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National Advisory Committee for Aeronautics, Report - Determination of Control Surface Characteristics from NACA Plain Flap and Tab Data

e data from previous NACA pressure-distribution
investigations of plain flaps and tabs with sealed gaps
have been analyzed and are presented in this paper in a
form readily applicable to the problems of control—surface
design. The experimentally determined variation of aero—
dynamic parameters with flap chord and tab chord are
given in chart form and comparisons are made with the
theory. With the aid of these charts and the theoretical
relationships for a thin airfoil, the aerodynamic character-
istics for control surfaces of any plan form with plain
flaps and tabs with sealed gaps may be determined. A
discussion of the basic equations of the thin-airfoil
theory and the development of a number of additional
equations that will be helpful in tail design are presented
in the appendices. The procedure for applying the data
is described and a sample problem of horizontal tail
design is included.

The data presented and the method of application set
forth in this report should provide a reasonably accurate
and satisfactory means of computing the aerodynamic
characteristics of control surfaces.

The need for an improvement in the method of pre-
dicting the aerodynamic characteristics of ail-foils with
multiple hinged flaps, such as horizontal. and vertical
tail surfaces, has long been realized. A number of
valuable contributions of both an experimental and a
theoretical nature have been made but the ultimate
objective has not yet been attained. With the inten-
tion of more closely approaching a satisfactory solution
of the problem the National Advisory Committee
for Aeronautics has undertaken a control-surface
investigation.

The theoretical expressions for the lift and the
pitching—moment coefficients of an airfoil and the hinge-
moment coefficients of any number of flaps about any
hinge position on the airfoil have been derived in refer.—
ences 1, 2, and 3.

Experiments have, however, failed to check the
theory, especially in the case of hinge—moment coeffi-
cients of small-chord flaps. It is for this reason that
the design of tail surfaces has depended largely on
experiments.