not eliminated by Vary~
ing the angle of afterbody keel from 4° to 8.50 and increas—
ing the length of afterbody from 161 to 311 percent of the
beam. '

INTRODUCTION

Several models of flying boats have been investigated
at the NACA tank in an effort to determine their longitudinal—
stability characteristics. Part I (reference 1) of this re~
port describes the methods that have been used at thertank.
The models usually represented specific designs; generally
either the full~size airplane had been built or the construc—
tion was at an advanced stage before tests of the model were
requested. The possible modifications were, therefore,
limited to small changes that were expected to improve the
stability characteristics without appreciably altering the
existing design..

With such an approach to the problem of longitudinal
stability, the greater part of the research has consisted
of a number of unrelated tests, each of which was made for
the specific purpose of improving the stability of a partic—
ular design. The investigations have been restricted to the
essentials because of the limited time that could be allotted
to any single test. A complete study of the effects of all
the modifications was therefore impossible, and in many in~
stances the data are incomplete. Repetition during the sev~
eral tests has been large, and the contribution of any single
test to the general problem has often been small.