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National Advisory Committee for Aeronautics, Technical Notes - Wind Tunnel Investigation of Boundary Layer Control by Suction on NACA 655-424 Airfoil with Double Slotted Flap

An investigation has been conducted at Reynolds numbers ranging
from 1.0 X 106 to 6.0 x 106 in the Langley two-dimensional low-
turbulence tunnels to determine the effectiveness of boundary-layer
control by suction and of suction-slot location in increasing the
maximum lift and decreasing the drag of the NACA 655-h2h airfoil section
equipped with a double slotted flap. Tests were made of the model with
a suction slot at 0.h5 chord and with a suction slot at 0.65 chord.
Measurements were made to determine the section lift, drag, and internal
pressure-loss characteristics for flow coefficients ranging from 0 to 0.03
for the airfoil with the flap extended and retracted, with and without
leading-edge roughness.

At a Reynolds number of 6.0 x 106, deflecting the flap increased
the maximum section lift coefficient of the smooth airfoil from l.h
to 3.h, and boundary-layer control at 0.65 chord with the flap deflected
further increased the value to h.2. For the model with the flap de-
flected, the maximum section lift coefficients obtained with the
boundary-layer control slot at 0.65 chord were generally higher than
those obtained with the slot at 0.h5 chord.

Boundary-layer control by means of a single suction slot at
0.65 chord resulted, for an extensive range of section lift coefficient,
in a section total-drag coeffiCient, which included a drag-coefficient
allowance for the boundary-layer control power, lower than that of the
plain airfoil. The maximum section lift-drag ratio of the airfoil with
flap retracted and leading-edge roughness was more than doubled at a
Reynolds number of 6.0 x 106 by the use of boundary~layer Control and
that for the smooth airfoil was increased by approximately 38 percent.

The use of airfoil sections having thicknesses greater than about,
21 percent of the airfoil chord has generally been avoided because of
the low maximum lift and high drag usually associated with such sections.