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Notes on the Effects of Trailing Edge Shapes of Low Drag Airfoils on Profile Drag and the Trim and Balance of Control Surfaces

By w. J. Underwood

Questions have arisen concerning the value of adher-
ing to the specified cusp trailingwedge shapes on low-
drag airfoils as opposed to trailing edges with straight-
.line elements and the effect of other unsymmetrical trail-
ine—edge shapes on the airfoil profile drag and the trim
and balance of control surfaces. '

Comparative drag tests of s'0.20c straight faired
aileron end a 0.20c cusp~type aileron, which adhered to
the specified wing contour, as shown in figure 1I were
made on a low-drag airfoil model (approximately NASA
66,2-116) in the NAGA lowmturbulenco tunnel. The model
with the cusp—type aileron had the lower drag coefficient.
The increment, Acdo, was equal to 0.000?.

Drag, lift, and aileron hinge moments were measured
on a 100-inch-chord model (approximately NACA 65,2-417)
with various modifications to the trailing edge of
the 0.20c Iriso type aileron as shown in figures 2
and 8. The profile drags with the aileron neutral for
the various modifications, with the drag of the original
shape used as a reference. are as follows:

Acdo
Modification no. 1 0.0001
Modification no. 2 0.0003
Modification no. 3 0.0009
Modification no. 4 0.0005

The effects of the same modifications on the aileron
effectiveness and hinge moments are given in figure 4.
The coefficients in all cases are based on the actual
chord of the model as tested with the various modifications.
From the lift curves in figure 4 it can be seen that
modifications no. 1 and no.-3 show little change in the
slope of their lift curves from that of the original ,
shape. Modifications no. 2 and no. 4, however, due to
the shortening of the aileron chord, show a decrease in
the slope of their lift curves.

0911-1

From the hinge-moment curves in figure 4 it can be
seen that the hinge moments for up deflections in modim
fications no. 1, no. 2, and no. 4 are lower than the
hinge moments of the original shape. This decrease in
the slope of the hinge—moment curve is attributed to the
partial relieving or unstalling of the flow near the
trailing edge of the aileron. Modification no. 3 failed
to show this decrease in the slope of the hinge—moment
curve because stalling at the trailing edge was present
throughout the angular range of the aileron due to the
high camber at the trailing edge.

The purpose of these data is to show qualitatively
the effects of comparatively small modifications to the
trailing edge of an aileron. The data are not suffi—
ciently complete for design purposes.