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National Advisory Committee for Aeronautics, Technical Notes - An Experimental Investigation of an NACA 631-012 Airfoil Section with Leading Edge Suction Slots

An NACA 631—012 airfoil section equipped with a single suction
slot near the leading edge was investigated to determine whether or
not the maximum lift coefficient could be increased by delaying the
separation of flow at the leading edge characteristic of the basic
section. The leading—edge separation was delayed and the linear
portion of the lift curve substantially extended until the turbulent
boundary layer separated from the rear portion of the airfoil. The
abruptness of the stall was thereby reduced.

The maximum lift increased with increasing flow through the slot,
rapidly at first, then at a diminishing rate. The effect on pitching
moment was negligible. The profile drag was increased for low values
of lift and reduced at high values of lift (for flow coefficients
greater than 0.002) over the corresponding drag of the basic airfoil
section.

It was found that the slot location and width are important.
Sixteen different slots were investigated without encountering the
optimum, but the results indicated that the leading edge of the slot
should be downstream of the point of separation of flow from the
leading edge of the basic airfoil immediately prior to its stall.

The efficacy of boundary—layer control as a means of delaying
separation of the turbulent boundary layer, and thereby increasing
the lift of airplane wings, has been demonstrated by numerous small—
scale experiments. Despite the favorable results of these experiments,
few, if any, practical applications to conventional wings of moderate
thickness have resulted because simpler high-lift devices were capable
of producing adequate lift.