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National Advisory Committee for Aeronautics, Report - Span Load Distribution as a Factor in Stability in Roll

This report gires the results of pressure-distribution
tests made to study the qfects on lateral stability of
changing the span-load distribution on a rectangular
monoplane wing model of fairly thick section. Three
methods of changing the distribution were employed:
variation in profile along the span to a thin symmetrical
section at the tip, twist from +5° to —15° at the tip, and
su'eepbaclc from +20“ to —20°. The tests were con-
ducted at the Langley hfemorial Aeronautical Laboratory
in the 5Joot closed-throat atmospheric wind tunnel.
The rolling moment due to roll at a rate of rotation
equicalent to that resulting from. the maximum rolling
disturbances encountered in normal flight is used as the
principal basis of comparison. NormalJorce curves are
girenjor the purpose of estimating the general efectireness
of each wing arrangement.
The problem of reducing the unstable rolling tend-
ency due to roll, characteristic of unyawed, stalled
flight, has been attacked in various ways. The earliest
methods consisted of attempts to improve lateral con;
trollability by increasing the rolling moments produced
by the ailerons. These methods enabled the pilot to
correctior disturbances more eflect-ively, but did not.
remove the source of the danger, the rolling tendency
itself. Subsequent developments resulted in the use
of such devices as the Handley Page automatic slot
and the wing tip floating aileron, both of which have
been shown not only to reduce the unstable rolling tend-
ency due to roll but also to improve the controllability.
Another method for reducing lateral instability is
the modification of the shape of the conventional
wing. The nature of the modifications best suited
to this purpose may be determined from a considera-
tion of the factors cont-rolling rolling moment due to
roll. Primarily, this moment depends upon the dis-
tribution of load along the span when the wing is
given an angular velocity in roll. This distribution
is dependent upon the variation of the chord along
the span, the angle of attack, and the slope of the
curve of normal force for each section. Also, the
nearer a section is to the tip the greater is its import-
ance, because of its longer moment arm and the larger
difference in angle of attack between it and the mid-
span section when the wing is rolling.