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National Advisory Committee for Aeronautics, Technical Notes - Exploratory Wind Tunnel Investigation of the Effectiveness of Area Suction in Eliminating Leading Edge Separation Over an NACA 641A212 Airfoil

An exploratory investigation was made in the Langley two—dimensional
low-turbulence pressure tunnel on an NACA 6hlA212 airfoil with various
extents of permeable surface area between the leading edge and 12.5 percent
chord to determine the effectiveness of area suction in eliminating
leading—edge separation at high lift coefficients. Lift and internal
pressure measurements were obtained at a Reynolds number of 1.5 X 106
for a range of flow coefficients from 0 to 0.008. Airfoil surface pressures
were measured over a range of angles of attack from h.lo to 18.30 with the
upper surface porous to h.5 percent chord.

The results obtained indicate that not only was leading—edge
separation prevented, but also turbulent separation moving forward from the
trailing edge was delayed. The maximum effectiveness was obtained at
a flow coefficient of 0.0018 with the upper surface porous to h.5 percent
chord. With more than h.5 percent chord permeable, the maximum section
lift coefficient cZ of the airfoil was not changed appreciably, but
the flow coefficient required to obtain c; was considerably increased.

It was also determined that for this airfoil at a similar Reynolds number
the maximum section lift coefficient is about the same as that for the
airfoil with a leading-edge slat.

The maximum lift coefficients of thin airfoil sections are low as a
result of separation of the laminar boundary layer near the leading edge.
Many types of leading—edge high—lift devices, such as flaps and slats,
have been investigated in an attempt to increase these naturally low
maximum lift coefficients. Single suction slots near the leading edge
also have been investigated but proved unsatisfactory because of changes
in the position of the laminar separation point with variations in angle
of attack. Area suction through a permeable surface near the leading
edge appeared to offer a method of applying boundary—layer suction to
control laminar separation.