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Wind Tunnel Investigation of Control Surface Characteristics - XVIII - A Linked Overhang Aerodynamic Balance

Wind-tunnel tests have been made in two-dimensional
flow to investigate the aerodynamic characteristics of a
flap balanced by a large overhang linked to deflect more
slowly than the flap. Three lengths of blunt-nose over—
hang were tested linked to a 0.30—airfoil-chord straight-
contour flap on an NASA 66-009 airfoil.

The test results indicated that the linked overhang
was capable of producing as highly balanced flap hinge
moments as can be obtained with other types of aerodynamic
balance. At the same time, the linked-balance flap pro—
duced slightly higher lift at large deflections than the
corresponding unbalanced or internally balanced flap and
much higher lift than flaps balanced by smaller conventional
type overhangs. Such characteristics can be obtained with
a linked balance because much balance can be obtained with-
out the nose of the overhang protruding sufficiently far
into the air stream to cause severe air-flow separation at
large de fle cti one .

Because the hinge—moment parameters are functions of
the rate of balance deflection, adjustment of the balancing
characteristics of a control surface can be made on each
individual airplane merely by changing the length of a link.

INTRODUCTION

Attempts to produce a highly balanced control surface
by providing a flap with a large overhang or internal
balance usually impair the maximum lift that would be pro-
duced by the surface without the balance. Flaps with
large overhangs generally encounter air—flow separation at
large deflections, and flaps with large internal balances
usually cannot be deflected to large angles because of space
limitations.
One possible means of obtaining the lift of an un—
balanced plain flap with the hinge momentsof a balanced
surface was suggested in reference 1, which preposed that
a control surface overbalanced by a large overhang be
provided with a tab to deflect in the same direction and
as some function of the control-surface deflection. By
this means, the control surface might be limited to low
deflections free from air—flow separation yet the lift
would be increased by the tab deflection. This arrange—
ment was tested on a finite—span model of a horizontal
tail surface (figs. 127 to 150 of reference 2) in the
LMAL 7— by 10-foot tunnel. 'The tab deflection, however,
increased the hinge moments of the control surface so
rapidly that the desired increments of lift caused by tab
deflection could not be achieved without excessively
large hinge moments.

The tab characteristics presented in figure 1A7 of
reference 2 indicate the optimum length of tab to use to
increase the lift of an overbalanced control surface re—
stricted in deflection range. An analysis of these data
and the data of figure lhl of reference 2 leads to the
conclusion that a tab with a chord equal to the chord of
the control surface should provide a maximum increment in
lift for a minimum increment in hinge moment. Such an
arrangement is the equivalent of deflecting the portion
of the movable surface ahead of the hinge axis at a slower
rate than the portion behind the hinge axis.

The desirability of linking an overhang balance to
deflect at a different rate from that of the control sur—
face to be balanced having been established, the problem
arises of the optimum length overhang and rate of deflec-
tion. Because the unporting angle and the resulting
separation over the nose of the balance vary;roughly as
the first power of the balance length whereas the bal—
ancing moment varies as some power of the balance length
higher than the square, it should be aerodynamically
advantageous to increase the balance length and to make
the balance deflect more slowly. Such a procedure must,
however, be limited by structural and practical consider-
ations. '

The current series of tests were therefore made to
determine the extent to which the-lift characteristics of
an unbalanced control‘surface could be maintained while
the control surface was provided with as great a degree
of aerodynamic balance as is commonly obtained on control
surfaces highly balanced by conventional means.